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Henry Pasternack Henry Pasternack is offline
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Default Diodes, triodes, and negative feedback

I noticed that the triode negative feedback argument is still going on. I'm a
bit surprised because I don't think the issue is really all that complicated.
Somewhat complicated, yes, but not beyond resolution.

The answer to the question, "Does a triode have internal negative feedback?",
is, "Yes, if you want it to, no if you don't." But I'll admit my bias and state
from the start that I don't see much use to the triode feedback viewpoint.

I'll explain what I mean. Negative feedback is a model. The system operates
the same regardless of the model. You can imagine an operational amplifier
with its inverting input connected to its output through a voltage divider. You
can model the system in terms of its forward and reverse transfer functions
and use the feedback equation to come up with the answer. Or you can just
solve for the response as a single system of equations, and never bother to
consider negative feedback. The amplifier doesn't care. It's up to you, the
designer, to decide what form of analysis you find most convenient at the time.

It's true that if you open and close the feedback loop, certain predictable
changes occur. The gain, bandwidth, transient response, and input/output
impedances change. You can say that the existence of these changes
"proves" that feedback is at work. But this is circular reasoning. You could
just as easily explain the difference by noting how the connection from
output to input connection modifies the signal input voltage.

With respect to triodes, you can model the internal behavior different ways.
The traditional model views the cathode as an electron emitter where the
probability of emission depends on the cathode temperature and the
electric field at the cathode surface. It assumes the electrons come out
with zero kinetic energy. The plate-cathode voltage establishes a boundary
condition and the subsequent density, distribution, and velocity of electrons
within the tube falls out from the solution of a relatively simple differential
equation.

The effect of the grid-cathode potential is to augment the plate-to-cathode
electric field, modifyinng the boundary conditions of the differential equation
and shifting the current-vs-voltage curve left or right according to the grid
voltage. The plate resistance is simply the slope of this curve, and the
effects of both plate and grid potentials are purely in the "forward" path.

Taking the other position, you could view both the plate and the grid as inputs.
Their voltages each produce an electric field at the cathode surface which,
summed together, determine the space charge density around the cathode.
The space change density, in turn, determines the plate current and, in
conjunction with the plate resistor, this creates a signal voltage at the plate.
The signal voltage "feeds back" to the cathode electric field, modulating the
space charge density and therefore the resulting current flow.

The second model is pretty complicated, and there are some serious
objectsions to be made to it. I'll get to that, but first I want to talk about
diodes and triodes.

It's been stated on this forum that diodes, unlike triodes, have no internal
negative feedback. Isn't this is a paradoxical claim? The grid in a triode
biases the space charge density, but It doesn't fundamentally change how the
tube operates. Indeed, if you connect the triode's grid to its cathode, the
E-field component due to the grid is zero and the tube behaves for all intents
and purposes, exactly like a diode. If a triode has internal negative feedback,
then by the exact same argument, so must a diode. This is an extremely
important point to understand.

If you have an operational amplifier in the classic non-inverting configuration
and you connect the input to ground, the negative feedback is still in control
and the amplifier's output impedance stays the same. But grounding the
non-inverting input takes that input out of the picture. Mathematically, it's
as though it no longer exists. The three-terminal op-amp has turned into a
two-terminal op-amp yet the negative feedback is unaffected. This is how a
regulated power supply works, incidentally.

The NFB argument says that the low output impedance of the triode is due
to the negative feedback between the plate and cathode, which, if we were to
put a screen grid into the tube, could be taken away, thereby significantly
raising the output impedance. Now, the plate curves of the diode and the
triode (with Vg=0) are exactly the same. You could put a screen grid in the
diode, apply a fixed potential, and greatly increase its plate resistance, just
like a triode. Overall, if you are comfortable saying that a triode is a pentode
without a screen grid, then you should be equally comfortable saying that a
diode is is a pentode without a screen grid or a control grid. Either they both
have feedback, or they don't.

Why is this important? One of the claims I have read on this newsgroup recently
is that the plate resistance of a diode is just an "ordinary" non-linear resistance
and has nothing to do with negative feedback. The mechanism that produces the
3/2 power plate curve is exactly the same for the diode as it is for the triode. The
Child-Langmuir law applies to both and the derivation is the same. If the plate
resistance of a diode is just an "ordinary" resistance, then so is the plate
resistance of a triode. The control grid is a complication, but it has no bearing
on this mechanism.

To understand whether or not a triode has negative feedback, we'd like to boil
the problem down to its barest essentials so that the underlying principles
are exposed clearly and without confusion. That's why it helps to show that
the control grid is irrelevant to the question. It's hard to visualize negative
feedback in a diode (or a resistor, for that matter, but more on that later)
because a simpler, open-loop model is just cleaner and more intuitive. When
you add a control grid in there, things get more complicated and it becomes
tempting to start waving hands. We'd like to avoid that.

Getting back to op-amps, it's very easy to separate out the forward path and
the reverse path. The forward path is everything inside the little triangle,
between the two input leads and the one output lead. The reverse path is the
external network connecting the output terminal to the inverting input terminal.
Being able to separate these two paths so cleanly makes it much easier to
visualize how the operational amplifier operates as a feedback circuit.

In the proposed diode NFB model, there is an inconvenient blurring between
the forward and reverse paths. The plate-cathode potential has two functions.
First, it establishes the electric field at the surface of the cathode, which in
turn modulates the space charge density. Second, the very same field
sweeps electrons out of the electron cloud and accelerates them to the
plate, establishing plate current. The first of these two functions may be
called feedback. The other determines the forward gain. To show that the
tube has negative feedback, you have to separate these two paths out, but
it's not such an easy thing to do. This is a problem for the feedback model.

There are two ways that tube feedback modelers have solved this problem.
The first is propose connecting the plate to a fixed DC potential. This lets
the tube keep conducting but eliminates any E-field feedback from the plate.
Fixing the plate potential raises the gain (transconductance) as predicted by
feedback theory. This is the essence of an argument that one member of
this forum put forth as "proof" of triode NFB.

But there is a glaring hole in this argument. There is absolutlely nothing
that external observation can prove conclusively about the internal operation
of the tube. There could just as easily be a nonlinear resistor inside the tube,
or little elves with voltmeters and a rheostat controlling the electron flow in
real time. All you can determine from external observations are the external
properties of the tube, and while these may suggest the action of negative
feedback, they do not by any means prove it.

The other, perhaps more convincing way that feedback modelers have tried
to prove their point is by postulating the existence of a fictitious screen grid
inside the tube. The screen grid takes away the feedback due to the varying
plate E-field while allowing the plate voltage to vary. With this fictitious screen
grid in place, the tube has higher gain, like a pentode.

There are several rebuttals to this argument. The simpler one is that the
fictitious screen grid is, indeed, a fiction. The triode is not a tetrode. It
should suffice to model the tube without the artifice of the fictitious grid.
Indeed, there is there is a perfectly good model that doesn't depend on
this complication. But the real problem with the fictitious grid argument
is that it pulls itself up by its own bootstraps. It says, a triode has NFB,
and a pentode is a triode with the NFB taken away by the screen. So
if we take a pentode and remove the screen, the feedback comes back
and this proves the triode has NFB. This is a brilliant example of circular
reasoning.

The triode NFB argument depends on another fiction, the "virtual grid", which
is the internal control point related to the electric field strength at the cathode
and space charge density. The model says the actual input to the tube is
this "virtual grid", which is related to but distinct from the external grid and
plate connections. This is an extremely awkward model because it relies
on an abstraction of the tube that is quite removed from its actual use in-circuit.
Contrast this with the op-amp where the input, output, and feedback connections
are quite clear and in the open.

One of the points made against the triode feedback model is that there is no
effect on the input impedance seen at the grid. In response, it's been noted
that even with conventional shunt feedback applied to the cathode, there is no
effect in the tube's input impedance. This isn't true. Shunt cathode feedback
definitely raises the input impedance seen at the grid. It's just that the tube's
input impedance is already so high and swamped by the bias resistor that we
don't notice the difference.

I mentioned resistors earlier, and I'd like to get back to that. Others have
pointed out that you can use the same kind of argument for triode NFB to
show that there is negative feedback in resistors, or automobiles, or electric
motors. It's actually kind of fun to derive the formula for a voltage divider in
terms of negative feedback, or even transmission line reflection coefficients.

Consider a resistor as a cylinder of material with a metallic cathode
bonded to one end and a metallic anode bonded to the other end. If you
apply a positive potential between the anode and the cathode, an electric
field will be established between the electrodes and electrons will begin to
drift inside the cylinder, creating a current flow. If the power supply has
significant output resistance, the supply voltage will drop, reducing the
electric field within the body of the resistor and modulating the current. In
other words, negative feedback. Or is it feedback?

