Common collector configuration, also known as an emitter follower. This diagram shows an NPN transistor circuit.
A common collector circuit (above figure) operates with the collector at signal ground. The input is applied at the base, just as it is with the common emitter circuit. The signal passes through C2 onto the base of the transistor. Resistors R2 and R3 provide the correct bias for the base. Resistor R4 limits the current through the transistor. Capacitor C3 keeps the collector at signal ground. A fluctuating direct current flows through R1, and a fluctuating dc voltage therefore appears across it.
The ac part of this voltage passes through C1 to the output. Because the output follows the emitter current, this circuit is sometimes called an emitter follower circuit.
The output wave of a common collector circuit is in phase with the input wave. This circuit is unique because its input impedance is high, while its output impedance is low. For this reason, the common collector circuit can be used to match high impedances to low impedances. When well designed, an emitter follower works over a wide range of frequencies, and is a low-cost alternative to a broadband impedance-matching transformer.