Q-POINT
The Q-point or operating point is established by the DC analysis which establishes the DC load line. The AC load line has a different slope then the DC load line and the two load lines intersect at the Q-point. In the common emitter amplifier, as you increase the frequency,the gain to go down and the phase shift to move away from -180 degrees. The Q-point remains unchanged.
Every amplifier has two loads: a dc Load and an ac load.
The dc load line represents all possible combinations of Ic and VCE
The ac load line represents all possible ac combinations of and ic & VCE.
The dc load line will not follow the path of the ac load line as shown to the left. This is because the ac signal “sees” the ac equivalent circuit that includes rC
Note that the Q point is shared by both lines.
The ac Load Line
The ac load line uses RC ||RL in its determination, and consequently, the ac load line will be than the dc load line.
The ends of the ac load line are found using the formulas:
ic(sat) =ICQ + VCEQ
rc
v ce(off) = VCEQ + ICQ
rC
Note that the Q point is in the centre of the dc loadline but not in the centre of the ac load line.
The ac load line tells us what the maximum output voltage swing will be for the given amplifier.