Re: Stability in Feedback Amplifiers Part 2B with badly needed corrections!



Chris Hornbeck wrote:

Thanks, friends, for all the helpful corrections.
Having, as I predicted!, botched the original post,
please permit me a do-over. Amateur rules, doncha
know.

Thanks to flipper, a picture is available on Wikipedia
that serves our purposes just fine. So let's draw
a pair of pictures, like the pair in Wikipedia, but with
a real x axis of useful frequency range, maybe a Hertz
or less up to a couple hundred thousand or a couple
MHz, of our amplifier's "open loop" response. This means
the response of the amplifier *without* its intended
feedback loop.

In a conventional valve amplifier the easiest way to
do this is to remove the feedback loop from the
output transformer winding and to connect that end to ground.
(The output of an amplifier would have been close enough to a
short to ground for our purposes, but not for all
porpoises, or even all papooses.)

Our first picture is of the amplifier's "magnitude"
(just a way of saying frequency response) response,
and is drawn log-log, meaning amplitude (y) is in dB's
and frequency (x) is in decades or octaves.

This is usually called the open loop gain response, or OLG response.

"magnitude" is the size of something or other.



Our second picture is of the amplifier's "phase"
response and is conventionally drawn with linear y
("degrees" of phase) and on the same log x scale as
our first picture. Thanks to Ian for correcting
my 90 degree memory shift.

This is the open loop phase response, OL phase response.

This is all that it takes to make a Bode diagram.
Anyone old enough to have made a Nyquist diagram
and to try to make sense out of it will appreciate
the power and intuitive simplicity of this kind.
I've mercifully forgotten brand N; good luck to
others. Arf!


I draw the OL gain and phase responses sometimes, but not with all new
amps
because there simply is no need.

I tend to draw stuff if its a real challenge, like making a Leak Point
One
stable while saddled with appalling tube choices and OPT.
But after amp no 10, they all have stopped being unique and different,
and drawing stuff doesn't much help to change what
you just know has to be done.


Now let's draw another pair of pictures for the feedback
path. To do this we drive the feedback network from
an appropriately low impedance source, leaving it
connected to the amplifier's summing junction (for
accurate loading; this could also be simulated if
loading is reasonably accurately modeled, but why bother?).

The network to drive the V1 cathode feedback entry port
needs to be low resistance, because the local current FB at V1
acts to reduce OLG, and cause HF phase shift because
of the effective higher Ra when Rk is high.

A diff pair with grid inputs is also doable, but very uncommon
on tube amps because the extra triode is costly.

And finally, and the reason for the whole exercize, we draw
a third pair of pictures for the *combination* of the two,
noting that you can simply algebraically add "linear" values
in the y axis to get the sum. Just exactly like you'd
overlay two opposed plate curves to draw a push-pull
loadline.

The resultant diagram is the relevant one when discussing
stability. Still small-signal, yada yada, as proposed
earlier, but will be equally applicable for large-signal
and extreme loading as we'll add later. It tells us
how the signal travels through our amplifier and back
again to the input as feedback, for small signal, and that's
where everthing must start.

From this picture we can begin to discuss stability.

So IMHO, you need to prevent the discussion endlessly going around a
circle
like a rat in a round running cage but getting nowhere.

So consider creating a Draft Document,
and which is modified into being wise as ppl contribute, but there as
the reference
document and quoted below all the bish bash text or hurley burley group
hot air.

You get to be the Keeper of Your Own Draft Document.

We just get to keep a version for ourselves if we want.

Internet wisdom isn't ever universally limited by
a single version produced by the only Gutenberg printing press.

What is to become of this effort you are making????


Patrick Turner.

Thanks to all for the desperately needed help and Feedback,

Chris Hornbeck

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