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Homework Help: Inverting Op Amp

  1. Mar 29, 2012 #1
    1. The problem statement, all variables and given/known data


    2. Relevant equations

    V(output) = V(input)(R2/R1)

    3. The attempt at a solution
    The equation above is valid if the back part of the upper circuit (involving R3,R4 and the earth wire) does not exist. I tried to improvise with it by using the equation V(output) = V(input)(R2+1/(1/R3+1/R4))/R1 however the answer is wrong. Just need some guidance as to what I am doing wrong.
  2. jcsd
  3. Mar 29, 2012 #2
    Can you get the voltage at the inverting (-) input of the opamp?
  4. Mar 29, 2012 #3
    That would be Vi/30 000= 3/30000 = 0.0001

    edit: Oh wait, that's the current. Sorry.
    Last edited: Mar 29, 2012
  5. Mar 29, 2012 #4
    Good, that's correct. So what is the voltage then?
  6. Mar 29, 2012 #5


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    Staff Emeritus
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    If you assume the things that you need to about ideal op-amps, then you basically just have a simple circuit to solve with one voltage source and a few resistors. I find it helpful to apply KCL at the node where R2, R4, and R3 all join. You can relate the voltage at this node to the input voltage, because you know what the voltage at the inverting input must be.
  7. Mar 29, 2012 #6
    You have 100 uA flowing into the junction of R1, R2 and the op amp - terminal. Where is that current flowing to from there?
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