The Long-Tailed Pair (LTP) is a circuit that consists of two bipolar transistors whose emitters are connected together, as shown in Figure 1.  Furthermore, the input of the circuit is applied across the bases of the two transistors, while the output is usually taken from across their collectors.  The long-tailed pair circuit is a very widely used circuit in linear applications, mainly as a difference amplifier.  Field-Effect Transistors (FET's) and vacuum tubes may also be used in implementing long-tailed pairs, wherein the FET sources and vacuum tube cathodes are the terminals connected together, respectively.

The term 'long-tail' came from the fact that the large resistor connecting the emitters of the transistors to ground or a power supply resembles a long tail. In Figure 1, wherein NPN transistors are used to form the pair, this resistor is tied to ground.  The long tail actually functions as a current source, and may in fact be replaced a more elaborate 'active' current source circuit. 

Figure 1.  The Long-Tailed Pair (LTP) Circuit

The long-tailed pair circuit is often used for amplifying any difference between the signals applied at the base of each transistor.   If the two transistors used are identical and balanced, a common-mode signal (i.e., a signal applied in the same phase to both inputs) applied to the bases of the transistors will not cause any significant differences between the voltages at the collectors of the two transistors.  In fact, any difference would only be due to a lack of balance between the transistors.  Thus, the output of this circuit (which is taken across the collectors) for a common-mode signal would ideally be zero.

On the other hand, even a minute difference between the base signals will be amplified considerably by the transistors, and will be reflected at the output as an amplified version of the difference between the signals.  Thus, the differential gain of this circuit is very high, whereas its common-mode gain is very low.  This is why this circuit is extensively used in the input circuitries of operational amplifiers.

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