How to determine the Input and Output Differential Impedance for an OPAMP ?

To determine the Input Impedance you can do the following:

         a. Place a 1A Current Source (IAC) at the Input and place a Differential Voltage Probe across the Input Terminals.

         b. Do a Frequency Sweep across the desired Frequencies for the AC current source to get the Differential (Input) Impedance as Vdiff/IAC in PSpice Probe window after the simulation run.
              As the current sources are ideal and have an infinite output impedance, you can also use a high-value resistor in parallel to avoid simulation errors.

DC and AC impedance: DC is measured at low frequencies, and AC is measured on the same curve at high frequencies. Although in reality, there is a nonlinear I-V relationship and no distinct resistance values for the OPAMP PSpice model. There are essentially three regions, a low-frequency resistance, a transition region, and a high-frequency resistance.

Similarly, the Output impedance is measured by sweeping the desired Frequencies for an AC current source at the output and then plotting Vdiff/IAC.

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