A transistor circuit in which the current in one side of the circuit is forced to be identical to the current in the other side of the circuit: a ‘mirroring’ action. The simplest current mirror comprises just two transistors, the diagram showing an example using bipolar junction transistors.

Because the bases and emitters of the two transistors are connected together, the base-emitter voltages must be identical. If the transistors are the same, then the collector currents must be the same. The input transistor has the base and collector connected together – a diode-connected transistor – allowing its base-emitter voltage to be determined explicitly. The output of the current mirror is taken as the collector load of the second transistor, providing a high-impedance current source output that is relatively insensitive to loads.

More complex current mirror circuits have been devised that provide even greater insensitivity to loading, an example being the Wilson current mirror.

Current mirror using two bipolar transistors