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Darlington Pair Transistor

 

How the circuit works…

The Darlington Pair is usually a single device, made up from two transistors as shown.

When an Input voltage greater than 1.2 volts (2 x 0.6 volts) is applied to the base of the Darlington pair, the transistors "switch" on.

Output current can now enter the collectors, flow through the transistors and leave through the emitters.

The output device (eg buzzer or lamp) is connected in the emitter/collector circuit where the output voltage arrow is shown.

The 1K resistor prevents too much current entering the base, when the input voltage is applied.

 

Circuit Diagram

In this circuit, two transistors have been drawn as the BCX38B Darlington Pair.

 

 

 

 




 

The overall amplification (Gain) of the Darlington Pair circuit is :-

Gain = hFE for BC548B x hFE for BC637

= 180 x 150 = 27000 times (typically)


 

Other useful Information …..

The BC637 transistor can power output devices with current up to 1 amp (Ic = 1 amp). This output will be sufficient to drive small motors and relays and outputs where devices are connected in parallel.

The BCX38B is a Darlington Pair in a single package. It can take the place of the two transistors shown in the dotted box in the circuit diagram above. It provides about 0.8 amps output current (Ic) and costs approximately 29p making it a good substitution. Please note the leg connections below.