Darlington Circuit

• Darlington circuit is made up of 2 transistors and is used to amplify weak signals.

• The current gain in this case is more than individual current gain taken separately.
• The two transistors here behave like a single transistor with high current gain.
• Darlington circuit provides very high input impedance for overall circuit.

Disadvantages 

• Since there are two base-emitter junction, the equivalent base-emitter voltage doubles.

• Therefore the base-emitter voltage required to turn on a darlington circuit is 2*0.7=1.4V
• The saturation voltage of Darlington circuit is about 0.7 V. This is higher than that of a single transistor, which is typically 0.1-0.2 in silicon. Because of this drawback power dissipation increases in device.
• The switching speed of Darlington pair is slow as compared to single transistor. It is because the first transistor can't suddenly inhibit the base current of the second. To resolve this problem a resistor is connected between the base and emitter of the second transistor. This resistor acts as a low impedance discharge path for the charge accumulated on the base-emitter junction. 

(Darlington circuit with resistor)

• Darlington circuit has more phase shift at higher frequency as compared to single transistor and can more easily become unstable with negative feedback.