Darlington transistor

Circuit diagram of a Darlington pair using NPN transistors

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 (written β, h_fe, or h_FE) 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 in transistor-like integrated circuit packages.

The Darlington configuration was invented by Bell Laboratories engineer Sidney Darlington in 1953. He patented the idea of having two or three transistors on a single chip (and sharing a single collector), but not that of an arbitrary number (which might have covered all modern integrated circuits).

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

Behavior

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:

$\beta _{\mathrm {Darlington} }=\beta _{1}\cdot \beta _{2}+\beta _{1}+\beta _{2}$

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

$\beta _{\mathrm {Darlington} }\approx \beta _{1}\cdot \beta _{2}$

A typical modern device has a current gain of 1000 or more, so that only a small base current is needed to make the pair switch on. However, this high current gain comes with several drawbacks.
Since there are two junctions between the base and emitter of the Darlington transistor, the equivalent base-emitter voltage is the sum of both base-emitter voltages:

$V_{BE}=V_{BE1}+V_{BE2}\!$

Thus, for silicon based technology, where there must be about 0.65 V across each base-emitter junction, the necessary base-emitter voltage of the pair for which the device is operating in the linear or saturated region is about 1.3 V. Another drawback of the Darlington pair is its saturation voltage. Indeed, if the compound transistor is operating in saturation (i.e. the collector-emitter voltage is lower than its base-emitter voltage), the first transistor is saturated and basically connects together the base and the collector of the output transistor. In this configuration, the output transistor acts like a diode and its collector-emitter voltage (equal with the pair's collector-emitter voltage) is approximatley the same with its base-emitter one, limiting the saturation voltage of the Darlington transistor to about 0.65V. (Note that the output transistor is not really saturated since its collector-base voltage is equal or larger than 0V). For equal collector currents (and same power rating), this drawback translates in an increase in the dissipated power for the Darlington transistor over a normal transistor.
Another problem is a reduction in switching speed, because the first transistor cannot actively inhibit the base current of the second one, making the device slow to switch off. To alleviate this, the second transistor often has a resistor of a few hundred ohms connected between its base and emitter terminals. 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 phase margin due to the extra transistor delay).

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.

See also

• Sziklai pair

External links

• U.S. patent 2,663,806 — Semiconductor signal translating devices. (ed., "Darlington Transistor")
• A Darlington Pair motor speed control circuit
• ECE 327: Procedures for Output Filtering Lab — Section 4 ("Power Amplifier") discusses Darlington pairs in the design of a BJT-based class-AB current driver in detail.