"John Stewart" wrote

Go To ABSE for the real explanation of NFB in triodes.
There are three pages. The file is about 1000K.


Real? It is a contrivance, surely. Lots of pages...what about kiss?
Who's knife was it?

As I have said, any system, including a perfect resistor, can be
expressed in the canonical form of a feedback control system. It is
a matter of definition, just maths, not science.

Whether it is useful to do so depends on delay. If there is no delay
between input and feedback at the summing point, then there is no
need for all the rigmarole of feedback analysis because a simple
formula can be used to express the relationship between input and
output.

The classic formula relating input to output of a triode includes no
term for time. There is no need to think of it as a feedback control
system any more than you would a perfect resistor.

That is why in practice, those who avoid nfb are happy with triodes.
There is no sensible meaning for "instantaneous feedback".

The Miller capacitance is however seen by the classic formula as
external, and being a connection from output to input, it qualifies
as feedback with respect to that formula, and it is convenient to
treat it as such, in spice modelling for example. The effect depends
on the input impedance, however, which then must also be considered
part of the system as a necessary component for the production and
summing of input and feedback voltages.

Once you include all the necessary circuit to define the Miller
effect, is it correct to say there is delay in the loop? And then is
it correct to say that this feedback control system is part of the
valve? I think yes and no.

I feel it is important to stress the importance of loop delay.
Without it, most objections to nfb disappear. Just wondering offhand
if all would...does the scattering of distortion products that
usually results from nfb depend on delay?

cheers, Ian