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Op-amp resister circuit analysis

  1. Mar 9, 2015 #1
    1. The problem statement, all variables and given/known data
    Given the ideal op-amp, determine the resistor R2 and the current I2 where Vin=500mV, Vout = 2.5 V, and R1=5kV.
    30u4xv5.png
    2. Relevant equations
    No current passes through op-amps, the op-amp tries to make V+ and V- equal.
    Ohm's law, voltage divider equation

    3. The attempt at a solution
    Since V+ is 500mV, I assumed that the node between R1 and R2 must also be 500mV, as it is connected to V-. So 500mV = 2.5V(R2/(5000Ω+R2)). I got R2 to be 1,250Ω.
    Then I found I2=Vout/Req = 2.5V/(1250Ω + 5000Ω) = .4mA.
    However, I am unsure because I am not sure if I did the part with the op-amp right; namely, the node between the resistors being set to 500mV.
     
  2. jcsd
  3. Mar 9, 2015 #2

    psparky

    User Avatar
    Gold Member

    You are 100% correct sir.

    V+ = V- in ideal opamps...

    Nice job.
     
  4. Mar 9, 2015 #3

    LvW

    User Avatar

    Yes - and for the future you can use the general closed-loop non-inverting gain formula (assuming an ideal opamp): Acl=1+R1/R2.
     
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