12.6 : 双极结型晶体管（BJT）的频率响应

A common-emitter amplifier's DC bias point is determined by the base-emitter voltage, base current, and collector current. The load line is determined by the applied voltage and load resistance.

When a small AC signal is added to the input voltage, the base current varies over time, resulting in a corresponding variation in the output current. This amplification effect occurs when the peak AC values are smaller than the DC values.

The low-frequency equivalent circuit includes resistances and transconductance as important parameters for AC analysis.

Transconductance represents the relationship between changes in i_C and V_BE, while input conductance relates to changes in i_B and V_BE.

At higher frequencies, the equivalent circuit undergoes modification by introducing depletion and diffusion capacitances for the emitter-base junction and a depletion capacitance for the collector-base junction.

Considering the base width modulation effect, there exists a finite output conductance.

The high-frequency equivalent circuit encompasses all elements, including resistances, offering a comprehensive view of the transistor's behavior at higher frequencies.