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Need some help with OP-AMP problems

  1. Apr 25, 2014 #1
    The problem statement, all variables and given/known data

    http://puu.sh/8mUJ5.png [Broken]
    here is the image
    the question is to find the A at Vo1 and Vo2

    The attempt at a solution
    I don't know how to calculate A at Vo1
    but at Vo2, what I need to do is just
    A= 1 + R/R
    is that right?

    At Vo1, what I think is only A= 4R/R
    but is that right?
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Apr 25, 2014 #2


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    Staff: Mentor

    Note that you've got feedback from the output of the final stage to the input of the first stage, so it's going to affect the gain of the overall circuit, including the first stage. So your guess about the gain of the first stage (Vo1/Vin) is not correct.

    Your expression for the gain of the second stage looks good. Note that R/R is 1 no matter the value of R. So what then is the equation relating Vo2 to Vo1?

    Can you write a node equation at the input terminal of the first stage?
    Last edited by a moderator: May 6, 2017
  4. Apr 26, 2014 #3
    thx for reply

    ok, so the Vo2 = 2Vo1 right?
    then, for the Vo1, Vo2 affects the Vo1
    is it what you mean?

    so i put the node before the first amp
    then the equation go like this?
    Vin/R = Vo1/4R + Vo2/4R
    so Vo1 + Vo2 = 4Vin
    because Vo2=2Vo1, then 3Vo1 = 4Vin
    the results is Vo1 = 4/3 Vin
    and Vo2 = 2Vo1

    is this right?
  5. Apr 26, 2014 #4


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    Staff: Mentor


    Be careful of the signs you assign to the currents when you write the node equation. If Vin/'R is the current flowing into the node then the other currents on the RHS of your equation should have signs that reflect that they are flowing out of the node.

    Once you've written the node equation you can substitute for Vo2 with the relationship Vo2 = 2Vo1 that you found, and then solve for Vo1 (in terms of Vin).
  6. Apr 27, 2014 #5
    thx for replying again

    so, the equation go like this?
    Vin/R = -Vo1/4R + -Vo2/4R
    because Vo1 and Vo2 going backward
    then -(Vo1 + Vo2) = 4Vin
    and Vo1 = -4/3 Vin???
    is it right??
  7. Apr 27, 2014 #6


    User Avatar

    Staff: Mentor

    Looks good.
  8. Apr 27, 2014 #7
    so Vo2 is -8/3 Vin
    and Vo1 is -4/3 Vin
    thanks for your assistance :D
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