The following are properties of an ideal amplifier, which to a good approximation are obeyed by an operational amplifier:
If these approximations are followed two rules can be used to analyze op-amp circuits:
To apply these rules requires negative feedback.
Feedback is used to control and stabilize the amplifier gain. The open-loop gain is too large to be useful since noise will causes the circuit to clip. Stabilization is obtained by feeding the output back into the input (closed negative feedback loop). In this way the closed-loop gain does not depend on the amplifier characteristics.