The purpose of ac equivalent circuit?

Guyz, i knew how to do the ac equivalent, Dc diagram and ac diagram. BUt i dont see the point doing these analysis ? wat is the importance in IC design?

AC equivalent circuit is used in ac analysis. For example, the MOS transistor is not linear, if we want to analyze it, we must use linear equivalent circuit . But the dc equivalent circuit and ac equivalent circuit of MOS transistor are not same. So we use them in different analysis.

The behavior of a transistor or a diode exhibits an extremely nonlinear characteristics. But, when an appropriate voltages are applied to the terminals(pins), the device will be at an electrical operating point. If the ac voltage small enough is applied additionally to a terminal, then, the ac responses of the other terminals are linear to the input ac voltage. That means ac parameters of a device can be obtained by some differentiation of the dc parameters of the device as far as the ac input voltage is small enough for the device to operate in a so called piece-wise linear region. It is apparent that, if the input ac voltage is large to go beyond the piece-wise linear region, the ac parameters will not predict correctly the respenses of the other terminals.