Q.What is meant by closed loop gain?

An amplifier's voltage gain is quoted differently depending on whether the feedback path is present, and distinguishing the two is a standard PUC board exam point.

With the feedback loop open (no feedback), the whole signal reaches the amplifier and the gain is the open-loop gain

where $V_i$ is the voltage appearing directly at the amplifier input and $V_o$ is the output. In this non-feedback system a change in the output does not affect the net input. The open-loop gain is high in value but unstable — it drifts with device parameters, supply voltage, temperature and ageing.

When the feedback network is connected (loop closed), it samples the output and returns a fraction to the input, so the effective input becomes the source voltage $V_s$. The gain measured from source to output is the closed-loop gain …