![]() Finally, the turn-off responsiveness decreases as the second transistor lags behind the shutoff of the first a resistor across the base-emitter junction of the second transistor helps expedite this process by providing a low-impedance dissipation pathway. Additionally, a Darlington transistor increases the saturation voltage drop across the collector-emitter junction of the second transistor by a factor greater than four (for silicon-based semiconductors), which can become a significant source of power loss and cause a mismatch between low-output levels and driven TTL logic. Despite its original formulation as a method to facilitate negative feedback, a Darlington transistor offers poor stability in these cases, a common-emitter transistor pair provides a better high-frequency response. A Darlington transistor functions best at low frequencies, as the phase shift of the pair is greater than the sum of its parts. ![]() Of course, there’s no such thing as a free lunch: the improved current gain comes at the cost of a higher turn-on voltage at the combined base-emitter junction, doubling the individual voltage requirements on a single BJT. A Darlington transistor also offers excellent impedance capabilities on both ends, with a high input impedance to minimize voltage drop (BJTs are, by nature, voltage-controlled devices) and a low output impedance to minimize power loss before reaching the load. In this sense, it offers a high amount of flexibility during circuit prototyping if layout space is ample, designers can employ two lesser (and potentially cheaper) transistors in the place of a more robust IC or realize unparalleled forward-operating current gain values with two high-performing BJTs. The Darlington transistor pair is readily implementable from discrete components or in a unified package format. Therefore, even a small base current can rapidly grow by three magnitudes or sometimes more. The performance improvement arises from the multiplicative effects of the β from the individual transistors: as the current gain of a single transistor already has a significant forward-operation current gain, the product of multiple transistors increases even further. By tying the emitter of the first transistor (for an NPN–note that PNP Darlington transistors are also a valid design) to the second transistor's base, the combined transistor network effectively acts as a single transistor. The Design and Performance of the Darlington TransistorĪ Darlington transistor configuration combines multiple (most often two) bipolar junction transistors (BJTs) for a multi-stage amplification effect.
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