In article ,
John Larkin wrote:

We're taking a flawed design, and adding band-aids on top of band-aids. You
can make a perfectly good amplifier with one opamp and two push-pull emitter
followers biased with a couple of diodes, similar to the first design
sketched in the OP's link. If that design draws too much idle current,
increase the emitter resistor. There's lots of stuff you can do to improve
that design without resorting to the demented design being proposed.


It's actually not a bad idea to have an opamp per output transistor,
if you do it right, which this guy clearly hasn't. I make a power amp
that uses 32 300-watt fets in the output (16 p-ch, 16 n-ch, +-200 volt
rails) and do just that. It forces essentially perfect current
sharing, nukes the device tc and part-part variations, and makes lots
of gate drive available.

Seems like a reasonable idea, *if* you do the feedback loops right.

In your case, I'd guess that you have a unity- or small-gain feedback
loop wrapped around each individual opamp-and-transistor (tapping off
between the transistor source and the ballast resistor?), and then an
outer feedback loop from the final output back to an earlier stage (a
pre-driver op amp which then feeds the driver op amps?). As long as
these loops are speed-compensated properly, this would probably be
quite safe... some local feedback and some global feedback.

The problemms come up when you have multiple op amps and feedback
loops in parallel in a way which allows them to fight, or have
"wrapped" feedback loops with inappropriate time constants.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!