Previously we considered a simple circuit composed of a source of and and a load of . Now we generalize this idea to a more complicated circuit containing an amplification circuit shown on top of the figure below, where everything, including the amplifier as well as and , inside the red box is treated as the source, while everything, including the amplifier as well as , inside the blue box is treated as the load. Given the amplifier as well as the source and , and the load , we need to find , and also the voltage gain .
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:
As shown above, this AC small signal equivalent circuit can be modeled by as an active circuit containing three components:
Here , , (), and the capacitors are large enough so that they can be considered as short circuit for the AC signals.
The DC load line is determined by the following two points:
; and ,
The intersection of the DC load line and the curve corresponding to is the DC operating point , and .
The AC load is
. The AC load line is a
straight line passing the DC operating point with slope
. The intersections of the AC load line with
and axes can be found by
Assume AC input voltage is
, the overall base voltage is
The output current is
This can be obtained graphically from the output i-v characteristics, based on , to be
Note that the output is in opposite phase (180 phase shift) with the input.
The circuit above can also be analyzed using the small-signal model.
Same as before,
, and we have the following DC variables: