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.
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