Common Collector
A common-collector amplifier is the bipolar-transistor stage in which the collector terminal is held at AC ground — usually tied directly to the supply, which is a signal ground because the supply is decoupled — so that the collector is common to the input and output signals. The signal goes into the base, and the output is taken from the emitter. It provides a voltage gain just under 1, a large current gain, a high input impedance and a low output impedance, which makes it the standard way to take a signal from a source that cannot supply much current and hand it to a load that demands it.
The entry is named for what the circuit connects, whereas its more famous nickname describes what it does. "Common collector" is a classification by terminal: of the three configurations (common emitter, common base, common collector), this is the one whose common terminal is the collector. "Emitter follower" is a description of behaviour — the emitter voltage follows the base voltage, offset by the base-emitter drop. The two names refer to the same circuit, and the detailed practical design of the buffer is described under the emitter-follower entry; this entry concentrates on the place of the stage in the family of configurations and on its small-signal numbers.
The voltage gain is just below 1 because the emitter resistor and the internal emitter resistance form a voltage divider. If the emitter resistor is RE and the transistor's own small-signal emitter resistance is re (about 26 millivolts divided by the emitter current), the gain is RE / (RE + re). At 1 milliampere re is about 26 ohms, so a 1 kΩ emitter resistor gives a gain of about 0.975. The output follows the input with no inversion, and a DC offset of about 0.6 volt for silicon sits between them. The reason the gain cannot exceed 1 is a form of 100 percent negative feedback: any rise at the emitter reduces the base-emitter voltage, reducing the current that caused the rise, so the output is pinned to the input. The same feedback explains the circuit's very low distortion compared with the common-emitter stage.
The impedance behaviour is the useful part. Looking into the base, the stage presents roughly β × (re + RE), or more precisely (β + 1) × (re + RE ∥ RL), so a 1 kΩ load and a beta of 100 can look like roughly 100 kΩ to the source — a hundred times larger than the resistance at the emitter. Looking back into the emitter, the output impedance is approximately re plus the source resistance divided by (β + 1); a 10 kΩ source driving a transistor with beta of 100 sees an output resistance of about 100 ohms plus a few tens of ohms. The current gain is about β + 1, so the stage gives genuine power gain even though voltage gain is below 1. It is the reason that a weak sensor or a high-impedance stage can drive a cable, a headphone or the next stage without losing its signal to loading.
Because the collector is grounded for signals, common-collector stages also have wide bandwidth and good thermal behaviour, but two cautions apply. The high input impedance depends on the base seeing only the transistor, and any bias divider in parallel with the base lowers it; designers use bootstrapping or large bias resistors to preserve it. A capacitive load on the emitter can make the follower oscillate at high frequency, because the stage's inductive-looking output impedance resonates with the load, and a small base stopper resistor is the usual cure. Another pitfall is believing that a stage with gain of 0.97 is "doing nothing": the current and power gain are exactly why it exists.
Modern relevance is wide, though often invisible. The common-collector connection, with its MOSFET twin the common-drain (source follower), forms the output buffers of op-amps and audio amplifiers, the output stages of ECL logic gates, level shifters, regulators (the series pass transistor in a linear regulator is a common-collector connection) and the outputs of video and line drivers. It is also what makes a Darlington pair a high-gain buffer.
Read the common collector as the configuration defined by its grounded terminal rather than by its job: a unity-voltage-gain stage whose significance is that it turns a high impedance into a low one, giving a source the current muscle it did not have.