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 iC and VBE, while input conductance relates to changes in iB and VBE. 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.