[citation needed] Circuits made with these components, called linear circuits, are governed by linear differential equations, and can be solved easily with powerful mathematical frequency domain methods such as the Laplace transform.
[citation needed] In contrast, many of the components that make up electronic circuits, such as diodes, transistors, integrated circuits, and vacuum tubes are nonlinear; that is the current through[clarification needed] them is not proportional to the voltage, and the output of two-port devices like transistors is not proportional to their input.
In these circuits a steady DC current or voltage from the power supply, called a bias, is applied to each nonlinear component such as a transistor and vacuum tube to set its operating point, and the time-varying AC current or voltage which represents the signal to be processed is added to it.
If the characteristic curve of the device is sufficiently flat over the region occupied by the signal, using a Taylor series expansion the nonlinear function can be approximated near the bias point by its first order partial derivative (this is equivalent to approximating the characteristic curve by a straight line tangent to it at the bias point).
The small signal model is dependent on the DC bias currents and voltages in the circuit (the Q point).
Any nonlinear component whose characteristics are given by a continuous, single-valued, smooth (differentiable) curve can be approximated by a linear small-signal model.
Manufacturers often list the small-signal characteristics of such components at "typical" bias values on their data sheets.