The difference between the diode feedback model and the resistor feedback
model is that in the resistor you don't have the additional complication of
the space space charge density. But the principle is the same. In either
case, the current flow that results from the application of an external
voltage is set by an equilibrium that develops within the device. You can
find zillions of similar examples in nature and engineering.

Parachutists are grateful for the negative feedback of air resistance that
reduces the "gain" of gravity accelerating them to the ground. Airplane
designers take pains to eliminate this feedback, maximizing the "gain" of
their engines by installing aerodynamic screens on the aircraft. Yes,
feedback is everywhere. And nowhere. It all depends on your point of
view.

Whether or not you choose to think of tube operation in terms of feedback
is your choice. It causes no harm to anyone. But you should be careful
to avoid insisting on the presence of feedback. Feedback is only as good
as the value it brings to engineering analysis. In practical terms, when it
comes to triodes the value isn't that great. I don't believe it gives us any
greater insight than we get from the standard models. In particular, as we
move from coarse generalities to much finer levels of detail, it remains to be
shown that feedback helps to predict the second- and third-order nonlinear
behavior of the tube.

It's been said, and I agree, the tube doesn't give a damn how you model it,
but goes about its business in blissful ignorance. Triode negative feedback
is a great conversation starter, but there is a risk, in invoking negative feedback,
of making things more confusing than they have to be. Out of confusion comes
bull****, and bull****, we all can agree, is something to be avoided.

Right?

Have a nice day.

-Henry


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Chris Hornbeck Chris Hornbeck is offline
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Default Diodes, triodes, and negative feedback

On Fri, 22 Sep 2006 16:06:35 -0400, "Henry Pasternack"
wrote:

Brilliant analysis, saved, but snipped 'cause you've
already read it.

Leaving only two questions: if it's feedback, where's
the change in bandwidth and input-referred noise?

And B: what do we gain from pretending that it's true?
IOW, what insights arise from stretching this analogy
into an assumption?


I'm very sorry to be so hard-assed about such a trivial
subject, but fuzzy thinking has become palpably dangerous
in the modern world. Fight the power. Fuzziness is great,
even if it gets in yer teeth, in some other circumstances,
of course.

Much thanks, as always,

Chris Hornbeck
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Phil Phil is offline
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Posts: 80
Default Diodes, triodes, and negative feedback

Henry,

VERY nice analysis, with many points that I and others missed (I've been
wanting to get back to this for days, but didn't know where to begin). I
think that you and I agree on one point above all others, namely that
even if you *can* model the low Zout of a triode as a negative feedback
mechanism, that doesn't mean that you *should*. An alternative model,
which provides the same predictions, may give a better insight into good
audio design, or may simply be easier to use, or may in fact be a
genuinely more accurate description of the actual physical processes in
the triode.

One of the most compelling of Patrick's arguments is the fact that a
change in the plate voltage changes the "electrostatic topology" from
the cathode to the plate. That is true, but as you point out, we can say
the same thing about a resistor, and the idea of throwing out the
concept of resistance -- where a larger force provides more acceleration
to a bunch of electrons, thereby causing greater throughput -- in favor
a a feedback mechanism that, in effect, merely makes it *easier* for
electrons to move, appears to be both arbitrary and counterproductive.

But let's get to "their" main point, the largely unspoken one, namely
that "triodes don't either have an advantage over pentodes and
transistors due to their ability to operate without the need for
feedback, because they do have feedback, it's just internal." Implicit
in this belief is the idea, probably correct, that feedback really does
add non-musical distortions in ways that are still hard to measure and
understand, which would explain the amazing ability of the better
single-ended triode amps to deliver MUSIC, as opposed to the sound of a
"low distortion" pentode or solid state feedback amp (something I didn't
believe for an *instant* until I actually heard one). The question then
becomes whether, if triodes "really do" have this feedback mechanism,
they should be regarded as being essentially the same as pentodes and
transistors.

An important point is that the electrostatic changes in the medium of
space -- the "space charge" -- caused by a change in the plate voltage
do *not* occur in the time required for an electron to move from the
cathode (or even from the electron cloud around the cathode) to the
plate, but rather at the speed of light. In the case of a plate 3 mm
from the cathode, this is 10 pico-seconds. Now even if this is related
to the quarter-wavelength of the equivalent frequency -- 4 times slower
than simply assuming it is the inverse of frequency -- that is still 25
GHz (giga-Hertz). Since such a triode can easily have a plate impedance
from 10 to 100 times less than a physically similar pentode, that
becomes 20 to 40 dB of feedback with a closed loop response of 25 GHz,
and an open loop response of 250 GHz to 2,500 GHz. The latter number is
out of the microwave region, and into the far infrared region! Nor can
we say that this only applies to a single triode (which it does),
*therefore* we should compare it to a single pentode/transistor, because
for triode amps we only need low output impedance in the last stage,
whereas virtually all transistor amps, and many pentode amps, require at
least 3 stages to get sufficiently low output impedance (using lots of
feedback), so that is what must be compared to the single output stage
of a zero-feedback triode amp (although this means the output
transformer will not be included in a feedback loop).

How many 40 dB pentode or transistor power amps have a closed loop
bandwidth of even 25 MHz, 1,000 times less than the triode described
here? None, that I know of (I think it's possible to make such a beast,
but it's probably *very* hard). So, the blanket statement that "triodes
are the same as pentodes, because they have an internal feedback" fails
to mention one *very* significant detail, namely that this "triode
feedback" occurs, when compared to a typical pentode/transistor amp with
a closed loop upper frequency limit of 100 KHz, at a speed 250,000 times
faster. Now, I am willing not only to accept, but to bet, that if you
could make a pentode/transistor amp with 40 dB of feedback, a clean
error signal (accurate to 100 to 120 dB), and a closed loop bandwidth of
25 GHz (implying an infrared open loop bandwidth of 2,500 GHz), that it
would sound just as good, if not better, to human ears, as *any* triode
amp in existence. But as the old farmer said when he first saw a
giraffe, "there ain't no such animal."

That's not exactly a minor difference, and it makes the whole idea of
lumping "triode feedback" in the same category as the pentode/transistor
feedback found in audio amps ridiculous. Perhaps in microwave amps we
could see some genuine effects (the bad ones) from "triode feedback,"
but for audio amps, it's basically just silly, and we really do get a
better understanding of what will happen, what we can expect, and the
*difference* between triodes and pentodes by modeling triodes as a
current source in parallel with a resistor, the plate resistance, just
as electronics engineers have done all along.

Now, having agreed with Henry, and (hopefully) ripped what little was
left of Patrick's idea of triode feedback into even smaller shreds, I'm
going to do something weird, and say that I'm beginning to wonder if
Patrick is in some ways RIGHT. It may be that a triode is *most*
accurately described as a resistance, but even if a change in plate
voltage *primarily* changes the velocity of the electrons, thereby
changing the current, the plate voltage does also affect the medium of
space around the grid and cathode, and this at least has a minor effect
on the current flow. The tremendous speeds -- and in more modern tubes,
we can be talking 0.3 mm, or 250 GHz, the low end of the infrared range,
even at closed loop speeds -- may make this "triode feedback effect"
totally insignificant as far as audio is concerned, but there's
something else that may be a total exception; the resistor.

During The Audio Critic's first run (until around 1980), I used to
believe that almost everything in it was true, but when I finally heard
some good tube amps, different cables, and CD players, it was obvious
that there were very audible differences between such devices which
*most* people could hear, and when TAC began preaching that "all amps
and cables sound the same" when properly ABXed, I realized he/they
either had a tin ear, or let their belief in theory override their ears
(in particular, they failed to realize that a standard ABX allows people
to memorize a musical passage, turning it into a *test* of a person's
memory, instead of a *comparison* between the two components). Since
then, the people who tweak their own systems, or at least get their
products from the best people out there, both homebrewers and companies,
have become my references (plural because in that range, tastes and
opinions vary). And one thing that virtually everyone with a high end
system agrees on, is that chokes and stepped transformers sound *much*
better than resistors and pots, respectively.

