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#1
Posted to rec.audio.tubes
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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 |
#2
Posted to rec.audio.tubes
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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 |
#3
Posted to rec.audio.tubes
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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 |
#4
Posted to rec.audio.tubes
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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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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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Diodes, triodes, and negative feedback
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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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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Diodes, triodes, and negative feedback
"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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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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Diodes, triodes, and negative feedback
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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Diodes, triodes, and negative feedback
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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Diodes, triodes, and negative feedback
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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Diodes, triodes, and negative feedback
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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Diodes, triodes, and negative feedback
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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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. |
#16
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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 |
#17
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Diodes, triodes, and negative feedback
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 |
#18
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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. |
#19
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Diodes, triodes, and negative feedback
"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 |
#20
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Diodes, triodes, and negative feedback
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 |
#21
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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". |
#22
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Diodes, triodes, and negative feedback
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 |
#23
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Diodes, triodes, and negative feedback
"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 |
#24
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Diodes, triodes, and negative feedback
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 |
#25
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Diodes, triodes, and negative feedback
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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