How to find input impedance for this op amp?
simulate this circuit &ndash; Schematic created using CircuitLab
By definition the input resistance is the resistance &quot;seen by the source&quot;. As far as the Op-amp is in the linear region we know that the inverting input voltage and non-inverting input voltage are equal to zero. So R1 is parallel to R4 and it can be easily seen that the resistance the source faces is (R4||R1)+R3 = 5.83 k ohm.
To calculate the gain it is easy to calculate the value of voltage in the (in1) and then simplify the circuit a bit, like you see in the picture