How? If a resistor has a very low temperature constant, and a low
voltage constant, then it *can't* mess up a sine wave, and it is hard to
see how it could possibly mess up a collection of signals, even when
there is a mix of signals with some signals being 100 to 120 dB lower
than the others. But a resistor can consist of several meters of wire on
a ceramic core that has both a high dielectric constant (which slows
transmission) and high dielectric absorption. In theory, the "closed
loop frequency limit" of a resistor could be 25 MHz, about the same as
the best pentode/transistor feedback amps. Furthermore, suppose for a
moment that it actually takes some time, when hit with a pulse, for the
current through a resistor to stabilize. It could have an initial surge,
followed by a bit of ringing as it settles to a final value. Now, by the
normal understanding of resistance, I think this is ridiculous, and it
is one of the reasons why, when it first became clear to me that "triode
feedback" requires us to view ordinary resistors as "feedback devices,"
that I concluded that the whole idea of a "triode feedback" was sheer
nonsense. But if it is not nonsense, then resistors do have a feedback
mechanism, and triodes are capable of being resistors of *much* higher
quality than a normal resistor (due to short lengths and a vacuum), and
chokes and tapped transformers, which basically override resistance with
an inductive impedance, can also handle mixed signals ranging 120 dB in
magnitude and 3 decades in frequency much more accurately than a
resistor. Now, maybe there is no feedback mechanism in a resistor, and
its inability to handle all the elements of the music spectrum
simultaneously is due to other phenomena, but the feedback idea would
explain the problems of resistors, although it should be noted that
merely "fitting the facts" is not in any way a *proof*.

Anyway, to summarize it seems to me that "triode feedback" is at best
inaudible, due to the ridiculous speeds involved, but when it comes to
resistors, I'm wondering if this really is the true explanation for the
lower audio capabilities of resistors relative to iron loads. Assuming
we know what feedback does wrong (it realistically has to be
phase-smearing, as Otala said, but that's an argument for another
post!), we may be able to design tests for it. Contrary to what Patrick
said elsewhere, it is naive in the extreme to believe that if feedback
problems exist, someone would have developed tests for it by now, and
even more naive to believe that the results (if feedback does have
problems) would have become common knowledge. There is a tremendous
intellectual inertia in the human race! The tests must check for
phase-smearing, a "blurring" of a sine wave signal on the 'scope when
two signals separated in frequency exist simultaneously in a feedback
amp or component, after one signal has been filtered out (a spectrum
analyzer won't work, because it sweeps the frequency). Since we don't
know how this process works, it must be tried when the two signals are
very different in magnitude, as well as the same, testing both the high
and low frequencies for time-smearing (note that the 'scope must be
triggered from the oscillator, not from the output of the amp, and then
compared to the same amp with no feedback). I don't know for certain
whether *any* of this will actually bear fruit. If the whole feedback
idea for both triodes and resistors is pure nonsense, then no fruit will
indeed be the result, and if we can't find 'scope tests for the
phenomena, then it will be difficult to verify even if it is true! But
it is an area that I believe has not been explored (other than Otala's
proof of phase-smearing), mainly because it initially sounds so stupid,
and it may yet result in both real insights, and real improvements, into
the science and understanding of audio.

Phil

Henry Pasternack wrote:
I noticed that the triode negative feedback argument is still going on. I'm a
bit surprised because I don't think the issue is really all that complicated.
Somewhat complicated, yes, but not beyond resolution.

The answer to the question, "Does a triode have internal negative feedback?",
is, "Yes, if you want it to, no if you don't." But I'll admit my bias and state
from the start that I don't see much use to the triode feedback viewpoint.

I'll explain what I mean. Negative feedback is a model. The system operates
the same regardless of the model. You can imagine an operational amplifier
with its inverting input connected to its output through a voltage divider. You
can model the system in terms of its forward and reverse transfer functions
and use the feedback equation to come up with the answer. Or you can just
solve for the response as a single system of equations, and never bother to
consider negative feedback. The amplifier doesn't care. It's up to you, the
designer, to decide what form of analysis you find most convenient at the time.

It's true that if you open and close the feedback loop, certain predictable
changes occur. The gain, bandwidth, transient response, and input/output
impedances change. You can say that the existence of these changes
"proves" that feedback is at work. But this is circular reasoning. You could
just as easily explain the difference by noting how the connection from
output to input connection modifies the signal input voltage.

With respect to triodes, you can model the internal behavior different ways.
The traditional model views the cathode as an electron emitter where the
probability of emission depends on the cathode temperature and the
electric field at the cathode surface. It assumes the electrons come out
with zero kinetic energy. The plate-cathode voltage establishes a boundary
condition and the subsequent density, distribution, and velocity of electrons
within the tube falls out from the solution of a relatively simple differential
equation.

The effect of the grid-cathode potential is to augment the plate-to-cathode
electric field, modifyinng the boundary conditions of the differential equation
and shifting the current-vs-voltage curve left or right according to the grid
voltage. The plate resistance is simply the slope of this curve, and the
effects of both plate and grid potentials are purely in the "forward" path.

Taking the other position, you could view both the plate and the grid as inputs.
Their voltages each produce an electric field at the cathode surface which,
summed together, determine the space charge density around the cathode.
The space change density, in turn, determines the plate current and, in
conjunction with the plate resistor, this creates a signal voltage at the plate.
The signal voltage "feeds back" to the cathode electric field, modulating the
space charge density and therefore the resulting current flow.

The second model is pretty complicated, and there are some serious
objectsions to be made to it. I'll get to that, but first I want to talk about
diodes and triodes.

It's been stated on this forum that diodes, unlike triodes, have no internal
negative feedback. Isn't this is a paradoxical claim? The grid in a triode
biases the space charge density, but It doesn't fundamentally change how the
tube operates. Indeed, if you connect the triode's grid to its cathode, the
E-field component due to the grid is zero and the tube behaves for all intents
and purposes, exactly like a diode. If a triode has internal negative feedback,
then by the exact same argument, so must a diode. This is an extremely
important point to understand.

If you have an operational amplifier in the classic non-inverting configuration
and you connect the input to ground, the negative feedback is still in control
and the amplifier's output impedance stays the same. But grounding the
non-inverting input takes that input out of the picture. Mathematically, it's
as though it no longer exists. The three-terminal op-amp has turned into a
two-terminal op-amp yet the negative feedback is unaffected. This is how a
regulated power supply works, incidentally.

The NFB argument says that the low output impedance of the triode is due
to the negative feedback between the plate and cathode, which, if we were to
put a screen grid into the tube, could be taken away, thereby significantly
raising the output impedance. Now, the plate curves of the diode and the
triode (with Vg=0) are exactly the same. You could put a screen grid in the
diode, apply a fixed potential, and greatly increase its plate resistance, just
like a triode. Overall, if you are comfortable saying that a triode is a pentode
without a screen grid, then you should be equally comfortable saying that a
diode is is a pentode without a screen grid or a control grid. Either they both
have feedback, or they don't.

Why is this important? One of the claims I have read on this newsgroup recently
is that the plate resistance of a diode is just an "ordinary" non-linear resistance
and has nothing to do with negative feedback. The mechanism that produces the
3/2 power plate curve is exactly the same for the diode as it is for the triode. The
Child-Langmuir law applies to both and the derivation is the same. If the plate
resistance of a diode is just an "ordinary" resistance, then so is the plate
resistance of a triode. The control grid is a complication, but it has no bearing
on this mechanism.

To understand whether or not a triode has negative feedback, we'd like to boil
the problem down to its barest essentials so that the underlying principles
are exposed clearly and without confusion. That's why it helps to show that
the control grid is irrelevant to the question. It's hard to visualize negative
feedback in a diode (or a resistor, for that matter, but more on that later)
because a simpler, open-loop model is just cleaner and more intuitive. When
you add a control grid in there, things get more complicated and it becomes
tempting to start waving hands. We'd like to avoid that.

Getting back to op-amps, it's very easy to separate out the forward path and
the reverse path. The forward path is everything inside the little triangle,
between the two input leads and the one output lead. The reverse path is the
external network connecting the output terminal to the inverting input terminal.
Being able to separate these two paths so cleanly makes it much easier to
visualize how the operational amplifier operates as a feedback circuit.

In the proposed diode NFB model, there is an inconvenient blurring between
the forward and reverse paths. The plate-cathode potential has two functions.
First, it establishes the electric field at the surface of the cathode, which in
turn modulates the space charge density. Second, the very same field
sweeps electrons out of the electron cloud and accelerates them to the
plate, establishing plate current. The first of these two functions may be
called feedback. The other determines the forward gain. To show that the
tube has negative feedback, you have to separate these two paths out, but
it's not such an easy thing to do. This is a problem for the feedback model.

There are two ways that tube feedback modelers have solved this problem.
The first is propose connecting the plate to a fixed DC potential. This lets
the tube keep conducting but eliminates any E-field feedback from the plate.
Fixing the plate potential raises the gain (transconductance) as predicted by
feedback theory. This is the essence of an argument that one member of
this forum put forth as "proof" of triode NFB.

But there is a glaring hole in this argument. There is absolutlely nothing
that external observation can prove conclusively about the internal operation
of the tube. There could just as easily be a nonlinear resistor inside the tube,
or little elves with voltmeters and a rheostat controlling the electron flow in
real time. All you can determine from external observations are the external
properties of the tube, and while these may suggest the action of negative
feedback, they do not by any means prove it.

