Darlington transistor

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Circuit diagram of Darlington configuration

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 have covered all modern integrated circuits.

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

1 Behaviour
2 See also
3 References
4 External links

[edit] Behaviour

A Darlington pair behaves like a single transistor with a very high current gain. This is beneficial as many commonly-used transistors with high gains have a low current threshold. The total gain of the Darlington is the product of the gains of the individual transistors:

$\beta_\mathrm{Darlington} = \beta_1 \times \beta_2$

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:

V BE = V BE1 + V 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 can also be made up 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 2400 at a collector current of 10 A and an included switch-off resistor.

A Darlington pair can be sufficiently sensitive to respond to the small current passed by skin contact, and can thus be used to make a touch sensitive switch.

[edit] See also

Sziklai pair

Transistor amplifiers
Bipolar junction transistor: Common emitter • Common collector • Common base Field effect transistor: Common source • Common drain • Common gate Multiple transistors: Darlington transistor • Cascode • Parallel circuit