Consider the dc equivalent circuit fig. 1.
Fig. 1
Assuming IC = IC(approx), the output circuit voltage equation can be written as
The slop of the d.c load line is
.
When considering the ac equivalent circuit, the output impedance becomes RC || RL which is less than (RC +RE).
In the absence of ac signal, this load line passes through Q point. Therefore ac load line is a line of slope (-1 / ( RC || RL) ) passing through Q point. Therefore, the output voltage fluctuations will now be corresponding to ac load line as shown in fig. 2. Under this condition, Q-point is not in the middle of load line, therefore Q-point is selected slightly upward, means slightly shifted to saturation side.
Fig. 2
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