Darlington Pair transistor configuration and features

The Darlington Pair transistor configuration of two bipolar transistors offers growth current switching for a given base. One transistor circuit configuration which can be used to quite a good effect in many instances is the Darlington Pair. The Darlington Pair provides a number of advantages. It is fundamentally used because it offers especially high gain and this also reflects in a high input impedance for the overall Darlington circuit when compared to a single transistor.

However, the Darlington Pair does have some defects and it is not suitable for all high-gain applications as a result. Nevertheless, where applicable, the Darlington Pair is able to provide many gains over a single transistor circuit configuration.

Darlington Pair transistor configuration and features

The Darlington Pair may sometimes also be referred to as a super-alpha pair, however, this name is used less these days and the circuit configuration was invented at Bell Laboratories by Sidney Darlington in 1953. The pin configuration of Darlington Pair transistor. Darlington Pair transistor.



  • A common Darlington transistor has a current advantage of 1000 or more, however, only a small base current is needed to make the pair switch on higher switching currents.
  • Another advantage involves providing a very high input impedance for the circuit, which also translates to an equal decrease in output impedance.
  • It can be designed easily, it’s also an advantage. The Darlington Pair transistor can be simply made with two different NPN transistors, and it is also available in the kinds of single packages.


  • One defect is an approximate doubling of the base-to-emitter voltage. However, there are two junctions between the base and emitter of the Darlington transistor, the equal base-emitter voltage is the sum of both base-emitter voltages:
{\displaystyle V_{BE}=V_{BE1}+V_{BE2}\approx 2V_{BE1}\!}

For silicon-on-based technology, where always(VBE) is about 0.65 V, &  when the device is operating in the saturated region or in turn, the base _ emitter voltage of the pair is 1.3 V.

  • Another defect of the Darlington pair Transistor is its growth “saturation” voltage. The output NPN transistor is not allowed to saturate,  since the first NPN transistor, This transistor when saturated, establishes fully 100% parallel negative feedback among the collector and the base of the second transistor.[2] since the collector & emitter voltage is equal to, the sum of its own base _ emitter voltage and the collector & emitter voltage of the first transistor, both positive amounts in normal operation, this transistor always exceeds the base-emitter voltage. (In symbols, {\displaystyle \mathrm {V_{CE2}=V_{CE1}+V_{BE2}>V_{BE2}} \Rightarrow \mathrm {V_{C2}>V_{B2}}} always.) hence, the “saturation” voltage of a Darlington transistor is one (VBE) (about 0.65 V in silicon) higher than a single transistor saturation voltage, which is commonly 0.1 – 0.2 V in silicon.

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