Behaviour

It behaves like a single transistor with a high current gain (approximately product of the gains of the 2 transistors). Actually integrated devices have 3 leads (B, C and E), broadly equivalent to those of the standard transistor.

A general relation in between the compound current gain and individual gains can be given by:

If β1 and β2 are high (hundreds), this relation can be approximated as follows:

A modern device has a current gain of 1000 or more, such that only the small base current is required to make the pair switch on. But, this high current gain comes with a number of drawbacks.

One limitation of it is an approximate doubling of base emitter voltage. As there are 2 junctions in between the base and emitter of Darlington transistor, equivalent base emitter voltage is sum of both base emitter voltages:

For the silicon based technology, where each VBEi  is around 0.65 V when device is operating in the active or saturated region, the required base-emitter voltage of the pair is 1.3 V.

The other drawback of the Darlington pair is its increased saturation voltage. The output transistor is not allowed to saturate (that is its base-collector junction should remain reverse-biased) because its collector-emitter voltage is now equal to the sum of its own base emitter voltage and the collector-emitter voltage of first transistor, both positive quantities in the normal operation. (In symbols, V_CE2  = V_BE2  + V_CE1, so V_C2  > V_B2  always.) Thus the saturation voltage of the Darlington  transistor  is  one  V_BE   (around  0.65  V  in  silicon)  higher  than  a  single  transistor saturation voltage, which is 0.1 - 0.2 V in silicon. For the equal collector currents, this drawback translates to an increase in dissipated power for Darlington transistor over a single transistor.

The other problem is a reduction in the switching of speed, as first transistor cannot actively inhibit base current of second one, making the device slow to switch off. To alleviate this, second transistor has a resistor of a few hundred ohms connected in between its base and emitter terminals. This resistor gives a low impedance discharge path for charge accumulated on base-emitter junction, allowing the faster transistor turn-off.

The Darlington pair has large phase shift at high frequencies than the single transistor and hence can become unstable more easily with  negative feedback (that is systems which use this configuration can have poor phase margin because of the extra transistor delay).

The Darlington  pairs  are  available  as  integrated  packages  or  can  be  made  from  2  discrete transistors; Q₁ (left-hand transistor in diagram) can be a low power type, but normally Q₂ (on right) will required to be of high power. The maximum collector current IC(max) of pair is that of Q₂. An integrated power device is 2N6282, which includes a switch-off resistor and has current gain of 2400 at I_C=10A.

A Darlington pair can be sensitive to respond to the current passed by skin contact even at the safe voltages. Hence, it can form the input stage of the touch sensitive switch. 

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