smallsignal

Small-signal analysis is a method in electronics used to analyze the linear response of circuits that contain nonlinear active devices by linearizing them around a chosen operating point, or Q-point. It uses incremental or small-signal parameters to describe how small input perturbations produce output changes.

The approach treats capacitors, resistors, inductors, and linearized models as linear elements and relies on superposition and phasor techniques for sinusoidal inputs. The DC operating point is determined by the bias network, and once fixed, the circuit is analyzed in the frequency domain to study how signals of different frequencies are amplified or attenuated.

In a bipolar junction transistor (BJT), the common small-signal model is the hybrid-pi or T-model, characterized by r_pi between base and emitter, a gm*v_pi current source from collector to emitter, and ro as the output resistance. For a MOSFET, the small-signal model includes a transconductance gm*v_gs, an output resistance ro, and parasitic capacitances between terminals (Cgs, Cgd, etc.). These models approximate the device behavior for small perturbations around the bias point.

Small-signal analysis yields transfer functions, input and output impedances, gain, phase response, and overall frequency response. Coupling and bypass capacitors, as well as source and load impedances, shape the circuit’s bandwidth and impedance seen by signals.

Applications are widespread, including the design and analysis of amplifiers, filters, and RF circuits, and are commonly supported by circuit simulators performing AC analysis. Limitations include the restriction to small perturbations around the Q-point; large-signal or transient behavior, bias shifts, and temperature effects are not captured by the linear small-signal models.