However the increased current gain often does not justify the sensitivity and saturation current problems, so this circuit is seldom used.ĭarlington pairs are often used in the push-pull output stages of the power audio amplifiers that drive most sound systems.
This gives current gain approximately equal to the product of the gains of the three transistors. The emitter of the second transistor in the pair is connected to the base of the third, as the emitter of first transistor is connected to the base of the second, and the collectors of all three transistors are connected together. Integrated Darlington pairs come packaged singly in transistor-like packages or as an array of devices (usually eight) in an integrated circuit.Ī third transistor can be added to a Darlington pair to give even higher current gain, making a Darlington triplet. Integrated devices can take less space than two individual transistors because they can use a shared collector. A typical integrated power device is the 2N6282, which includes a switch-off resistor and has a current gain of 2400 at I C=10 A. The maximum collector current I C(max) of the pair is that of Q 2. 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 performance due to the extra transistor delay).ĭarlington pairs are available as integrated packages or can be made from two discrete transistors Q 1, the left-hand transistor in the diagram, can be a low power type, but normally Q 2 (on the right) will need to be high power.
This resistor provides a low-impedance discharge path for the charge accumulated on the base-emitter junction, allowing a faster transistor turn-off. To alleviate this, the second transistor often has a resistor of a few hundred ohms connected between its base and emitter terminals. The increased low output level can cause troubles when TTL logic circuits are driven.Īnother problem is a reduction in switching speed or response, because the first transistor cannot actively inhibit the base current of the second one, making the device slow to switch off.
For equal collector currents, this drawback translates to an increase in the dissipated power for the Darlington transistor over a single transistor.
Β D a r l i n g t o n = β 1 ⋅ β 2 + β 1 + β 2 always.) Thus the "saturation" voltage of a Darlington transistor is one V BE (about 0.65 V in silicon) higher than a single transistor saturation voltage, which is typically 0.1 - 0.2 V in silicon. It typically creates a high current gain (approximately the product of the gains of the two transistors, due to the fact that their β values multiply together).Ī general relation between the compound current gain and the individual gains is given by: A Darlington pair behaves like a single transistor, meaning it has one base, collector, and emitter.
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