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I tried measuring the input impedance of Opamp LT1128 Buffer using LTSpice. And from the simulation then maximum impedance is showing only 20k.

This particular opamp has 300MEG common mode input resistance, 20K differential mode input resistance and 5pF input capacitance. Coupled with the fact that the opamp is configured in a unity gain follower configuration, shouldn't the input impedance of this opamp measure in Mega Ohms?

Opamp input impedance measurement in LTSpice

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Looks like you don't have the power rails connected for this amplifier. That is, they are both connected to ground.

Here's the same exact configuration you have, except the opamp is connect to +/-15V, fairly standard. I'm getting roughly 600 Megs, which sounds about right for an opamp. SPICE Simulation of Opamp

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  • \$\begingroup\$ Wow. Thats looks right. But don't we need to short circuit the voltage sources to ground, before measuring impedance? \$\endgroup\$ – Sandhan Sarma Jul 6 '20 at 8:58
  • \$\begingroup\$ @SandhanSarma The opamp would be made of transistors. Would they work if they are not biased correctly? The model tries to get as close as possible to the real life. Why would the model function differently than the opamp? \$\endgroup\$ – a concerned citizen Jul 6 '20 at 9:31
  • \$\begingroup\$ @Sandhan Sarma In addition to the above response, DC voltage sources look like short circuits at AC. You need to the DC sources to bias the transistors of the opamp into some operation region. Then you superimpose an AC signal (that is, the input signal to the opamp) which is amplified by the transistors in the opamp. Using the property of linear superposition, we can consider each voltage source separately (there are some assumptions we make when doing this, but it is a good assumption for this circuit). But again, we need the DC sources there in order to bias the transistors correctly. \$\endgroup\$ – LetterSized Jul 6 '20 at 18:56
  • \$\begingroup\$ I understood now. I tried the same circuit with DC power for the op-amp, and I did get the Input impedance plot. \$\endgroup\$ – Sandhan Sarma Jul 27 '20 at 14:31

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