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Find voltage gain and input resistance for op-amp

  1. Oct 4, 2013 #1
    1. The problem statement, all variables and given/known data
    Find the input resistances and voltage gains for those ideal op amps
    GRwpV5O.png
    XcewVfX.png

    2. Relevant equations



    3. The attempt at a solution
    The original inverting circuit look like this :
    jt260xU.png
    we already have the equations :
    input resistance = 10k
    voltage gain = -r2/r1 = -10
    For the first circuit :
    it still a inverting op amps, does the red marked 10k resistor get involved with input resistances ? I think it's not because it connected to the ground (virtual ?). R2 is 100k so the equation for voltage gain remains the same as the original circuit .
    Second circuit:
    there is no current in red marked 10k resistor, input resistance is unchanged (10k), voltage gain remains (-10k)

    I find it is difficult to calculate using op amps characteristics, can I use voltage node method to find the voltage gain, which node should I choose. Are those ground connected resistor have no effect on the circuit input resistances and voltage gain at all ?
     
  2. jcsd
  3. Oct 4, 2013 #2

    berkeman

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    Staff: Mentor

    You are correct that because of the virtual ground property (especially with ideal opamps), the input resistances of these circuits is 10kOhms. You have a typo in the gains, however. Can you see what your typo is?
     
  4. Oct 4, 2013 #3
    Yes, for the second circuit the gain is -10.
     
  5. Oct 6, 2013 #4
    So can we say in those cases : In an inverting op-amp, any resistors connected to the ground can be ignored when calculate ?
     
  6. Oct 6, 2013 #5

    berkeman

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    Staff: Mentor

    I wouldn't say that -- it's a bit too simplified and not always true. Instead, understand what the "virtual ground" means. Can you tell us what is going on with the "virtual ground" property of a high-gain opamp with negative feedback means?
     
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