Darlington transistor

In electronics, the Darlington transistor (often called a Darlington pair) is a semiconductor device which combines two bipolar transistors in a single device so that the current amplified by the first is amplified further by the second. This configuration gives a high current gain (written β, h_fe, or h_FE) and can take less space than two separate transistors because the two transistors can use a shared collector. Integrated circuit packages are available, but it is still common also to use two separate transistors.

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 (the product of the gains of the two transistors):

$beta\_mathrm\{Darlington\}\; =\; beta\_1\; cdot\; beta\_2$

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

The base-emitter voltage is also higher. It is the sum of both base-emitter voltages:

$V\_mathrm\{BE\}\; =\; V\_mathrm\{BE1\}\; +\; V\_mathrm\{BE2\}$

Thus, for Si based technology, there must be about 0.7 V across both base-emitter junctions (connected in series in the device), so that we need about 1.4 V in total to turn on the device. The saturation voltage of a Darlington pair is about 0.7 V, which can cause substantial power dissipation. Another drawback is a reduction in switching speed, because the first transistor cannot actively inhibit the base current of the second, which makes the device slow to switch off. To alleviate this, the second transistor often has a base resistor of a few hundred ohms.

The Darlington 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

References

External links

• — 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.