A small-signal model is the linear equivalent circuit that replaces a non-linear device for signal analysis, valid for small swings about a fixed Operating point. It’s the concrete output of Linearisation around an operating point: once you know the tangent slopes of the device’s characteristics at Q, you draw a little circuit of resistors and dependent sources that reproduces those slopes, and that circuit is the device as far as the signal is concerned.
Why it exists
A diode or transistor is non-linear, so it can’t be analysed with the resistor-and-source machinery of linear circuit theory. But after Linearisation around an operating point, the device’s response to a small signal is fully described by a handful of constants: derivatives of its – laws evaluated at the bias point. A small-signal model packages those constants into an equivalent circuit so the rest of the analysis is pure linear-circuits work. This is Step 2 of Small-signal analysis: kill the DC sources, drop in the small-signal model, solve the linear circuit.
The technique is universal; only the parameters change
The procedure is identical across devices. What differs is the internal make-up of the equivalent circuit and the names of its parameters:
- Diode: a single resistor, the Diode small-signal resistance .
- MOSFET: a voltage-controlled current source with an output resistance . The gate draws no current, so the input side is an open circuit. See MOSFET small-signal model.
- BJT: the same controlled source plus an input resistance (the base draws current) and an output resistance . See BJT small-signal model.
Every one of these parameters is a slope of a non-linear curve at Q, and every one depends on the DC bias. That’s why DC analysis (finding the Operating point) comes first: you can’t draw the small-signal model until you know the bias currents that set its parameter values. Master the diode case and the MOSFET and BJT cases are the same idea with more parameters.