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R1 and R2 bias the
transistor in class A,
so that a steady
collector current flows.
R3 is the load resistor.
Look at the signal into
C1.
During the first half
cycle it is positive,
and increases the
forward bias on the base
of the transistor.
This increases the base
current and thereby the
collector current
through the load
resistor.
Therefore the voltage
across the load
increases, and the
collector voltage, with
respect to the zero
line, decreases.
The collector voltage
falls as the base
voltage rises.
During the second half
cycle, the input signal
goes negative, reducing
the forward bias on the
base.
This reduces both base
and collector currents.
The voltage across the
load resistor falls and
the voltage on the
collector rises.
Again, the collector
voltage has done the
opposite of the base
voltage.
The transistor inverts
the input signal as well
as amplifying it.
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