What is Darlington pair?

Darlington pair consists of two cascaded emitter followers used to achieve high input resistance. Darlington pair is a better buffer compared to single stage emitter follower circuit.

The typical Darlington pair is shown below

Darlington pair

Following are some of the characteristic parameters of Darlington pair

Rin = (1+β)^2 Re/(1+βRe/ro)

Ai = (1+β)^2/(1+ β*Re/ro)

Av = 1-β*(2+β*Re/ro)/((1+β)*Re)

Ro = Rsig/(1+β)^2+(2*β)/(1+β)