Editing Equivalent circuit (section)

===DC and AC equivalent circuits===
In [[linear circuit]]s, due to the [[superposition principle]], the output of a circuit is equal to the sum of the output due to its DC sources alone, and the output from its AC sources alone.  Therefore, the DC and AC response of a circuit is often analyzed independently, using separate DC and AC equivalent circuits which have the same response as the original circuit to DC and AC currents respectively.  The composite response is calculated by adding the DC and AC responses:
*A DC equivalent of a circuit can be constructed by replacing all capacitances with open circuits, inductances with short circuits, and reducing AC sources to zero (replacing AC voltage sources by short circuits and AC current sources by open circuits.)
*An AC equivalent circuit can be constructed by reducing all DC sources to zero (replacing DC voltage sources with short circuits and DC current sources with open circuits)
This technique is often extended to [[Small-signal model|small-signal]] nonlinear circuits like tube and transistor circuits, by linearizing the circuit about the DC bias point [[Q-point]], using an AC equivalent circuit made by calculating the equivalent ''small signal'' AC resistance of the nonlinear components at the bias point.