Article : Andy Collinson
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Transistor Configurations
The bipolar junction transistor (BJT) has three terminals, so can be used in three different configurations with one terminal common to both input and output signal. The circuits shown here are drawn without biasing and power supplies for clarity.

The common emitter configuration has the emitter terminal 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.

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 realizes the best high frequency performance, and finds dominant use in RF amplifiers and high frequency circuits.

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 impedance's 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 both in phase. In 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.

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