What is the difference between open-loop gain (A0) and closed-loop gain (AV)?
If you take an operational amplifier (op-amp) and connect an input signal with no negative feedback, the output voltage will be A0 times larger than the difference between the input voltages. The control loop that might otherwise set the gain is not present (the feedback loop is open-circuit), hence the term open-loop gain. If you now add a negative feedback network, the voltage gain of the circuit will usually be reduced to a value dictated by the external components. These external components close the feedback loop, hence the term closed-loop gain, or AV for short. Note that the op-amp output voltage is still A0 times bigger than the difference between its inputs, but the negative feedback forces this difference to be a vanishingly small value, thereby ensuring a finite output voltage.