The other, perhaps more convincing way that feedback modelers have tried
to prove their point is by postulating the existence of a fictitious screen grid
inside the tube. The screen grid takes away the feedback due to the varying
plate E-field while allowing the plate voltage to vary. With this fictitious screen
grid in place, the tube has higher gain, like a pentode.

There are several rebuttals to this argument. The simpler one is that the
fictitious screen grid is, indeed, a fiction. The triode is not a tetrode. It
should suffice to model the tube without the artifice of the fictitious grid.
Indeed, there is there is a perfectly good model that doesn't depend on
this complication. But the real problem with the fictitious grid argument
is that it pulls itself up by its own bootstraps. It says, a triode has NFB,
and a pentode is a triode with the NFB taken away by the screen. So
if we take a pentode and remove the screen, the feedback comes back
and this proves the triode has NFB. This is a brilliant example of circular
reasoning.

The triode NFB argument depends on another fiction, the "virtual grid", which
is the internal control point related to the electric field strength at the cathode
and space charge density. The model says the actual input to the tube is
this "virtual grid", which is related to but distinct from the external grid and
plate connections. This is an extremely awkward model because it relies
on an abstraction of the tube that is quite removed from its actual use in-circuit.
Contrast this with the op-amp where the input, output, and feedback connections
are quite clear and in the open.

One of the points made against the triode feedback model is that there is no
effect on the input impedance seen at the grid. In response, it's been noted
that even with conventional shunt feedback applied to the cathode, there is no
effect in the tube's input impedance. This isn't true. Shunt cathode feedback
definitely raises the input impedance seen at the grid. It's just that the tube's
input impedance is already so high and swamped by the bias resistor that we
don't notice the difference.

I mentioned resistors earlier, and I'd like to get back to that. Others have
pointed out that you can use the same kind of argument for triode NFB to
show that there is negative feedback in resistors, or automobiles, or electric
motors. It's actually kind of fun to derive the formula for a voltage divider in
terms of negative feedback, or even transmission line reflection coefficients.

Consider a resistor as a cylinder of material with a metallic cathode
bonded to one end and a metallic anode bonded to the other end. If you
apply a positive potential between the anode and the cathode, an electric
field will be established between the electrodes and electrons will begin to
drift inside the cylinder, creating a current flow. If the power supply has
significant output resistance, the supply voltage will drop, reducing the
electric field within the body of the resistor and modulating the current. In
other words, negative feedback. Or is it feedback?

The difference between the diode feedback model and the resistor feedback
model is that in the resistor you don't have the additional complication of
the space space charge density. But the principle is the same. In either
case, the current flow that results from the application of an external
voltage is set by an equilibrium that develops within the device. You can
find zillions of similar examples in nature and engineering.

Parachutists are grateful for the negative feedback of air resistance that
reduces the "gain" of gravity accelerating them to the ground. Airplane
designers take pains to eliminate this feedback, maximizing the "gain" of
their engines by installing aerodynamic screens on the aircraft. Yes,
feedback is everywhere. And nowhere. It all depends on your point of
view.

Whether or not you choose to think of tube operation in terms of feedback
is your choice. It causes no harm to anyone. But you should be careful
to avoid insisting on the presence of feedback. Feedback is only as good
as the value it brings to engineering analysis. In practical terms, when it
comes to triodes the value isn't that great. I don't believe it gives us any
greater insight than we get from the standard models. In particular, as we
move from coarse generalities to much finer levels of detail, it remains to be
shown that feedback helps to predict the second- and third-order nonlinear
behavior of the tube.

It's been said, and I agree, the tube doesn't give a damn how you model it,
but goes about its business in blissful ignorance. Triode negative feedback
is a great conversation starter, but there is a risk, in invoking negative feedback,
of making things more confusing than they have to be. Out of confusion comes
bull****, and bull****, we all can agree, is something to be avoided.

Right?

Have a nice day.

-Henry


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Ian Iveson Ian Iveson is offline
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Default Diodes, triodes, and negative feedback

Hooray, Henry!

The wrecking crew realised they are wrong some time ago, but have
invested too much of their sordid reputations in nonsense to say so.

..."Yes, if you want it to, no if you don't."...


You can't say that about real feedback. If it's optional, it's in your
head. If you must take it into account in design, then it is in the
circuit.

Thanks, Henry, and welcome back!

cheers, Ian


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Andre Jute Andre Jute is offline
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Default Diodes, triodes, and negative feedback


Chris Hornbeck wrote:
On Fri, 22 Sep 2006 16:06:35 -0400, "Henry Pasternack"
wrote:

Brilliant analysis, saved, but snipped 'cause you've
already read it.


I've skimmed Plodnick's exhaustive -- and exhausting! -- summary of the
60 or so posts, many of them long, in the thread "Negative Feedback in
Triodes: The Logical and Experimental Proof" which I started on 15
August 2006:
http://groups.google.ie/group/rec.au...dff8d8ed263a35

Leaving only two questions: if it's feedback, where's
the change in bandwidth and input-referred noise?

And B: what do we gain from pretending that it's true?
IOW, what insights arise from stretching this analogy
into an assumption?


The first is a metaphysical question that Plod has answered
surprisingly well: "Negative feedback is a model. The system operates
the same regardless of the model."

The answer to your second question arises from acceptance of the
qualities of a model as an operational tool in a practical design, and
also as a weapon in our hand against the barbarians at the gate of RAT.


One model, yours, models the triode according to Thevenin or Norton, or
whatever turns up your wick, without the feedback. This model is liable
to frivolous attack from the sort of engineers and hangers-on who have
NFB signs in their eyeballs. It simply gets boring listening to them,
and watching Patrick actually make a serious argument (the same one)
every time I tweak their noses.

One model, among several that Patrick and I have described and other
parties have elaborated, models the pentode as a triode with the
internal feedback removed, the pentode requiring the negative feedback
to be reapplied externally in order to function, the rationale being
that something invisible is happening inside the triode which resembles
NFB as applied to pentodes to a large enough extent to be called NFB
and so to be modelled (see the second part of my original exposition
referenced above). This is a useful model for non-electronic purposes
as well in that it preempts some arguments particularly against SET
amps made by the NFB neanderthals.

Electronically it is irrelevant whether or not you believe, as Patrick
appears sincerely to do, that actual NFB takes place inside the triode,
or whether you believe, as I do, that the net effect at the output end
of the triode is similar enough to view the process inside as so close
to NFB as to be calculable as NFB, indeed to be shorthanded (at least
in the privacy of our fractious family on RAT) without qualification as
"xdB NFB". It's a black box effect and arguments in the dark inside the
box about who's got the torch do not affect the electrical outcome. Or,
as Plod bluntly puts it for those of duller mind than you, "Negative
feedback is a model. The system operates the same regardless of the
model."

I'm very sorry to be so hard-assed about such a trivial
subject, but fuzzy thinking has become palpably dangerous
in the modern world.


Of course I agree with you.The Fuzzy Thinking and Arrogant Hubris of a
minority pressure group that led to the banning of DDT, which in turn
caused an ongoing genocide by starvation and the return of a previously
eradicated wasting disease (malaria) of those poor black people least
able to defend themselves, is matter of principle for which it is worth
standing up and offending or even losing friends.

But whether inside a triode there is negative feedback or some other
mechanism with the same net effect on the sound-- is, as you say, "a
trivial subject", not worth fighting about, except if we can use it as
a club to beat the enemies of fidelity around the ears.

Fight the power. Fuzziness is great,
even if it gets in yer teeth, in some other circumstances,
of course.


Deliberate fuzzy thinking is a major tool of the enemies of society and
should be stomped wherever it is encountered.

Much thanks, as always,

Chris Hornbeck


Andre Jute
Habit is the nursery of errors. -- Victor Hugo



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Andre Jute Andre Jute is offline
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Ian Iveson quotes Pompass Plodnick on whether NFB is built into
triodes:
..."Yes, if you want it to, no if you don't."...


and then concludes:
You can't say that about real feedback. If it's optional, it's in your
head. If you must take it into account in design, then it is in the
circuit.


Or of course the NFB could be sealed into the component itself, eh
Iveson.

See my reply to Chris Hornbeck in this thread about the beneficial
effect of black box thinking.

Andre Jute
Put your mind in gear, sonny

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Patrick Turner Patrick Turner is offline
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Default Diodes, triodes, and negative feedback



Henry Pasternack wrote:

I noticed that the triode negative feedback argument is still going on. I'm a
bit surprised because I don't think the issue is really all that complicated.
Somewhat complicated, yes, but not beyond resolution.

The answer to the question, "Does a triode have internal negative feedback?",
is, "Yes, if you want it to, no if you don't." But I'll admit my bias and state
from the start that I don't see much use to the triode feedback viewpoint.

