Darlington Pair

Darlington Pair is transistors used in cascade, giving high gain and high input impedance.

The maximum input impedance one can expect from an emitter follower, is limited by the finite gains of individual transistors (~ 50 to ~ 350). However, there is a way to increase the effective gain or transistors by using two transistors. The total gain of this transistor pair is Gv1 x Gv2 = Gvtotal (Gv ~ 2k - 100k). This is achieved by arranging the transistors such that the emitter of one is driving the base of the next and connecting the collectors together. This is known as a Darlington pair, and can be used as any single transistor would be: common emitter, emitter follower, etc.

The down side of this arrangement, is reduced speed: because of the very high gain's effect on the collector to base capacitance, C_o ( Ctotal = Co x Hfe ).