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Resistance input of amplifier

  1. Feb 21, 2013 #1
    I have an un ideal OA aplifier(Amax=-9.975).
    I have to evaluate its Rinput--> it has to be something like 10*10^10 Ω

    I need to put a potenciometr after Vin and to "play" with it when I get half of Vout/vin.
    when do I need to put it?
    and what is my voltage equations? thanks.

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  3. Feb 21, 2013 #2


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    Staff: Mentor

    Your text does not match your figure, as far as I can see.

    What do you mean when you say "Rin has to be something like 10*10^10 Ω"? In the figure, Rin1 is given as 400 kΩ.
  4. Feb 22, 2013 #3
    the resistance named Rin is 400K.
    But: the input resistance of the circuit is not 400. thats what I have to find
  5. Feb 22, 2013 #4


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    Staff: Mentor

    So you'll redraw the circuit, this time replacing the triangle with a 400k resistor to ground at the input, and show a voltage source with a gain of 20000 and so on. Surround it with the external circuit resistances, then analyze that.

    I don't know anything about putting a potentiometer somewhere and playing with it. Maybe that's something you do in the lab?
  6. Feb 22, 2013 #5

    rude man

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    Homework Helper
    Gold Member

    Given the actual gain of the circuit = 9.975 vs. the ideal op amp gain of 10.000 you can compute the open-loop gain of the amplifier.

    You then compute the input current for a given inut voltage to get the circuit's input impedance.

    You can then put a potentiometer in series with the 1K input resistor and determine the value needed to halve the output.

    That is, if I understood what you are trying to say, which is dubious.
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