Darlington pair realized with two NPN BJT transistors is given in figure bellow.
Simulate Darlington pair.
Darlington pair, also called Darlington transistor is actually two stage current amplifier with very high current gain. Input current is firstly amplified with input transistor Q1. As emitter current of Q1 is also base current for Q2 transistor (see fig.), output current is additionally amplified by Q2 transistor. If current gains are Î²1 and Î²2 for transistors Q1 and Q2 respectively, input current is amplified 1+ Î²1 times in Q1 (as the emitter current of Q1), and then emitter current IE1 is amplified Î²2 times in Q2 collector branch. So if Î²1>>1, we can say that overall gain of Darlington transistor is Î²1Â·Î²2. To have such high current gain, collectors are connected, emitter of the input transistor is connected to the base of the output transistor, while base of the input transistor and emitter of the output transistor are open. Usually, input transistor Q1 is smaller in power and size, but with higher current gain Î²1>Î²2. Since emitter current of Q1 is not negligible, output transistor Q2 is more powerful with smaller current gain. Darlington transistor is applied in situations when input current is very small, such as current from photo diode, or when some small current have to be amplified enough to drive relay. Since there are two transistor in the scheme, in order to run Darlington transistor, input voltage on the base has to be at least 2Â·0.6=1.2V. Darlington pair can’t operate on high frequencies, since output transistor can’t be turned off instantly. Darlington transistor, can be speed-up a bit by adding resistor between base and emitter of the output transistor Q2. Resistor also stabilizes operating point in sense of temperature changes.
Go to Darlington pair simulation.