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.
The differential amplifier uses two matched transistors that share a common emitter connection. The emitter connection is often connected via a single resistor but for best performance should be fed via a constant current source as shown in the diagram. This circuit is also known as a "long tailed pair". The circuit employs split supplies and input can be between either transistor base and earth or applied simultaneously to both bases. Used in this manner the input signal will be greatly amplified and appear at the collectors. If a balanced input signal is applied, from say long cables, then any interference will be common on each wire. A signal with same polarity applied to both differential inputs will not be amplified and this property is known as "common mode rejection". The output can be taken from both collectors (balanced) or between either collector and earth (unbalanced or single ended). Examples include DC amplifiers, instrumentation amplifiers and audio circuits, example circuit.
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.Sziklai Pair
The Sziklai pair (also known as a "compound transistor") is a configuration of two bipolar transistors, similar to a Darlington pair. In contrast to the Darlington arrangement, the Sziklai pair has one NPN and one PNP transistor, and so it is sometimes called the "complementary Darlington". Current gain is similar to that of a Darlington pair, which is the product of the gains of the two transistors. The configuration is named for its inventor George C. Sziklai.
One advantage over the Darlington pair is that the base turn-on voltage is only about 0.6V or half of the Darlington's 1.2V nominal turn-on voltage. Like the Darlington, it can saturate only to 0.6V, which is a drawback for high-power stages.
A low value resistor (e.g. 100Ω − 1k) is usually connected between Q2 emitter and base to improve its turn-off time and better high frequency performance.