I'll explain what I mean. Negative feedback is a model. The system operates
the same regardless of the model. You can imagine an operational amplifier
with its inverting input connected to its output through a voltage divider. You
can model the system in terms of its forward and reverse transfer functions
and use the feedback equation to come up with the answer. Or you can just
solve for the response as a single system of equations, and never bother to
consider negative feedback. The amplifier doesn't care. It's up to you, the
designer, to decide what form of analysis you find most convenient at the time.

It's true that if you open and close the feedback loop, certain predictable
changes occur. The gain, bandwidth, transient response, and input/output
impedances change. You can say that the existence of these changes
"proves" that feedback is at work. But this is circular reasoning. You could
just as easily explain the difference by noting how the connection from
output to input connection modifies the signal input voltage.

With respect to triodes, you can model the internal behavior different ways.
The traditional model views the cathode as an electron emitter where the
probability of emission depends on the cathode temperature and the
electric field at the cathode surface. It assumes the electrons come out
with zero kinetic energy. The plate-cathode voltage establishes a boundary
condition and the subsequent density, distribution, and velocity of electrons
within the tube falls out from the solution of a relatively simple differential
equation.

The effect of the grid-cathode potential is to augment the plate-to-cathode
electric field, modifyinng the boundary conditions of the differential equation
and shifting the current-vs-voltage curve left or right according to the grid
voltage. The plate resistance is simply the slope of this curve, and the
effects of both plate and grid potentials are purely in the "forward" path.

Taking the other position, you could view both the plate and the grid as inputs.
Their voltages each produce an electric field at the cathode surface which,
summed together, determine the space charge density around the cathode.
The space change density, in turn, determines the plate current and, in
conjunction with the plate resistor, this creates a signal voltage at the plate.
The signal voltage "feeds back" to the cathode electric field, modulating the
space charge density and therefore the resulting current flow.

The second model is pretty complicated, and there are some serious
objectsions to be made to it. I'll get to that, but first I want to talk about
diodes and triodes.

It's been stated on this forum that diodes, unlike triodes, have no internal
negative feedback. Isn't this is a paradoxical claim? The grid in a triode
biases the space charge density, but It doesn't fundamentally change how the
tube operates. Indeed, if you connect the triode's grid to its cathode, the
E-field component due to the grid is zero and the tube behaves for all intents
and purposes, exactly like a diode. If a triode has internal negative feedback,
then by the exact same argument, so must a diode. This is an extremely
important point to understand.


In a thermionic diode the anode voltage determines the current flow.
The effect of the anode electrostatic field upon the space charge of electrons is no
different
to what occurs in the triode, but a triode has a GRID, and so you have TWO
things which JOINTLY conrol electron flow in the triode, and this interation
involves a simple shunt or summing electrostatic NFB effect which does not exist in a
diode.
A triode can be used as a diode if the grid voltage is kept fixed; the Ra curves
of a triode at fixed value for Eg-k = 0.0V, ie, grid is tied to cathode are little
different to the Ra curves
of a diode. But where you have the grid controlling the Ia with interaction with Ea
there is, IMHO, a NFB effect.


If you have an operational amplifier in the classic non-inverting configuration
and you connect the input to ground, the negative feedback is still in control
and the amplifier's output impedance stays the same. But grounding the
non-inverting input takes that input out of the picture. Mathematically, it's
as though it no longer exists. The three-terminal op-amp has turned into a
two-terminal op-amp yet the negative feedback is unaffected. This is how a
regulated power supply works, incidentally.


The non-inverting opamp used with NFB taken to its inverting input
can be considered a unit witth low Rout. But its low Rout because of the NFB.
if you remove the NFB by ground the inverting FB input port
then the gain goes sky high and the Rout is high, and bandwidth very poor as the NFB
is no longer connected.
The opamp becomes like a pentode with little NFB.



The NFB argument says that the low output impedance of the triode is due
to the negative feedback between the plate and cathode, which, if we were to
put a screen grid into the tube, could be taken away, thereby significantly
raising the output impedance. Now, the plate curves of the diode and the
triode (with Vg=0) are exactly the same. You could put a screen grid in the
diode, apply a fixed potential, and greatly increase its plate resistance, just
like a triode. Overall, if you are comfortable saying that a triode is a pentode
without a screen grid, then you should be equally comfortable saying that a
diode is is a pentode without a screen grid or a control grid. Either they both
have feedback, or they don't.


Its not as simple as that Henry.
if a screen is placed in a diode, then it becomes a triode, and thre Ia can be controlled
by the screen
which works as a grid which draws input current and has a low gm and low µ and gives the
Ra a high value.
There is little NFB because the anode has such a small effect on the space charge.

The position of the screen electrode is such that its distance between cathode and anode
is MUCH
further away from the cathode than a normal control grid.

There are some beam tetrodes and pentodes where the g1 is tied to the cathode
or kept at a fixed voltage slightly negative and all the control of the the
anode voltage is by the g2 screen, and some such tubes can be very linear
and there is enough linearizing NFB within even though Ra seems rather high for such a
triode.
The µ of such tubes is usually very low.

The use of the g2 as a control grid is evident with UL operation where the screen
accpts a % of the anode signal which is in fact an injection of local NFB
normal triode connection with screen strapped to anode is merely where ALL
the anode voltage is fed back into the tube which is a large amount of applied
shunt NFB but employed in an electostatic voltage divider and where the effective or
virtual
control point is the summed ecct of the anode and grid 1 voltages.
Such summing effects just don't happen in diodes.




Why is this important? One of the claims I have read on this newsgroup recently
is that the plate resistance of a diode is just an "ordinary" non-linear resistance
and has nothing to do with negative feedback. The mechanism that produces the
3/2 power plate curve is exactly the same for the diode as it is for the triode. The
Child-Langmuir law applies to both and the derivation is the same. If the plate
resistance of a diode is just an "ordinary" resistance, then so is the plate
resistance of a triode. The control grid is a complication, but it has no bearing
on this mechanism.


The applied NFB in a triode is applied via a 3/2 rule, so the NFB mechanism isn't
linear where a current change occurs in the triode.
But it becomes VERY linear when voltage change occurs between anode and cathode
but WITHOUT any current change; ie, where the load line is horizontal.



To understand whether or not a triode has negative feedback, we'd like to boil
the problem down to its barest essentials so that the underlying principles
are exposed clearly and without confusion. That's why it helps to show that
the control grid is irrelevant to the question. It's hard to visualize negative
feedback in a diode (or a resistor, for that matter, but more on that later)
because a simpler, open-loop model is just cleaner and more intuitive. When
you add a control grid in there, things get more complicated and it becomes
tempting to start waving hands. We'd like to avoid that.


We are not waving hands.
We are having a technical discussion.



Getting back to op-amps, it's very easy to separate out the forward path and
the reverse path. The forward path is everything inside the little triangle,
between the two input leads and the one output lead. The reverse path is the
external network connecting the output terminal to the inverting input terminal.
Being able to separate these two paths so cleanly makes it much easier to
visualize how the operational amplifier operates as a feedback circuit.

In the proposed diode NFB model, there is an inconvenient blurring between
the forward and reverse paths.


A diode has no amplification and no gain.
Its just a non-linear resistance.

The plate-cathode potential has two functions.
First, it establishes the electric field at the surface of the cathode, which in
turn modulates the space charge density. Second, the very same field
sweeps electrons out of the electron cloud and accelerates them to the
plate, establishing plate current. The first of these two functions may be
called feedback. The other determines the forward gain.


The diode has no forward gain. Current flows due to one single effcect,
anode voltage is raised by some external active device and current flows.
Solid state diodes are no different, except that their on resistance curve is
very much more abrupt and of lower resistance than a thermionic diode.

To show that the
tube has negative feedback, you have to separate these two paths out, but
it's not such an easy thing to do. This is a problem for the feedback model.


Its not a problem to show there are two interacting fields of control in a triode.



There are two ways that tube feedback modelers have solved this problem.
The first is propose connecting the plate to a fixed DC potential. This lets
the tube keep conducting but eliminates any E-field feedback from the plate.
Fixing the plate potential raises the gain (transconductance) as predicted by
feedback theory. This is the essence of an argument that one member of
this forum put forth as "proof" of triode NFB.


With no change in Va -k, there is no voltage gain and no NFB applied.
The triode NFB is purely a voltage caused phenomena .
There is current gain and is simply gm x Eg applied.



But there is a glaring hole in this argument. There is absolutlely nothing
that external observation can prove conclusively about the internal operation
of the tube. There could just as easily be a nonlinear resistor inside the tube,
or little elves with voltmeters and a rheostat controlling the electron flow in
real time. All you can determine from external observations are the external
properties of the tube, and while these may suggest the action of negative
feedback, they do not by any means prove it.

