"Rich Grise" wrote in message
news
On Wed, 17 Nov 2004 00:18:38 -0800, Watson A.Name - "Watt Sun, the
Dark Remover" wrote:
Opinions on this, especially the 2.5W amp schem at the bottom.
http://www.intio.or.jp/jf10zl/EF.htm
When the amp is idle, and there's no(?) current flowing in either
output
transistor because there's no forward bias, then the V drop
'resistance'
of the E-B junction adds to the 100k feedback resistor, so the loop
gain
increases to the open loop gain. So it would seem that the amp
would
attempt to hunt in this region, possibly oscillating?
Maybe a 1k resistor E to B on the output transistors would 'bypass'
this. Or should the amp be biased to work class AB.
Hey, Watson. :-)
I'm going to level with you, I'm not an expert, I just play one on the
internet. But it looks to me like the gain of the output stage -
you're
talking about the one with an opamp per each output tranny, right? -
is
strapped such that the opamp's loop gain predominates, and I would not
be
a bit surprised to see the circuit behave just as you describe
(emitter
followers are notoriously fast), with two caveats: The slew rate of
the
opamps, and something about a pole or a zero in the complex impedance
at
that horrendous huge output cap.
But that's just a butt-level[0] feeling, so take it for what it's
worth, and let any uselessness go. :-)
Cheers!
Rich
[0] i.e., seat-of-the pants driving by a bench tech. ;-)
One never sees this configuration used in comm'l designs, so I figure
there must be a reason, such as problems with stability. I would do a
few things, like put fuses in the emitters of the power output
transistors.