[John Byrns]

In article ,
Patrick Turner wrote:

On Jun 24, 3:34*pm, John Byrns wrote:

I want to see a schematic with all test results before I make up my
mind on Alex's FB "trick." It could be a clever trick, or a swindle.

However the network introduces both a zero and pole into the response, with
the
zero at a higher frequency than the pole. *Remember this network is just
another
tool in your toolbox; it is not a cure all and requires some sophistication
in
its application. *Now the one thing I know about stabilizing the low
frequency
response of a feedback system is that it is all about correctly placing the
poles, and zeros if there are any.

In any amp where there are say 2 CR coupled stages and a final stage
with LR then you have a recipe for LF instability and a poor margin of
stability at LF.

OK, I have worked through some of the math and understand more fully what is
going on with Alex's feedback network. As I said the network introduces both a
zero and a pole in the loop gain.

Ignoring the added pole for a moment, the zero can be placed so that it exactly
cancels the effect of one of the three poles in the amplifier you describe
above, with 2 CR coupled stages and a final stage with LR, effectively reducing
the number of low frequency poles by 1, making the LF stability problem easier
to deal with. The zero effectively cancels both the phase shift and amplitude
roll off caused by the pole that is being canceled. Unfortunately it is
impossible, at least so far as I know, to build a network with an isolated zero
such as I have described, so an actual network, such as Alex's, must include a
pole at a lower frequency. Hopefully this new pole won't cause us too much
trouble if we place it at a very low frequency where the loop gain has already
fallen well below 1.0 as a result of the other two remaining poles that weren't
canceled.

Now the obvious question is, why bother with this extra complexity when we could
simply directly move one of the 3 poles to a very low frequency, as would
probably be part of the normal pole staggering process anyway? I will leave
that for others to comment on as I have not personally mucked about in my
workshop with amplifiers that have 3 LF poles. I suspect that one reason may
have to do with LF overload when using a Bean Counter approved OPT.

I can see how Alex's network has the potential to resolve a problem I have
encountered when mucking about with simpler amplifiers having only 2 poles.
When using OPTs designed by Bean Counters, especially SE OPTs, there is a
tendency towards LF overload in the OPT and final tube(s). I have attempted to
mitigate this problem by choosing a relatively high pole frequency for the
interstage coupling network to keep LF signals out of the OPT and final tube(s).
This puts the interstage pole too close to the pole caused by the OPT which then
causes a bump in the ³CLG² low frequency response, plus of course it isn't
really a very good solution to the LF overload problem. It occurs to me that
Alex's feedback network might also offer a solution to the OPT saturation
problem in Bean Counter designed OPTs, just as it offers a solution to the input
stage problem.

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/