Operating point ($Q$)

The steady electric potential differences and electric currents about which signals vary in an electrical circuit. A small-signal model approximates nearby behavior.

A first-order Taylor expansion of an electrical circuit's response about an operating point. It describes small changes in electric potential difference and electric current; the neglected higher-order terms limit the useful signal range.

The change of output electric current per change of input electric potential difference, evaluated at an operating point with the other terminal conditions specified. In a small-signal model, $i_{\mathrm{out}}=g_m{v}_{\mathrm{in}}$ describes this contribution; transconductance has the dimensions of electrical conductance.