Experiment 4 - Gain of the Darlington Pair

A Darlington circuit has a much greater gain than a single transistor. But how big is the gain?

Build the circuit shown. With two multimeters you measure the current into the base of transistor T1 and the collector current of the Darlington pair.

Measuring the gain of a Darlington pair. (Enlarge)

Circuit diagram measuring the gain of a Darlington pair.

You'll have difficulty to measure a current into the base of transistor T1 at all. Turn the potentiometer P1 to a point where the Darlington pair is not yet saturated and you might measure the following currents...

I_B = 0.1 μA I_C = 4.0 mA

Hence the gain β_D of the Darlington pair calculates...

β_D = I_C ÷ I_B = 4.0 mA ÷ 0.0001 mA = 40,000

β of the Darlington Pair

The gain β_D of the Darlington pair is approximately the product of the gains of the individual transistors...

β_D = β_T1 × β_T2

(You derive this formula in Experiment 16...)

Experiment 5 - Darlington Pair IV