A pair of matched transistors, either two BJTs or two FETs, connected as an emitter-coupled or source-coupled pair, with a constant current source providing the bias at the common connection. The collector (drain) output current is related to the difference between the input voltages at the base (gate) terminals of the two transistors. This arrangement is the basis of a differential amplifier, which amplifies differential-mode signals only. The common-mode performance of the amplifier is related to the conductance of the constant current bias source. Any signals appearing across this source will be effectively common to both inputs, but will cause a change in the output current; this is unwanted common-mode gain.

This source can be realized using a large resistor; this requires a large voltage to maintain the bias current, and signal voltages will be much smaller than the steady bias voltage, giving good rejection of the common-mode signals. The larger this resistance, the longer the ‘tail’ of the differential pair. In integrated circuits the current source is more likely to be realized using active devices, in the form of a current mirror.