Here is a simple audio amplifier circuit. The BC337 transistor acts as the signal amplifier.

Rc = 130 ohms

RE = 26 ohms

R1 = 3.4 kOhms

R2 = 1 kOhms

C1 = C2 = 10 microfarad

Here, the transistor is acting as an amplifier. It amplifies the voltage/signal that comes out of the capacitor (c1). See, the input signal is connected to a capacitor which is connected to a voltage divider (created with the two resistors: r1 and r2). And as the signal changes, the charge on the other end of the capacitor changes with the signal. And it is this signal change that is amplified by the transistor.

The transistor amplifies the signal because as the signal changes on it’s base, the amount of current it “allows” to flow through the collector to the emittor changes proportionally to the input signal on the base. Thus, “amplifying” the signal (or changing a constant voltage accordingly so that it becomes a dilated version of an input signal).

Hope this helps!

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*I don’t know exactly when I did this, but it was around this time.