The operating point of a device, also known as a bias point, quiescent point or Q-point, is the steady-state DC voltage or current at a specified terminal of an active device such as a transistor with no input signal applied.

The term ‘bias point’ is used because, sometimes, an electronic device like a transistor must be biased with a steady DC input so that it can operate correctly and without saturation when performing its role as a processor of a time-varying AC signal. For example, a transistor could be biased to set its Q-point output to, say, 5 V to sit half way between 0 and 10 V supply rails. An AC input then superimposed onto the DC level at the transistor input will generate a corresponding, amplified waveform at the output. Biasing provides room for this waveform either side of the Q-point without clipping or saturation.

Stated another way, biasing in a transistor allows it to operate within a specified region of its transconductance curve. Distortion is avoided, as neither the positive nor negative output swing drives the transistor into a nonlinear operating region.

It should be noted that a Q-point does not necessarily remain stable after it has been set up. If a transistor’s junction temperature is allowed to increase, thermal runaway may occur. This will shift the Q-point into the saturation region, causing signal distortion and possible damage to the transistor.