The AC load line is a straight line with a slope equal to the AC impedance facing the nonlinear device, which is in general different from the DC resistance. The ratio of AC voltage to current in the device is defined by this line. Because the impedance of the reactive components will vary with frequency, the slope of the AC load line depends on the frequency of the applied signal. So, there are many AC load lines, that vary from the DC load line (at low frequency) to a limiting AC load line, all having a common intersection at the DC operating point. This limiting load line generally referred to as the AC load line, is the load line of the circuit at "infinite frequency", and can be found by replacing capacitors with short circuits, and inductors with open circuits. The AC load line is used to tell you the maximum possible output voltage swing for a given common-emitter amplifier. In other words, the AC load line will tell you the maximum possible peak-to-peak output voltage (Vpp ) from a given amplifier.
