12.9 : 双极结型晶体管（BJT）放大器

In the active region of a common emitter NPN BJT, the emitter current is the sum of the base current and the collector current. Here, the base voltage controls the collector current in the BJT.

The output voltage across the collector is derived by subtracting the voltage drop across the load resistance from the supply voltage, producing an inverted output relative to the supply voltage.

The relation between collector voltage and base voltage gives the voltage-transfer characteristic curve of the common-emitter BJT amplifier.

It highlights the amplifier's active region, characterized by a steep slope for maximum gain but a nonlinear output voltage.

By applying a base-emitter DC voltage bias at the quiescent point within the active region, the BJT typically achieves nearly linear amplification.

When a small, time-varying signal is applied to the amplifier, it is superimposed on the DC bias voltage around the quiescent point.

So, the BJT operates within a short, nearly linear segment of the characteristic curve, resulting in an amplified output collector voltage.