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In Reply to: RE: I think my point of view and Henry's are still both correct... posted by kurt s on October 27, 2009 at 21:22:33
Interesting points, Kurt. This discussion has probably gotten too esoteric to be of interest to most readers, so I'll try not to go on too long.
I agree that "white box" models based on direct internal testing or knowledge of device physics can help us do more accurate simulations and hopefully create better designs.
I'm a bit hung up on whether the "triode internal NFB" model is really a white box model, or if it's just another black box model based on externally observable behavior that doesn't really tell us anything useful about how the triode works internally.
The paper that was discussed on r.a.t., as I recall, wasn't derived from the physics equations (i.e., Child-Langmuir). I do know if you crack open a genuine triode and go looking for that hidden screen grid to do white box testing, you won't find it.
So, let's not pretend the triode has a fictitious internally connected screen grid. Instead, let's treat it as a test probe that we use, in fact or in a thought experiment, to disconnect the hypothetical feedback loop inside the tube. I can't claim that this is really all that different from opening up a feedback amplifier and using a test probe to short out an internal feedback node for just the same purpose. Except, of course, in the case of the amplifier the internal feedback node is a tangible internal structure, whereas in the triode it's harder to visualize. But to a mathematician, I suppose, it's clear enough.
Getting back to the introduction to my long essay, I can't argue that there is no way to model internal NFB in the triode. I just don't see what you gain from it versus, say, more detailed physics-based models. I know that internal negative feedback is a good way to model grid-plate capacitance, for instance, but I've never seen a practical problem where the kind of feedback model we're talking about here made the solution easier.
I know that Mark Kelly, whom I respect as a math genius with a strong interest in triode modeling, looked at this question and came away unimpressed. Though I've had some strenuous arguments, notably with one fellow from Australia, nobody's ever framed it in such a way that I said to myself, "Holy moly, now I see da light!"
In the end, I'm waiting for people to take this triode internal NFB thing and start solving problems with it that nobody's been able to solve by conventional methods. Or taking previously hard problems and making them simple. That will convince me.
-Henry
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