Hello, I'm simulating the following circuit with LT Spice:

http://www.geocities.ws/debussy/noninvertingopamp.jpg

I'm doing an .ac analysis to calculate the input resistance of the op amp, which should be very high over the whole bandwidth (which is approximately 0-100 kHz), but I get the following Vin/Iin plot, which makes no sense:

http://www.geocities.ws/debussy/noninvertingopamp_plot.jpg

I see, by using a cursor, that the input resistance decreases from 157 GigaOhm (at 0 Hz) to 1.5 MegaOhm (at 100Khz): how is this possible? Is it possible for the input current of the op amp to so rapidly grow with frequency?

I'm puzzled because I did the same simulation on the inverting configuration and the input resistance was 1 kiloOhm over the whole bandwidth as expected.

Thanks in advance for your help.

 

Any op amp also have some input capacitance. And this is why input resistance decreases when frequancy increase.

 

Actually the resistance is likely essentially constant with frequency. It's the input impedance, consisting of the resistance in parallel with the parasitic capacitance that decreases with frequency.

 

I got it, thanks