Editing Differential amplifier (section)

==== Emitter constant current source ====
The quiescent current has to be constant to ensure constant collector voltages at common mode. This requirement is not so important in the case of a differential output, since although their two collector voltages will vary simultaneously their difference (the output voltage) will not vary. But in the case of a single-ended output, it is extremely important to keep a constant current since the output collector voltage will vary. Thus the higher the resistance of the current source <math>R_{\text{e}}</math> in the original circuit of Fig.&nbsp;2, the lower (better) is the common-mode gain <math>A_{\text{c}}</math>.

The constant current needed could be produced by connecting an element (resistor) with very high resistance between the shared emitter node and the supply rail (negative for NPN and positive for PNP transistors), but that requires a high supply voltage. So in more sophisticated designs, an element with high differential (dynamic) resistance approximating a [[constant current]] source/sink (the bottom of Fig.&nbsp;3) is substituted for the “long tail”. It is usually implemented by a current mirror because of its high compliance voltage (small voltage drop across the output transistor).