What is the input resistance of the following inverting op amp?
I have illustrated the &quot;magic&quot; virtual short connection between the op-amp inputs by the conceptual picture below. I have extracted it from
my answer to a recent similar question.

The op-amp output is represented by a variable voltage source producing a voltage VOA. It follows the voltage drop across the resistor R2; so VOA = VR2 (as magnitudes). Since it is connected in series to R2, it destroys the voltage drop across it (VR2 - VOA = VR2 - VR2 = 0)... and the combination of the two elements in series behaves just as a &quot;piece of wire&quot; (&quot;virtual short connection&quot; between the two op-amp inputs).
Now, if you connect the third resistor in parallel to this virtual short, it will be shunted... and nothing will change. The equivalent circuit input resistance will be determibed only by R1.