Small signal modelling is an analysis method used in electrical engineering, to describe nonlinear devices in terms of linear devices.
Motivation
Electronic circuits generally involve small time-varying signals carried over a constant bias. This suggests using an method akin to approximation by differentials to analyze relatively small perturbations about the bias point.
Any nonlinear device which can be described quantitatively using a formula can then be linearized about a bias point by taking taking partial derivatives of the formula with respect to all governing variables. These partial derivatives can be associated with physical quantities (such as capacitance, resistance and inductance), and a circuit diagram relating them can be formulated. Small signal models exist for diodes, field effect transistors and bipolar transistors.