Input Resistance - LM741 Circuit

Discussion in 'Homework Help' started by Zaraphrax, Sep 4, 2009.

  1. Zaraphrax

    Thread Starter Active Member

    Mar 21, 2009
    47
    3
    G'day,

    I'm trying to calculate the input resistance of an op-amp circuit. I already know that an op-amp by itself has an input resistance of around 2 mega-ohms.

    I have a 50 kilo-ohm resistor, hooked up to a voltage follower (LM741 with the output fed back to the negative input - giving no voltage gain), with the output from this hooked up to an inverting amplifier that looks like this:

    [​IMG]

    Where R1 = 22 kilo-ohms and R2 = 220 kilo-ohms, this gives a voltage gain of -0.1 (-R1/R2).

    I can't find any info in my text books aside from that the value itself is around 2 Mega-ohms, and that inverting amplifiers can reduce the input impedance. However I'm not sure how to go about calculating this.

    Thanks.
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Gain=-R2/R1

    Thus, if R2=220K and R1=22K then the gain is -10.

    The inverting input is very close to 0V.

    Thus the input current is:

    Iin=Vin/R1

    The input resistance is:

    Rin=Vin/Iin=Vin/(Vin/R1)=R1
     
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