EmitterCoupled

EmitterCoupled refers to a transistor configuration in which two transistors share a single emitter connection that is biased by a current source. This arrangement forms an emitter-coupled pair, or differential pair, used as the input stage of many high-speed amplifiers and logic families. The emitters are tied together and fed by a tail current; each transistor's base receives one input signal, and the current through each transistor varies inversely with the other. The common-emitter connection provides negative feedback and high common-mode rejection.

In operation, the pair acts as a differential amplifier: the difference between the input voltages steers the tail current between the two collectors, generating two output currents that can be converted to voltages via the collector loads. Small-signal behavior is characterized by transconductance gm ≈ Ic/VT and by a high differential gain. The impedance at the common emitter node is low, while the differential input impedance is determined by the transistor base resistances and biasing.

Applications include differential amplifiers in analog front ends, and emitter-coupled logic (ECL), a high-speed digital logic family that relies on the fast switching of the differential pair and uses current-steering loads. Advantages of emitter-coupled designs include high speed and good linearity over small signals; downsides include relatively higher power consumption and the need for precise current biasing and temperature tracking.