Based on the previous discussion, a single transistor AC amplification circuit is given as shown in the figure.
If the capacitances of the coupling capacitors and the emitter by-pass
capacitor are large enough with respect to the frequency of the AC signal
in the circuit is high enough, these capacitors can all be approximated as
short circuit. Moreover, note that the AC voltage of the voltage supply
is zero, it can be treated the same as the ground. Now the AC
behavior of the transistor amplification circuit can be modeled by the
following small signal equivalent circuit:
AC Input Impedance: For AC signals, the input of the amplification
circuit is shown below, where
is the internal resistance of the signal
source, and the input impedance of the circuit is the three resistances
,
and
in parallel:
AC Output Impedance: This is simply the resistance of the resistor
AC Amplification Gain: Given the AC input voltage
, the base
voltage can be found (voltage divider) to be
Example 1:
This figure shows a common-emitter amplification circuit of npn transistor.
Assume
,
,
,
(assuming
). Also we assume the capacitors are large enough so
that they can be considered as short circuit for the AC signals.
The circuit above can also be analyzed using the small-signal model.
Same as before,
, and we have the following DC variables: