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The positive power
supply rail is joined to
the zero volts rail by
C3. As far as ac is
concerned, both rails
are joined together.
Therefore they, and the
collector, are common
to both input and
output.
Since the emitter
voltage follows the base
voltage, it is also
called the emitter
follower.
Current gain is
Ie/Ib
which is quite high,
typically 50.
Voltage gain is only 1
because of the
undecoupled emitter.
The input impedance is
high, typically 500k,
requiring only low power
to drive it.
The output impedance is
low, typically 20 ohms.
The output signal
follows the input. There
is no inversion.
It is often used to
match high impedances to
low ones.
It can be used to drive
several high impedance
loads.
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