Darlington transistor

In electronics, the Darlington transistor (often called a Darlington pair) is a compound structure consisting of two bipolar transistors (either integrated or separated devices) connected in such a way that the current amplified by the first transistor is amplified further by the second one. This configuration gives a much higher current gain than each transistor taken separately and, in the case of integrated devices, can take less space than two individual transistors because they can use a shared collector. Integrated Darlington pairs come packaged singly in transistor-like packages or as an array of devices (usually eight) in an integrated circuit.

The Darlington configuration was invented by Bell Laboratories engineer Sidney Darlington in 1953. He patented the invention of having two or three transistors on a single chip sharing a collector.

A similar configuration but with transistors of opposite type (one NPN and one PNP) is the Sziklai pair, sometimes called the "complementary Darlington".

A Darlington pair behaves like a single transistor with a high current gain (approximately the product of the gains of the two transistors). In fact, integrated devices have three leads (B, C, and E), broadly equivalent to those of a standard transistor.

A general relation between the compound current gain and the individual gains is given by:

If β₁ and β₂ are high enough (hundreds), this relation can be approximated with:

Darlington pairs are available as integrated packages or can be made from two discrete transistors; Q₁ (the left-hand transistor in the diagram) can be a low power type, but normally Q₂ (on the right) will need to be high power. The maximum collector current I_C(max) of the pair is that of Q₂. A typical integrated power device is the 2N6282, which includes a switch-off resistor and has a current gain of 2400 at I_C=10A.

A Darlington pair can be sensitive enough to respond to the current passed by skin contact even at safe voltages. Thus it can form the input stage of a touch-sensitive switch.

Darlington transistors can be used in circuits involving motors, relays, or other current-hungry components connected to computers. The current is amplified from the normal low level of the computer output line to the amount needed by the connected device.

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