BJT Common-Emitter Amplifier

A common-emitter amplifier uses a bipolar NPN transistor to turn a small base signal into a larger voltage change at the collector. The bias divider establishes an operating point, while the collector and emitter resistors set current and stabilise the stage. An emitter bypass capacitor can increase AC gain without changing the DC bias. The output is inverted relative to the input. This schematic is a teaching example rather than a finished audio design: the required bias depends on the transistor, supply, source level, and desired output swing. Measure or simulate the collector voltage and adjust values so the transistor has room to amplify both halves of the waveform.