Darlington Transistor

Darlington transistors are special types of transistors which has high current gain and sensitivity which is owned to the fact that it contains two bipolar junction transistors connected together to act as a single piece of semiconductor device. It was named after its inventor "Sidney darlington". 

A darlington transistor is made by joining two PNP transistors to form a PNP darlington transistor or joining two NPN transistors to produce an NPN darlington transistor. The emitter of one BJT is connected to the base of the other one whose  emitter is connected to a negative power source and the  collector of the two transistor connected to the positive power source as shown below. The device is fabricated from a solid piece of silicon by process of lithography forming a single piece of transistor with an emitter, base and collector. 

Circuit symbol of a Darlington transistor.

Working principle of a Darlington transistor.

Darlington transistors functions as an amplifier and a very sensitive solid state switching device because the first transistor A amplifies the input signal while the second transistor B furtherly amplifies the signal amplified by the transistor A thus giving a double signal amplification.

The image shown above is that of an NPN darlington transistor. Positive voltage is applied to the base of the first transistor along with a resistor in series, and negative voltage is applied to the emitter of the second transistor while current flows out of the collector. The base current switches the flow of amplified current from the emitter to the collector.

Applications of darlington transistors.

This type of transistors can only function as either a switch or an amplifier.

•Rain sensor.

A Darlington can be used as a rain sensor because of it's amplifying power.

•As an audio amplifiers.

It can also be use to amplify low audio signals because of it high amplifying power. An example of an amplifier is shown above.