The other, perhaps more convincing way that feedback modelers have tried
to prove their point is by postulating the existence of a fictitious screen grid
inside the tube. The screen grid takes away the feedback due to the varying
plate E-field while allowing the plate voltage to vary. With this fictitious screen
grid in place, the tube has higher gain, like a pentode.


There is no need for fictitious screens to be considered.

There are TWO voltage fields which sum together to control Ia.
As RL at the anode becomes lower, the nfb becomes lower, but with a CCS
connected NFB is at a maximum applied amount, and thus maximally
linearizes the tube.


There are several rebuttals to this argument. The simpler one is that the
fictitious screen grid is, indeed, a fiction. The triode is not a tetrode. It
should suffice to model the tube without the artifice of the fictitious grid.
Indeed, there is there is a perfectly good model that doesn't depend on
this complication. But the real problem with the fictitious grid argument
is that it pulls itself up by its own bootstraps. It says, a triode has NFB,
and a pentode is a triode with the NFB taken away by the screen. So
if we take a pentode and remove the screen, the feedback comes back
and this proves the triode has NFB. This is a brilliant example of circular
reasoning.


Just connect the screen to anode and you have a triode because the anode voltage is
allowed
to work against the operation of the grid1, so gain, Ra and thd reduces.



The triode NFB argument depends on another fiction, the "virtual grid", which
is the internal control point related to the electric field strength at the cathode
and space charge density. The model says the actual input to the tube is
this "virtual grid", which is related to but distinct from the external grid and
plate connections. This is an extremely awkward model because it relies
on an abstraction of the tube that is quite removed from its actual use in-circuit.
Contrast this with the op-amp where the input, output, and feedback connections
are quite clear and in the open.


Nothing hard to understand about a virtual controlling field in a triode.
Grid goes +10V, anode goes -200V.

The effects of the two voltage changes sum together according to the
distances between cathode and grid and cathode and anode.

The triode operates with a virtual input similar to the virtual
input of an inverting opamp with its non-inverting input taken to 0V,
and fitted with a two resistor shunt FB network.

But the arms of the network in a triode are electrostatic fields, not resistances.


One of the points made against the triode feedback model is that there is no
effect on the input impedance seen at the grid.


Well in fact there is, and its the Miller effect.
If the input source signal resistance is many meg-ohms, then the miller effect
has considerable input Z effect at audio F.


In response, it's been noted
that even with conventional shunt feedback applied to the cathode, there is no
effect in the tube's input impedance. This isn't true. Shunt cathode feedback
definitely raises the input impedance seen at the grid. It's just that the tube's
input impedance is already so high and swamped by the bias resistor that we
don't notice the difference.


Shunt FB normally lowers Rin, while series FB such as the case of a cathode follower
will raise Rin.

But triodes in AF circuits have very high Rin up to 20kHz regardless
of the NFB because the arms of the electrostatic shunt FB network are
very high Z reactances.






I mentioned resistors earlier, and I'd like to get back to that. Others have
pointed out that you can use the same kind of argument for triode NFB to
show that there is negative feedback in resistors, or automobiles, or electric
motors. It's actually kind of fun to derive the formula for a voltage divider in
terms of negative feedback, or even transmission line reflection coefficients.


The argument in favour of triode NFB CANNOT be used for resistors etc.



Consider a resistor as a cylinder of material with a metallic cathode
bonded to one end and a metallic anode bonded to the other end. If you
apply a positive potential between the anode and the cathode, an electric
field will be established between the electrodes and electrons will begin to
drift inside the cylinder, creating a current flow. If the power supply has
significant output resistance, the supply voltage will drop, reducing the
electric field within the body of the resistor and modulating the current. In
other words, negative feedback. Or is it feedback?

The difference between the diode feedback model and the resistor feedback
model is that in the resistor you don't have the additional complication of
the space space charge density. But the principle is the same. In either
case, the current flow that results from the application of an external
voltage is set by an equilibrium that develops within the device. You can
find zillions of similar examples in nature and engineering.

Parachutists are grateful for the negative feedback of air resistance that
reduces the "gain" of gravity accelerating them to the ground. Airplane
designers take pains to eliminate this feedback, maximizing the "gain" of
their engines by installing aerodynamic screens on the aircraft. Yes,
feedback is everywhere. And nowhere. It all depends on your point of
view.

Whether or not you choose to think of tube operation in terms of feedback
is your choice. It causes no harm to anyone. But you should be careful
to avoid insisting on the presence of feedback. Feedback is only as good
as the value it brings to engineering analysis. In practical terms, when it
comes to triodes the value isn't that great. I don't believe it gives us any
greater insight than we get from the standard models. In particular, as we
move from coarse generalities to much finer levels of detail, it remains to be
shown that feedback helps to predict the second- and third-order nonlinear
behavior of the tube.

It's been said, and I agree, the tube doesn't give a damn how you model it,
but goes about its business in blissful ignorance. Triode negative feedback
is a great conversation starter, but there is a risk, in invoking negative feedback,
of making things more confusing than they have to be. Out of confusion comes
bull****, and bull****, we all can agree, is something to be avoided.

Right?


I don't think you quite have it right yet.

Patrick Turner.



Have a nice day.

-Henry


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Henry Pasternack Henry Pasternack is offline
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"Patrick Turner" wrote in message ...
In a thermionic diode the anode voltage determines the current flow. The effect
of the anode electrostatic field upon the space charge of electrons is no
different to what occurs in the triode, but a triode has a GRID, and so you have
TWO things which JOINTLY conrol electron flow in the triode, and this interation
involves a simple shunt or summing electrostatic NFB effect which does not exist
in a diode.


I spent a significant part of my posting explaining why this argument is wrong. The
conclusion you draw, that the presence of the control grid is a prerequisitie for the
NFB model, is just plain wrong.

[A whole lot of stuff deleted.]


I'm sorry, Patrick, but I just can't respond to this. Some of what you say makes
sense, some of it is irrelevant, other parts are incomprehensible, and a few bits are
just plain wrong.

Happily, the sun will continue to rise in the east and set in the west regardless of
whether or not we debate the subject to its conclusion. You've had your say, I've
had my reply, and people can read and decide on their own.

I don't think you quite have it right yet.


You're entitled to that opinion.

Cheers.

-Henry


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Henry Pasternack Henry Pasternack is offline
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Default Diodes, triodes, and negative feedback

"Chris Hornbeck" wrote in message news
Leaving only two questions: if it's feedback, where's
the change in bandwidth and input-referred noise?

And B: what do we gain from pretending that it's true?
IOW, what insights arise from stretching this analogy
into an assumption?


I think I can do a little better job of responding to this than that other guy.

Child-Langmuir says nothing about the bandwidth or time-varying behavior
of the tube. But clearly it takes time for the space charge density to reach
a new equilibrium in response to an input change I propose that this time
constant is equivalent to the time constant of the integrator in the classic
op-amp block diagram. In both cases, therefore, these time constants set
the open-loop bandwidth, and also create a dominant-pole that insures
stability when the feedback loop is closed.

The closed-loop bandwidth will vary in proportion to the gain reduction. But
there is so much plate-to-grid capacitance that you will probably never see
the effects of "virtual tube" bandwidth in real-world circuits.

To the best of my knowledge, no one has ever proposed this idea before
on this forum. I'm not saying this proves there is negative feedback in the
triode. I still believe the supposed internal NFB is a fiction. This is just the
mental gymnastics you have to go through to make the feedback model
work out.

With respect to input-referred noise, this subject has been debated before
on the newsgroup. Input-referred noise is output-referred noise divided by
gain. Negative feedback reduces both by an equal amount. So, whether it's
an op-amp or a triode, you don't expect NFB to have any effect on the noise
figure of the amplifier. See the archives for more detailed discussions.

And concerning your question B, I don't think we get any benefit whatsoever
from stretching the triode feedback analogy. If this model has any value, it
should make things clearer, not more confusing. As far as I can see, the
proponents of triode NFB have never been able to explain themselves clearly
and convincingly. That includes today's postings.

I'm very sorry to be so hard-assed about such a trivial subject, but fuzzy
thinking has become palpably dangerous in the modern world. Fight the
power. Fuzziness is great, even if it gets in yer teeth, in some other
circumstances, of course.


How true. But I've never been able to figure out how to convince a fuzzy
thinker that his thinking is, indeed, fuzzy. It seems to be a corollary to
Catch-22.

-Henry


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Chris Hornbeck Chris Hornbeck is offline
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On Sat, 23 Sep 2006 17:47:29 -0400, "Henry Pasternack"
wrote:

Child-Langmuir says nothing about the bandwidth or time-varying behavior
of the tube. But clearly it takes time for the space charge density to reach
a new equilibrium in response to an input change I propose that this time
constant is equivalent to the time constant of the integrator in the classic
op-amp block diagram. In both cases, therefore, these time constants set
the open-loop bandwidth, and also create a dominant-pole that insures
stability when the feedback loop is closed.

