Circuit Problem

[FanBlade]

Can someone please explain how this amplifier generally works? Why are the resistors and capacitors needed?

[r4tch3t]

The 1k is to limit the current through the transistor and also the speaker, the 100k is to bias the transistor so it's half on so when the input goes up and down it doesn't go below 0. The capacitors are there to stop dc current flowing.

Think that's all correct.

[r4tch3t]

Hmm on second glance it's not a class A amp, since there would need to be another resistor between the base and ground to bias it halfway.

It still works in the same kind of way, the input from the microphone changes the current flowing in the transistor thus changing the output voltage to the speaker.

The signal from the microphone can pass through the capacitor as the frequency is high enough that the capacitor can't fully charge before the signal voltage changes again, so a variable voltage can allow current to flow through a capacitor but with a static voltage the capacitor will charge up to that voltage and current will stop flowing.

[cameroncoats]

You have a common-emitter amplifier with collector feedback biasing; this design is not temperature compensated and the beta/Hfe compensation is relatively poor - potentiometer biasing is generally better.

IIRC the 1kΩ resistor sets the collector current and the 300kΩ resistor sets the base current to a fraction of this in order to bias the transistor. The capacitors block any DC bias whilst still allowing the AC signal to pass.