Three lesser known BJT configurations are the differential amplifier, current mirror and the darlington amplifier. These circuits all require two transistors, which should be the same type and have closely matched parameters for best performance,. For clarity, all circuits are drawn without bias circuits. Note that the same circuits can also be made with FET's and MOSFET's. If PNP transistors are used, power supply polarity should be reversed.
Sometimes also known as a programmable current mirror this unusual configuration uses two closely matched transistors with their bases in parallel. The base current is supplied via R1 and both transistors will have identical base currents. Because the transistors are matched, and have the same forward current gain, then collector current I2 will be the same as I1.
The darlington amplifier is a useful circuit and has the advantage of providing a very high current gain, high input impedance and higher output power. (It is not necessary to use matched transistors here) and often you see a smaller signal transistor driving a larger power transistor. The current gain is approximately the product of both Q1 and Q2 forward current gains. One point to note is that as Q1 emitter is connected to Q2 base the bias voltage required is Vbe1 + Vbe2. This must be taken into account when designing bias circuits for the darlington amplifier.
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