The closed-loop bandwidth will vary in proportion to the gain reduction. But
there is so much plate-to-grid capacitance that you will probably never see
the effects of "virtual tube" bandwidth in real-world circuits.


Certainly an extraordinary leap. I'll need more time than available
this evening to ponder it and respond properly.


With respect to input-referred noise, this subject has been debated before
on the newsgroup. Input-referred noise is output-referred noise divided by
gain. Negative feedback reduces both by an equal amount.


A reductio ad absurdum is to make output loading a very small number,
reducing both triode and multigrid valves to transconductance stages,
with identical gains for identical transconductances.

Observing that they *do not* have different output noise despite
the proposed "feedback" and identical gains should be compelling.

But that's just me,

Oh yeah, apparently nobody but me thought this was funny:
" Fuzziness is great, even if it gets in yer teeth, in some other
circumstances, of course."

We all need to get out more. Arf.

Much thanks, as always,

Chris Hornbeck


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Andrew Jute McCoy, in the face of the evidence proved once again:

That it is full of dung-beetle rejected excrement.


Peter Wieck
Wyncote, PA

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On 23 Sep 2006 08:52:49 -0700, "Andre Jute" wrote:

It's a black box effect and arguments in the dark inside the
box about who's got the torch do not affect the electrical outcome. Or,
as Plod bluntly puts it for those of duller mind than you, "Negative
feedback is a model. The system operates the same regardless of the
model."


But *this* is the part that bugs me. It's *not* the same
and it's not a real model. It's disprovable.

I'm sorry to be so ****y about such a ****-anty thing,
but bad, or at least incorrect, models have become
elevated to the status of "equivalent truth" in my
country, and eternal vigilance hasn't been maintained
here. Fight the power.

Much thanks, as always,

Chris Hornbeck
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Henry Pasternack wrote:
"Chris Hornbeck" wrote in message news
Leaving only two questions: if it's feedback, where's
the change in bandwidth and input-referred noise?

And B: what do we gain from pretending that it's true?
IOW, what insights arise from stretching this analogy
into an assumption?



I think I can do a little better job of responding to this than that other guy.

Child-Langmuir says nothing about the bandwidth or time-varying behavior
of the tube. But clearly it takes time for the space charge density to reach
a new equilibrium in response to an input change I propose that this time
constant is equivalent to the time constant of the integrator in the classic
op-amp block diagram. In both cases, therefore, these time constants set
the open-loop bandwidth, and also create a dominant-pole that insures
stability when the feedback loop is closed.

The closed-loop bandwidth will vary in proportion to the gain reduction. But
there is so much plate-to-grid capacitance that you will probably never see
the effects of "virtual tube" bandwidth in real-world circuits.

To the best of my knowledge, no one has ever proposed this idea before
on this forum. I'm not saying this proves there is negative feedback in the
triode. I still believe the supposed internal NFB is a fiction. This is just the
mental gymnastics you have to go through to make the feedback model
work out.

With respect to input-referred noise, this subject has been debated before
on the newsgroup. Input-referred noise is output-referred noise divided by
gain. Negative feedback reduces both by an equal amount. So, whether it's
an op-amp or a triode, you don't expect NFB to have any effect on the noise
figure of the amplifier. See the archives for more detailed discussions.

And concerning your question B, I don't think we get any benefit whatsoever
from stretching the triode feedback analogy. If this model has any value, it
should make things clearer, not more confusing. As far as I can see, the
proponents of triode NFB have never been able to explain themselves clearly
and convincingly. That includes today's postings.


I'm very sorry to be so hard-assed about such a trivial subject, but fuzzy
thinking has become palpably dangerous in the modern world. Fight the
power. Fuzziness is great, even if it gets in yer teeth, in some other
circumstances, of course.



How true. But I've never been able to figure out how to convince a fuzzy
thinker that his thinking is, indeed, fuzzy. It seems to be a corollary to
Catch-22.

-Henry


Don't get me wrong, I'm not trying to say that triodes have infrared
feedback, which "proves it's true." I'm saying that IF you want to claim
that triodes have internal feedback, then it has such a ridiculously
high bandwidth that for audio, it might as well not exist. It gives us
no advantages in terms of designing circuits, and gives the misleading
impression that triode circuits are basically the same as pentode
circuits with feedback. It is misleading because of the massive
difference in the bandwidths of pentode feedback circuits vs. "triode
feedback." The whole argument for triode feedback seems to me to be (1)
useless (2) misleading (3) a method for saying that triodes "Do not
either have an advantage over pentodes/transistors, SO THERE!" It's a
childish attempt to say that not only do triodes have some disadvantages
compared to pentodes/transistors -- which they do -- that they also have
no advantages.

If triodes did have internal negative feedback, and it had the same
bandwidth as pentode circuits with feedback, then you could probably
make that argument, but that "if," the idea that they have the same
bandwidth, isn't true. Does the most accurate physical model of a triode
include at least some feedback, perhaps in parallel with a genuine plate
resistance? I'm still not sure. Does the inclusion of a triode feedback
mechanism help the design of audio circuits? No, and furthermore, it
does some harm by being more complex and misleading. It really is like
saying that "we are not using analog, since current consists of discrete
charges called electrons, so we should be using digital techniques,
including the Nyquist theorem, to design circuits." That would be a
purely debating smear against analog, and "triode feedback" is just a
useless smear against triodes.

However, it MIGHT be the case that taking this same approach and using
it to examine resistors, might actually tell us something useful about
how to design better audio circuits. Not likely, but possible, and if
that is the case, well, the subject of "triode feedback" will wind up
doing some good! Just not when it comes to triodes.

Phil
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On Sun, 24 Sep 2006 15:20:57 GMT, Phil
wrote:

It really is like
saying that "we are not using analog, since current consists of discrete
charges called electrons, so we should be using digital techniques,
including the Nyquist theorem, to design circuits."


Without agreeing or disagreeing about any of the issues
you've raised recently,

You may be interested in the M.J. Hawksford paper "Fuzzy
Distortion in Analog Amplifiers: A Limit to Information
Transmission?", JAES Vol 31, No 10, October 1983, which
touched on several topics you've raised, with an emphasis
on the correlation between slewing issues and monotonicity
(according to my notes; the paper itself is lost).

I can quote a bit to possibly whet your appetite to dig
for it: "A direct consequence of amplifier nonlinearity
and signal interation is partial rectification, which
produces a dynamic shift in the quiescent bias state. If
an amplifier incorporates energy storage elements (such
as AC coupling and bypass capacitors), then the error
signal is filtered and exhibits 'overhang'".

Just the flavor, but possibly something along the lines of
what you're searching for.

All good fortune, and much thanks,

Chris Hornbeck
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Default Diodes, triodes, and negative feedback


"Chris Hornbeck" wrote in message
...

You may be interested in the M.J. Hawksford paper "Fuzzy
Distortion in Analog Amplifiers: A Limit to Information
Transmission?", JAES Vol 31, No 10, October 1983, which
touched on several topics you've raised, with an emphasis
on the correlation between slewing issues and monotonicity
(according to my notes; the paper itself is lost).

I can quote a bit to possibly whet your appetite to dig
for it: "A direct consequence of amplifier nonlinearity
and signal interation is partial rectification, which
produces a dynamic shift in the quiescent bias state. If
an amplifier incorporates energy storage elements (such
as AC coupling and bypass capacitors), then the error
signal is filtered and exhibits 'overhang'".

Just the flavor, but possibly something along the lines of
what you're searching for.


The flavor would be wrong, because bias shift doesn't happen with odd order
nonlinearities.




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Default Diodes, triodes, and negative feedback

In article ,
Chris Hornbeck wrote:

On Fri, 22 Sep 2006 16:06:35 -0400, "Henry Pasternack"
wrote:

Brilliant analysis, saved, but snipped 'cause you've
already read it.

Leaving only two questions: if it's feedback, where's
the change in bandwidth and input-referred noise?



We can't begin to discuss the change in bandwidth when feedback is
applied until someone proposes a mechanism in the model that will
actually limit the bandwidth. As it stands now the bandwidth of the
model is infinite, with or without feedback. If the bandwidth of the
model were finite, we would see that the feedback inside the triode
actually does increase the bandwidth.

As far as feedback changing the input-referred noise goes, wasn't there
a big argument here six or seven years ago about that? As I remember it
the conclusion was that feedback does not change the input-referred
noise, although it does reduce the output-referred noise.


Regards,

John Byrns
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Andrew Jute McCoy blathered pretentiously:

Deliberate fuzzy thinking is a major tool of the enemies of society and
should be stomped wherever it is encountered.


Which is why it is such a fat, tempting, inviting and nearly
irresistable target.

