We can now use the AC equivalent circuit to calculate the AC voltage gain between the base and collector. The base voltage is developed across the input resistor and . The collector voltage can be similarity expressed as the voltage drop across the resistor : . Eliminating , we can write the amplifier voltage transfer function between the base and collector as
The minus sign indicates that the voltage signal at the collector is 180 out of phase with the signal at the base.
The input impedance to this amplifier circuit is just the parallel combination of and , and since is usually much smaller than , the input impedance generally reduces to just the input impedance of the transistor itself, namely, . The circuit output impedance is the collector resistance .
The high-frequency operation of the common emitter amplifier is limited by the parasitic capacitance between the collector and base. This capacitance provides a path by which the large and inverted signal at the collector drives a feedback current into the base. The base-to-collector voltage gain of this amplifier looks like a low-pass filter.