Aug 282014
 

Darlington transistor

The Darlington transistor is a special type of transistor which has a high current gain. It consists of two internally bipolar transistors being connected in cascade.

Bipolar Transistor

The T1 transistor delivers on its emitter terminal, a current that goes to the base of the T2 transistor. The current gain equation of a typical transistor is:  IE = ßxIB (collector current is equal to beta times the base current).

The picture shows a common transistor with its pin configuration. Analysing the picture:

  • On the first transistor: IE1 = ß1 x IB1 (1),
  • On the second transistor is: IE2 = ß2 x IB2 (2)

Darlington transistor internal configuration

From the picture, we can see that the emitter current of the T1 transistor is the same as the base current of the T2 transistor. Then IE1 = IB2 (3).

The picture shows a Darlington transistor with its pin configuration and its internal structure. Using the equation (2) and the equation (3) we obtain: IE2 = ß2 x IB2 = ß2 x IE1. Replacing the value of IE1 on the last equation (see equation (1)) we obtain the the Darlington transistor gain equation: IE2 = ß2 x ß1 x IB1.

We can see, that this current amplifier gain is much greater than the one of a single transistor, since it takes advantage of the current gain of the two transistors. (The gains are multiplied).

If we have two transistors with gain of 100 each (ß = 100), connected as a darlington transistor using the above formula, the final theorical gain would be: ß2 x ß1 = 100 x 100 = 10000. This seems to be a very big gain (the ideal one) but it is actually a lower gain. Darlington transistors are widely used in circuits where it is necessary to control large loads with very small currents.

Very important: When we add the base-emiter voltage of the first transistor B1 to E1 (0.7 volts) plus the base-emitter voltage of the second transistor B2 to E2 (0.7 volts), we get as a result a voltage drop of 1.4 volts between the base and the emitter of the Darlington transistor. VB1E1 + VB2E2 = 0.7 + 0.7 = 1.4 volts.


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