[LT Spice] Input resistance of noninverting op amp

Discussion in 'General Electronics Chat' started by eliotsbowe, Jun 23, 2013.

  1. eliotsbowe

    Thread Starter New Member

    Sep 11, 2012
    2
    0
    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.
     
    Last edited: Jun 23, 2013
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,962
    1,097
    Any op amp also have some input capacitance. And this is why input resistance decreases when frequancy increase.
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,053
    3,244
    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.
     
  4. eliotsbowe

    Thread Starter New Member

    Sep 11, 2012
    2
    0
    I got it, thanks :)
     
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