On Tue, 22 Dec 2015 09:31:10 +0000 (GMT), Jim Lesurf
wrote:

In article ,
Don Pearce wrote:
http://srv2.umlib.com/1129f1c8443296...r-sm.pdf-3.png


I've never seen that topology before. There doesn't seem to be a
dominant pole in the voltage amplifiers - how did they keep it stable?

It does have what I used - a snubber across the first long-tail pair(s).
Also a cap across them *and* some later caps. If anything it seems to have
roll-off applied in multiple places. Which one may dominate I haven't
checked. But it doesn't seem to lack HF roll-off.

Interesting that it apparently omits having any output inductor. I tended
to find one useful if only for helping to reduce any RF injection coming in
via the speaker leads.

Jim

This is my problem. A single dominant pole placed so that the phase
doesn't wander too far from 90 degrees until you are comfortably clear
of unity gain gives you a warm comfortable feeling. Now I know this is
easiest when you have open loop gain by the bucketload, as you have in
a modern integrated op amp. But this is still an op amp and it seems
strange to dot bits of this and bits of that around it.

I have used the snubber circuit when adding discrete transistors to
the front end of an op amp for low noise work. You have to do this
because the extra voltage gain completely screws the phase margin of
the bigger loop. If you could get inside the IC and beef up the
dominant pole cap that wouldn't help, because the current in the tails
would be insufficient to drive it at high frequencies, and you'd get
slew rate limiting.

Anyway, I'm sure this works, but it has a look of an amplifier that
oscillated on the bench, so they just added bits until it stopped.

d

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