Darlington transistor

A Darlington pair behaves like a single transistor, meaning it has one base, collector, and emitter.

It can be simply made with two separate NPN (or PNP) transistors, and is also available in a variety of single packages.

The increased low output level can cause troubles when TTL logic circuits are driven.

To alleviate this, the second transistor often has a resistor of a few hundred ohms connected between its base and emitter terminals.

[1] This resistor provides a low-impedance discharge path for the charge accumulated on the base-emitter junction, allowing a faster transistor turn-off.

The Darlington pair has more phase shift at high frequencies than a single transistor and hence can more easily become unstable with negative feedback (i.e., systems that use this configuration can have poor performance due to the extra transistor delay).

A typical integrated power device is the 2N6282, which includes a switch-off resistor and has a current gain of 2400 at IC=10 A.

Darlington pairs are often used in the push-pull output stages of the power audio amplifiers that drive most sound systems.

In a fully symmetrical push-pull circuit two Darlington pairs are connected as emitter followers driving the output from the positive and negative supply: an NPN Darlington pair connected to the positive rail providing current for positive excursions of the output, and a PNP Darlington pair connected to the negative rail providing current for negative excursions.

A Darlington pair can be sensitive enough to respond to the current passed by skin contact even at safe zone voltages.