Behavior
A Darlington pair behaves like a single transistor with a high current gain (approximately the product of the gains of the two transistors). In fact, integrated devices have three leads (B, C and E), broadly equivalent to those of a standard transistor.
Darlington pair of transistor is also called as super-beta transistor. Since it has the capacity to amplify the current output many a times a normal transistor configuration.
A general relation between the compound current gain and the individual gains is given by:
If β1 and β2 are high enough (hundreds), this relation can be approximated with:
Darlington pairs are available as integrated packages or can be made from two discrete transistors; Q1 (the left-hand transistor in the diagram) can be a low power type, but normally Q2 (on the right) will need to be high power. The maximum collector current IC(max) of the pair is that of Q2. A typical integrated power device is the 2N6282, which includes a switch-off resistor and has a current gain of 2400 at IC=10A.
A Darlington pair can be sensitive enough to respond to the current passed by skin contact even at safe voltages. Thus it can form the input stage of a touch-sensitive switch.
A typical modern device has a current gain of 1000 or more, so that only a small base current is needed to make the pair switch on. However, this high current gain comes with several drawbacks.
The basic Darlington transistor circuit is formed by taking the emitter of the input transistor and connecting it such that its emitter drives the base of the second and then connecting both collectors together
Read more about this topic: Darlington Transistor
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