opamp open loop gain

[shinulalu6]

Why we are using RC network in the negative feedback to measure the op-amp open loop gain?

 

[LvW]

Without negative feedback the finite offset voltage - together with the very large open-loop gain of the opamp - will drive the output into saturation, thus disabling any small-signal gain measurement.
Fot this reason we provide dc feedback (which drastically reduces the dc gain) but in addition we use a capacitor for shorting the feedback signal for rising frequencies.
As a consequence, we cannot directly measure or simulate the gain at DC - however, if the capacitor is large enough the signal feedback (ac) is not effective for frequencies below the open-loop cut-off frequency.
Hence, we can extrapolate the gain down to dc.