The circuit shown in figure 5.8a is the basic common
emitter amplifier using the simplest biasing method.
Because it is constant, the power supply voltage 
is an AC
ground indistinguishable from the normal ground of the circuit.
We can therefore relocate the upper end of 
and 
to the
common ground line as shown in figure 5.8b.
The transistor symbol is ideal and 
is shown explicitly as the
input impedance and hence 
.
Figure 5.8: a) Basic CE amplifier and b) AC equivalent
circuit drawn using an ideal transistor symbol with shown
explicitly.