Compensating multi-stage feedback (was: RE: all tranny vca+ )

[Don Tillman]

   From: Haible Juergen <Juergen.Haible at nbgm.siemens.de>
   Date: Fri, 30 Jun 2000 14:10:30 +0200

   So, if someone has a straight forward design guide for stabilizing
   3-stage or even 4-stage discrete amplifiers, I'd be interested.

I learned what I know from audio power amp design...

You want one major pole at loop gains above unity for stable
operation.  The problem is that each stage contributes a pole, and
these poles are too close together, you have too much phase shift
above unity loop gain and the thing oscillates.

In audio power amps the output stage usually has the lowest frequency
pole, but it's not sufficiently lower than the other poles for
stability.  So we a capacitor is added to the voltage gain stage to
make that the major pole.

But for stable operaion this would leave the major pole at a very low
frequency.  So a zero is also added to the voltage gain stage to
counteract the output stage pole.  This approximates a reasonable
frequency single pole in a 3- or 4-stage circuit.

That's the traditional approach, anyway.  (And no, it's not perfect!)

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com