Darlington Pair

Two transistors may be combined to form a configuration known as the Darlington pair which behaves like a single transistor with a current gain equivalent to the product of the current gain of the two transistors. This is especially useful where very high currents need to be controlled as in a power amplifier or power-regulator circuit. Darlington transistors are available whereby two transistors are combined in one single package. The base-emitter volt-drop is twice that of a normal transistor.

In practical circuits resistor R (shown dotted in the illustration) is often included to prevent leakage current through the first transistor biasing the second into conduction. The Darlington pair was invented by Sidney Darlington who was a researcher at Bell Labs in the nineteen-fifties and who invented his eponymous pair at the weekend in his home laboratory!

A similar configuration is termed the complementary Darlington, the Sziklai pair, in which two transistors of different polarity are combined to form a composite transistor as illustrated. The advantage of this configuration is that the base-emitter volt-drop is limited to that of a single transistor. The complementary Darlington is very popular in audio power amplifiers.

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