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Homework Help: Finding the output current in an op-amp.

  1. Nov 9, 2011 #1
    1. The problem statement, all variables and given/known data

    The problem is gives an ideal op-amp and wants me to find the output current of it. Here is a picture of the problem.
    http://i.imgur.com/NfO6q.png

    2. Relevant equations

    vp=vn
    in=ip=0

    3. The attempt at a solution
    So what i did is take KCL at the Vn node....

    (Vn-10)/5K + (Vn-Vout)/10K = 0

    Since vp=0 then vn must equal zero

    -10/5k-Vout/10k=0

    So Vout=-20

    But because it's out of the amplifiers range which is from -15 to 15 volts, then Vout is 15 volts. So I then do KCL at the final node to solve for iout....
    (15-0)/10K + 15/10K = -iout
    -3mA=iout
    But the answer to the question is -3.167mA. Please help!
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Nov 9, 2011 #2

    The Electrician

    User Avatar
    Gold Member

    Re-consider your assumption "Since vp=0 then vn must equal zero".
     
  4. Nov 9, 2011 #3

    berkeman

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    Staff: Mentor

    Nice!
     
  5. Nov 9, 2011 #4
    Ok so I instead I did Vout=(-Rf/Rs)*Vs
    So (-10K/5K)*10
    Vout=-20
    But since Vout is still not with the linear range of the power rails, isn't Vout=15?
     
  6. Nov 9, 2011 #5
    NVM I got it. So my mistake was that since Vout is -20 and outside the linear range of the op-amp then Vout is -15. From there I did KCL at the Vn node with the new Vout.
    (Vn-10)/5K + (Vn+15)/10K = 0
    Vn=5/3
    From there I did KCL at the output Vout:
    (-15-5/3)/10K - 15/10K = Iout
    Iout=-3.167 mA
     
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