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
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)