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