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In electronics, the Darlington transistor is a semiconductor device which combines two bipolar transistors in tandem (often called a "Darlington pair") in a single device so that the current amplified by the first is amplified further by the second transistor. This gives it high current gain (written β or h_FE), and takes up less space than using two discrete transistors in the same configuration. The use of two separate transistors in an actual circuit is still very common, even though integrated packaged devices are available.

This configuration was invented by Bell Laboratories engineer Sidney Darlington. The idea of putting two or three transistors on a single chip was patented by him, but not the idea of putting an arbitrary number of transistors, which would cover all modern ICs.

A similar transistor configuration using two transistors of opposite type (NPN and PNP) is the Sziklai pair, sometimes called the "complementary Darlington".

Behaviour

A Darlington pair behaves like a single transistor with a very high current gain. The total gain of the Darlington is the product of the gains of the individual transistors:

${\displaystyle \beta _{\mathrm {Darlington} }=\beta _{1}\times \beta _{2}}$

A typical modern device has a current gain of 1000 or more, so that only a tiny base current is required to make the pair switch on. Integrated devices have three leads (B, C and E) which are equivalent to the leads of a standard individual transistor.

The base-emitter voltage is also higher; it is the sum of both base-emitter voltages:

${\displaystyle V_{\mathrm {BE} }=V_{\mathrm {BE1} }+V_{\mathrm {BE2} }}$

To turn on there must be ~0.6 V across both base-emitter junctions which are connected in series inside the Darlington pair. It therefore requires more than 1.2 V to turn on. When a Darlington pair is fully conducting, there is a residual saturation voltage of 0.6 V in this configuration, which can lead to substantial power dissipation. Another drawback is that the switching speed can be slow, due to the inability of the first transistor to actively inhibit the current into the base of the second device. This can make the pair slow to switch off. To alleviate this, a resistor of a few hundred ohms between the second device's base and emitter is often used. Integrated Darlington pairs often include this resistor.

It has more phase shift at high frequencies than a single transistor, and hence can become unstable with negative feedback much more easily.

Darlington pairs are available as complete packages but you can make up your own from two transistors; Q₁ (the left-hand transistor in the diagram) can be a low power type, but normally Q₂ (the right-hand transistor) will need to be high power. The maximum collector current I_C(max) for the pair is the same as I_C(max) for Q₂. A typical integrated type of power device is the 2N6282, with a current gain of 2,400 at a collector current of 10A and an included switch-off resistor.

A Darlington pair is sufficiently sensitive to respond to the small current passed by your skin and it can be used to make a touch-switch.

External link

• Darlington’s Contributions to Transistor Circuit Design
• Free IC DataSheet Search Site : http://www.Datasheet4U.com

• Transistors