BJT Configurations

There are plenty of texts written about transistor theory, so this page describes a brief look at three popular bipolar junction transistor (BJT) configurations. In each case, one terminal is common to both the input and output signal. All the circuits are shown here are without bias circuits and power supplies for clarity.

Common Emitter Configuration



In the common emitter configuration, the emitter terminal is common to both the input and output signal. The arrangement is the same for a PNP transistor, except that the power supplies (not shown) will have the opposite polarity. Used in this way the transistor has the advantages of a medium input impedance, medium output impedance, high voltage gain and high current gain.

Common Base Configuration



When the base is used as the common terminal, the transistor will have a low input impedance, high output impedance, unity (or less) current gain and high voltage gain. This configuration also realises the best high frequency performance, and finds dominant use in RF amplifiers and high frequency circuits.

Common Collector Configuration



This last configuration is also commonly known as the emitter follower. This is because the input signal is applied to the base and passes out at the emitter with little loss. Stage properties are high input impedance, a very low output impedance, a unity (slightly less) voltage gain and high current gain. The circuit is also used extensively as a "buffer" converting impedances or for feeding or driving long cables or low impedance loads.

A note about Phase Shifts
In both the the common base and emitter follower configurations, the input and output signals are in phase, but with the common emitter configuration only, the input and output signals are phase inverted, a positive input resulting in a negative output and vice versa. This is also known as phase displacement.