Never mind the "metaphysical" questions.

Peter Wieck
Wyncote, PA

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Default Diodes, triodes, and negative feedback



John Byrns wrote:

In article ,
Chris Hornbeck wrote:

On Fri, 22 Sep 2006 16:06:35 -0400, "Henry Pasternack"
wrote:

Brilliant analysis, saved, but snipped 'cause you've
already read it.

Leaving only two questions: if it's feedback, where's
the change in bandwidth and input-referred noise?


We can't begin to discuss the change in bandwidth when feedback is
applied until someone proposes a mechanism in the model that will
actually limit the bandwidth. As it stands now the bandwidth of the
model is infinite, with or without feedback. If the bandwidth of the
model were finite, we would see that the feedback inside the triode
actually does increase the bandwidth.

As far as feedback changing the input-referred noise goes, wasn't there
a big argument here six or seven years ago about that? As I remember it
the conclusion was that feedback does not change the input-referred
noise, although it does reduce the output-referred noise.

Regards,

John Byrns




But if you observe a pentode with very high Ra and gain, its bandwidth
into a given load containing shunt L and shunt C components is like an arch
but when connected as a triode the same pentode with the same load suddenly
has a much wider BW,
lower Ra, less gain and less THD etc. All because of the NFB
introduced from the anode into the tube via the screen electrode.

To increase bandwidth in the pentode case we must lower the R component of
the load.
Thus the Rout becomes dominated by RL.

In power amps where pentodes are used the high Ra gives rise to a very
arched
response without NFB applied.

One has the option of applying 20db of global NFB, or 50% UL connection
and 16dB for about the same measured result or triode connection which is
100% UL, and maybe 8 dB of global NFB is then needed for low Rout,
perhaps not though, depending on OPT ratio.

The input noise cannot be reduced with NFB and all devices have
an amount of noise which is unavoidable.

Triodes have an equivalent input noise resistance
stated in RDH4 being = 2.5/gm .
Very few triodes conform to this formula, because flicker noise also occurs.

pentodes are always noisier with additional partition noise at the screen.

For really low input noise, don't use a tube; use a j-fet like 2SK369.

Fet input noise resistance is approx 0.7/gm, and since
the gm of a 2SK369 is about 40mA/V at 5mA the EINR is very much lower than a
triode or pentode
because the gm is so much greater than any tube.
The fet has very little spurious LF noises. It makes a splendid input device

for MC carts or a microphone, with no reliance on an input transformer that
must be used
with a tubed input to keep a decent SNR.

Patrick Turner.




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"Patrick Turner" wrote in message ...
But if you observe a pentode with very high Ra and gain, its bandwidth into a
given load containing shunt L and shunt C components is like an arch but
when connected as a triode the same pentode with the same load suddenly
has a much wider BW, lower Ra, less gain and less THD etc. All because of
the NFB introduced from the anode into the tube via the screen electrode.


A statement about external feedback around a pentode, even if it's true, tells
us very little about the existence, or lack thereof, of reputed internal NFB in
a triode.

Riddle me this, Batman: If you take a pentode and connect the screen grid to
the plate, you end up with a triode, and the negative feedback greatly lowers
the effective plate resistance.

On the other hand, if you take a triode and connect the control grid to the plate,
you end up with a diode, and again the resulting negative feedback greatly
lowers the effective plate resistance.

It follows that if a triode is a pentode with negative feedback, then a diode is a
triode with negative feedback. QED.

Get back to me, Patrick, when you have accepted the indisputable reality that
the NFB mechanism you propose for triodes, if it exists, also exists in diodes.
Then we can continue on, step by step, with the dismantling of the rest of your
theory.

-Henry


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In article ,
"Henry Pasternack" wrote:

"Patrick Turner" wrote in message
...
But if you observe a pentode with very high Ra and gain, its bandwidth into
a
given load containing shunt L and shunt C components is like an arch but
when connected as a triode the same pentode with the same load suddenly
has a much wider BW, lower Ra, less gain and less THD etc. All because of
the NFB introduced from the anode into the tube via the screen electrode.


A statement about external feedback around a pentode, even if it's true,
tells
us very little about the existence, or lack thereof, of reputed internal NFB
in
a triode.

Riddle me this, Batman: If you take a pentode and connect the screen grid to
the plate, you end up with a triode, and the negative feedback greatly lowers
the effective plate resistance.

On the other hand, if you take a triode and connect the control grid to the
plate,
you end up with a diode, and again the resulting negative feedback greatly
lowers the effective plate resistance.

It follows that if a triode is a pentode with negative feedback, then a diode
is a
triode with negative feedback. QED.

Get back to me, Patrick, when you have accepted the indisputable reality that
the NFB mechanism you propose for triodes, if it exists, also exists in
diodes.


Sure it exists in diodes, but how does that demonstrate that the theory
is wrong?


Regards,

John Byrns


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Andre Jute Andre Jute is offline
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Default Diodes, triodes, and negative feedback


Henry Pasternack wrote:
"Patrick Turner" wrote in message ...
But if you observe a pentode with very high Ra and gain, its bandwidth into a
given load containing shunt L and shunt C components is like an arch but
when connected as a triode the same pentode with the same load suddenly
has a much wider BW, lower Ra, less gain and less THD etc. All because of
the NFB introduced from the anode into the tube via the screen electrode.


A statement about external feedback around a pentode, even if it's true, tells
us very little about the existence, or lack thereof, of reputed internal NFB in
a triode.

Riddle me this, Batman: If you take a pentode and connect the screen grid to
the plate, you end up with a triode, and the negative feedback greatly lowers
the effective plate resistance.

On the other hand, if you take a triode and connect the control grid to the plate,
you end up with a diode, and again the resulting negative feedback greatly
lowers the effective plate resistance.

It follows that if a triode is a pentode with negative feedback, then a diode is a
triode with negative feedback. QED.

Get back to me, Patrick, when you have accepted the indisputable reality that
the NFB mechanism you propose for triodes, if it exists, also exists in diodes.
Then we can continue on, step by step, with the dismantling of the rest of your
theory.

-Henry


This is the sort of fatuous "logic" that will not be permitted to even
the known plodders among the newbies down at the Union (1) at any
decent tertiarty institution.

That a result A is true of a set of circumstance B does not ipso facto
exclude the possibility that it is also true of circumstances C.

Let me give you an example that you see in your shaving mirror every
morning. That one P. Plodnick was born with a personality defect that
makes him an arrogant **** in later life does not preclude the
possibility that he would choose to study engineering which also
appears to turn (at least some) people into arrogant ****s. Result A
(behaviour of an arrogant ****) is here caused by circumstance B (born
with personality defect tending to...) and definitely not excluded by
circumstance C (arrogant ****tiness reinforced by choice of engineering
as a profession).

QED.

Andre Jute (2)
Habit is the nursery of errors. -- Victor Hugo
Shaving is the most egocentric habit of all. -- Abraham Maslow

(1) At the better universities the vernacular name for the debating
society, in fact the place (the Students' Union hall) where debates are
staged.

(2) Described in the Northern Echo as "a great big charming, cuddly
bearded bear of a man".

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Andrew Jute McCoy wants:

Nothing to do with an actual knowledge-based discussion, and so resorts to its usual lies, innuendos, obfuscations and whining personal attacks.


Peter Wieck
Wyncote, PA

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"John Byrns" wrote in message ...
Sure it exists in diodes, but how does that demonstrate that the theory
is wrong?


Formal logic says Patrick can't be right because his premises lead to inconsistent
conclusions.

-Henry


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Henry Pasternack wrote:

Consider a resistor as a cylinder of material with a metallic cathode
bonded to one end and a metallic anode bonded to the other end. If you
apply a positive potential between the anode and the cathode, an electric
field will be established between the electrodes and electrons will begin
to
drift inside the cylinder, creating a current flow. If the power supply
has significant output resistance, the supply voltage will drop, reducing
the
electric field within the body of the resistor and modulating the current.
In
other words, negative feedback. Or is it feedback?


I don't believe anyone would write such rubbish let alone believe it - one
of the worst examples of pseudo-science I have come across.

Ian
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Ian Bell wrote

Consider a resistor as a cylinder of material with a metallic
cathode
bonded to one end and a metallic anode bonded to the other end. If
you
apply a positive potential between the anode and the cathode, an
electric
field will be established between the electrodes and electrons will
begin
to
drift inside the cylinder, creating a current flow. If the power
supply
has significant output resistance, the supply voltage will drop,
reducing
the
electric field within the body of the resistor and modulating the
current.
In
other words, negative feedback. Or is it feedback?


I don't believe anyone would write such rubbish let alone believe
it - one
of the worst examples of pseudo-science I have come across.



It reads like a question to me. Perhaps you can explain your answer?

Ian


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