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Approximate AC Model

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.

Doug Gingrich
Tue Jul 13 16:55:15 EDT 1999