Load line (electronics)

It represents the constraint put on the voltage and current in the nonlinear device by the external circuit.

The diode, a nonlinear device, is in series with a linear circuit consisting of a resistor, R and a voltage source, VDD.

A load line drawn on this graph shows how the base current will affect the operating point of the circuit.

The intersections of the load line with the transistor characteristic curves represent the circuit-constrained values of IC and VCE at different base currents.

The transistor is said to be cut off, passing only a very small leakage current, and so very nearly the entire supply voltage appears as VCE.

Adjusting the base current so that the circuit is at this operating point with no signal applied is called biasing the transistor.

The same technique is applied to other types of non-linear elements such as vacuum tubes or field effect transistors.