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Homework Help: Opamps -- Unknown values

  1. Jun 20, 2016 #1
    • Poster has been reminded to post schoolwork in the Homework Help forums (moved from EE)
    Hi everybody!

    This is my first post as I am needing some help in a question I am struggling with. Not looking for answers, just some guidance in the right direction.

    1. The problem statement, all variables and given/known data

    I need to calculate the unknown, highlighted, voltages on the op amp circuits (attached).

    Assume all op-amps are ideal

    2. Relevant equations

    See below

    3. The attempt at a solution

    I have done the following so far:

    a) Inverting
    A=-Rf/Ri =(-100)/10=-10
    Vi =I1*R1 =0.00001*100000=1 V
    Vo=(-Rf/Ri )*Vi
    Vo =1*-10
    Vo =-10 V

    b) Summing
    Vo=-(Rf/R_1 V_1+Rf/R_2 V_2 )
    -2=(Rf/R_1 1+Rf/(R/2) V_2 )

    c) Non-inverting
    V1 =V2
    V1 =100 mV
    V2 =100 mV

    d) Current to Voltage
    Vo =-0.0002*10000=-2
    Vo =-2 V

    I am struggling the (b). I cant see how to calculate given only one value, and also with (c)

    Many thanks in advance, happy to discuss the methods.


    Attached Files:

    Last edited: Jun 20, 2016
  2. jcsd
  3. Jun 20, 2016 #2
    For A, Note that the voltage at both inputs to the op amp are equal, and at ground.
    For B, I have 1R-2R+x(R/2)=0
    For C & D, we agree.
  4. Jun 22, 2016 #3

    rude man

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    Homework Helper
    Gold Member

    You have R1 wrong.
    correct (I took out the "Rf" , should be just "R" everywhere)
    The R's cancel. There is nothing more to do with (c) than you have already done.
  5. Jun 22, 2016 #4
    Thank you very much for your help
  6. Oct 9, 2016 #5
    b) Summing
    Vo=-(R/R*V_1+R/R*V_2 )
    -2=(R/R*1+R/(R/2)*V_2 )

    So is there another step to this because this equation doesn't give V_2. do you transpose it ?
    Last edited: Oct 9, 2016
  7. Oct 9, 2016 #6

    rude man

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    It doesn't?
  8. Oct 9, 2016 #7
    can you elaborate please
  9. Oct 9, 2016 #8

    rude man

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    Gold Member

    Well let's see - you have one equation with 1 unknown.
    If this is difficult I really am at a loss as to what to hint next, so I must perforce cease from further such.
  10. Nov 1, 2016 #9
    b is 1/2 volt
  11. Nov 2, 2016 #10


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    Staff: Mentor

    As a check, substitute your answer into the original equation to ensure that your answer does indeed make both sides of the equation equal.
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