Consider the dc equivalent circuit **fig. 1**.

**Fig. 1 **

Assuming I_{C} = I_{C}(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 R_{C} || R_{L} which is less than (R_{C} +R_{E}).

In the absence of ac signal, this load line passes through Q point. Therefore ac load line is a line of slope (-1 / ( R_{C} || R_{L}) ) 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 **