Amplifier input stage vs output stage

Can someone explain the difference?

The input stage is what you connect your music sources to, either directly or through a preamplifier. The output stage is what you connect your speakers to. The output stage is built to handle the relatively high voltages and currents required by speakers. In between the input and output stages you normally have a Voltage Amplification Stage, which transforms the low voltages of your music sources into the larger ones needed to drive the speakers. The output stage then just has to be able to supply the current.

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My question is more technical.

The simplified version is an amp takes a low level signal and amplifies it. Do both current and voltage amplification happen in the output stage? If so, what’s the purpose of the input stage? If not, which stage amplifies what (current/voltage)?

I thought my original reply lightly covered that. The amp input stage takes the signal, adds in feedback from the output, and then feeds it to the voltage amplification stage. That stage transforms the relatively small voltage into a much larger one. The output stage takes this larger signal and makes it available to the speakers with the ability to supply a much larger current. large voltage and large current means lots of power.

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Thanks. So a super simplification of an amp would be: input stage -> voltage gain -> current supply (output stage). Is this correct?

And if so, then the only reason for the input stage’s existence is as reference for the feedback loop?

Not all amplifiers use feedback. That’s a design choice.



Also you might check the amplification section of


It is a little more complex than that. I feel the input stage provides a suitable ‘receiver’ for the signal from the source which doesn’t place too many demands for current or voltage on it. Since most amplifiers use negative feedback that is the point at which it will be applied. The 3 stage amplifier model is most relevant for class AB amps, and is implemented in most of them. The way a class D amp provides current for the speakers is very different. Follow up Ted’s links for more detail about amplifiers, but you will be getting a LOT more detail!