[HomeBrew] Power amp for LM324

[Steven Weber]

>
> I have a LM324 amplifier which has a 0 to 30VDC output. The chip is only 
> capable of like 15ma output. I would like to drive a circuit that will put 
> out 100ma with the same 0 to 30 volts. The LM324 amp I am using is a X3 
> amplifier that converts 0 to 10 to 0 to 30 volts.
>
> Does anyone have ideas for a simple DC power amp? Thanks!

You need to add some buffer transistors. What you use and how they are 
configured depends on what your driving. If your use it as a variable power 
supply, a simple NPN common emitter follower will do. If your pushing the 
full 100 ma at a low voltage output setting and using a 30 V supply, you 
will need to use at least a 5W power transistor with heat sinking.


If you need source/sink capability, like for say an AC audio amp, you need a 
push pull NPN/PNP follower. If cross over distortion in not important, you 
can tie both bases together and drive direct from the op amp. If cross over 
distortion needs to be minimized, you need to add bias resistors and diodes 
to the base of the transistors.

Note that you will not be able to go all the way to 0V with the push pull 
output, to do that you need a slightly negative supply for the op amp. And 
either way, you won't get the full 30 volts out. The LM324 max output is 
Vcc-1.6 volts, then add the base-emitter drop for the buffer output 
transistor.

Steve KD1JV