[sdiy] Buffering an op amp's output

[Byron G. Jacquot]

> Does anyone have a favorite circuit for buffering an op amp's output,
> to drive low impedance loads such as a 4 ohm speaker? I thinking of
> perhaps adding buffer transistors to the output of the op amp, and
> including the transistor buffer in the op amp's feedback loop. Would
> that be the recommended way to approach this?

I don't know that it'll drive a 4-ohm load, but there are a couple of 
class-B ideas here:

http://www.prodigy-pro.com/forum/viewtopic.php?t=838&start=30

Including the transistors within the feedback loop can help linearize 
any crossover distortion, as in that first schematic.

The second schematic (with the biasing diodes) is a better behaved by 
itself, but I've not put it through any cirtical listening tests.  It 
works alright outside the feedback loop.

A web search for Walt Jung's "Super Buffer" might turn up a similar 
idea for driving heavy loads.  Similarly, if you can find the 
schematic for an Opamp-labs power opamp, there will be some more 
ideas there...the opamp labs power opamps are all similar, often 
differing mostly in the amount of heatsinking povided...more aluminum 
on the higher-wattage ones.

If the to-3 transistors are overkill, you might look into some 
to220s.  I've been having good luck with TIP31 & TIP32 pairs.

Before digging in Go re-read Horowitz & Hill's discussion on class-B 
thermal runaway.  The small value emitter resistors are essential!

Putting resistors on the collectors allows you to see when each 
transistor is conducting, to help see how crossover distortion creeps 
in.

Byron Jacquot