Experimentally determining the input impedence of an op amp

Discussion in 'Homework Help' started by abhaymv, Oct 31, 2012.

  1. abhaymv

    Thread Starter Active Member

    Aug 6, 2011
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    4
    Hello everyone,

    What should I do to determine the input impedence of an opamp? I want to find the input impedence of a noninverting amplifier and it is the same as that of the opamp. I know to measure various parameters like slew rate and offset voltage, but I can't find anything on Input impedence. I know it's in the datasheet, about a few megaohms, but what circuit can I use to determine it?

    Thank you in advance:D
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,175
    1,798
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  3. abhaymv

    Thread Starter Active Member

    Aug 6, 2011
    104
    4
    Thank you for the reply.

    The impedence w.r.t GND I think. I found a circuit that I think works based on voltage divider after one hour of searching:
    http://my.safaribooksonline.com/boo...asics/measurement_of_operational_amplifier_pa

    The circuit is at the end of the page. However, the statement
    "
    With the switch open, Rs = 0, some amount of input is given to obtain a convenient value of V0. Rs is adjusted till V0 falls to [​IMG]. Then Rs = Rin."
    confused me. How is Rs= 0 when switch is open? Is it Rf=0?
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Don't you think the resistors that bias the opamp input affect the input impedance?

    If you want a very high input impedance then use an opamp with Jfet inputs like a TL081 or TL071. The bias resistors can be bootstrapped or be a very high value.
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,368
    Get a signal generator and set it to a desired frequency (say 1kHz) and amplitude (say 100mV).

    Connect a resistor in series between the signal generator and the input of the amplifier.
    Measure the amplitude at the input of the amplifier.
    Adjust the value of the resistor until you get half the amplitude (50mV) at the input.
    That resistance value would be equal to the input impedance of the amplifier.
     
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  6. abhaymv

    Thread Starter Active Member

    Aug 6, 2011
    104
    4
    Thank you. That was really helpful.
     
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