Darlington transistors or Darlington pairs are actually pairs of transistors in a single package that looks like a regular transistor. The pair is connected in such a way that the current amplified by the first transistor is amplified further by the second one.

NPN Darlington

NPN Darlington

The arrangement leads to a high current gain (hfe or β, thousands or more) so Darlington transistors can be used to increase the current you switch by applying a very small current to the base of the transistor.

Darlington transistors behave in a similar way as regular (bipolar junction) transistors but with a high current gain. Drawback is an approximate doubling of the base–emitter voltage compared to a normal transistor. This relates to the two junctions between the base and emitter of the Darlington transistor so the equivalent base–emitter voltage is the sum of both base–emitter voltages of the individual transistors. Also the collector – emitter voltage in saturation of a Darlington transistor is higher compared to a regular transistor (1 V or more compared to 0.1 to 0.2 V for a regular transistor). This leads to higher power dissipation in the transistor.

Most important specifications

  • hfe or β: the DC current gain (several (ten)thousands)
  • Vbe,saturation: the base – emitter saturation Voltage (around 1.4 V depending on configuration)
  • Ic,max: the maximum collector current that can be switched by the transistor (A)
  • Vce,max:the maximum collector – emitter Voltage that the transistor can handle when switched off (Ib = 0A).