Note that as you make the open-loop gain larger and larger, the differential input voltage Vdiff and the differential input current Id get smaller and smaller. Also, the actual closed-look gain becomes closer and closer to the nominal gain given by the ratio of R2 to R1. Real operational amplifiers have open loop gains that are so high (50,000 or more) that is a good approximation to assume that it is infinite, and therefore that the differential input voltage Vdiff and the differential input current Id are both zero. This is not exactly true, of course, but the accuracty of this approximation is better that the accuracy of the resistors (even of precision resistors).
In this simulation you can change the values of R1, R2 and the differential input resistance (typically 1 to 100 Megohm).
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