Hey Guys,

I understand that a Op amp has a high input impedance and a low output impedance. A microphone has a high impedance, and a speaker has a low impedance. Couldn't an op amp be used for a audio amplifier? If this is possible, how would this be done?

Thanks!

 

It can be used a preamplifier and then feed its output to a push-pull pair to drive the speaker. This is because an op amp it self cannot power a speaker directly connected on its output. You need to boost its output current.

 

Oh I see. So preamplifiers generally just amplify the signal and the second amplifier amplifies the current?

 

Yes, a preamplifier will amplify the signal's voltage and the push-pull pair, which is actually a voltage follower, will boost (amplify the current). Thus you have both the voltage and current amplified and thus the output power is amplified.

 

Most op amps can't generate enough power to drive a speaker, though they can satisfactorily drive an earphone (not a headphone).

If you want an integrated circuit amplifier to drive speakers there are plenty of these on the market from 0.5 watts to 500 watts.

Or as MIK says you can boost the output of an op amp by adding extra components to make a hybrid amplifier.

 

There are examples that can do this however. They aren't your common parts however, it is easier to build a driver after an op amp stage and incorporate it into the op amp feedback, which effectively makes it part of the op amp.

http://forum.allaboutcircuits.com/showthread.php?t=10631

http://forum.allaboutcircuits.com/showthread.php?t=18042

 

Couldn't I put a microphone on the input of an op amp and then to my common emitter circuit amplifier? Wouldn't this make a better impedance match?

 

A power amplifier that drives a low impedance speaker has two complimentary emitter-follower transistors at the output making a push-pull class-AB pair, not a single common-emitter class-A transistor.

 

since the impedances are so different, you can put the audio from a mic through a couple op amps for amplification, but at the output, use a miniature 1K to 8 ohm audio output transformer to match the impedance to an 8 ohm output.