Q. Write a short note on the working of Darlington Amplifier?

A Darlington Amplifier is one that is employing the Darlington pair of transistors. It is a cascaded emitter follower circuit.

The arrangement is such that the first transistor's emitter is connected to the base of the second transistor. Further the two collectors are tied together and are connected to the collector supply. Thus the second transistor forms the load impedance of the first amplifier.

Assuming that the ‘h' parameters of the two transistors, Q1 and Q2 are the same, it can be shown that the voltage gain of the Darlington stage is less than unity. The input resistance of the complete Darlington Emitter Follower Circuit is much higher than the input resistance of the emitter follower circuit. The output resstance will be low.

The input signal varies the base current of the first transistor. This produces the variation in the collector current in the first transistor. The emitter load of the first stage is the input resistance of the second stage. The emitter current of the first transistor is the base current of the second transistor.

The circuit is therefore a cascade of two emitter followers. Hence the gain is less than the gain of the emitter follower circuit which is less than unity. To provide a reasonable operating current in the first stage the second stage should be a power stage.