Editing Common collector (section)

== Applications ==
[[Image:Voltage follower.svg|thumb|302px|Figure 4: NPN voltage follower with [[current source]] biasing suitable for integrated circuits]]

The common collector amplifier's low output impedance allows a source with a large [[output impedance]] to drive a small [[load impedance]] without changing its voltage. Thus this circuit finds applications as a voltage [[buffer amplifier|buffer]]. In other words, the circuit has current gain (which depends largely on the ''h''<sub>FE</sub> of the transistor) instead of voltage gain. A small change to the input current results in much larger change in the output current supplied to the output load.

One aspect of buffer action is transformation of impedances. For example, the [[Thévenin's theorem|Thévenin resistance]] of a combination of a voltage follower driven by a voltage source with high Thévenin resistance is reduced to only the output resistance of the voltage follower (a small resistance). That resistance reduction makes the combination a more ideal voltage source. Conversely, a voltage follower inserted between a small load resistance and a driving stage presents a large load to the driving stage—an advantage in coupling a voltage signal to a small load.

This configuration is commonly used in the output stages of [[power amplifier classes#Class B|class-B]] and [[power amplifier classes#Class AB|class-AB]] amplifiers.  The base circuit is modified to operate the transistor in class-B or AB mode. In [[power amplifier classes#Class A|class-A]] mode, sometimes an active [[current source]] is used instead of ''R''<sub>E</sub> (Fig. 4) to improve linearity and/or efficiency.<ref name=Elliot>[https://sound-au.com/project10.htm Rod Elliot: ''20 Watt Class-A Power Amplifier'']</ref>