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Op-Amp Circuit - Find R

  1. Sep 1, 2012 #1
    1. The problem statement, all variables and given/known data
    Calculate the value of R2 required to deliver 5W to RL if Vin equals 5V.

    oacircuit-1.jpg

    2. Relevant equations
    Vout=iRL
    P=Ri^2

    3. The attempt at a solution
    I started by using KVL for the input loop, noting that current is going counter-clockwise:

    Input loop: iR2-Vin=0→R2=Vin/i

    I found i by using P=Ri^2:

    i=sqrt(5/50)
    =0.316A

    Then, I used i from above to solve for R2:

    R2=Vin/i
    =5/0.316
    =15.8Ω

    The answer should be 23.1Ω according to the book (Engineering Circuit Analysis 8ed). Where did I go wrong? I have tried so many different approaches, but I can never get to the answer of 23.1Ω.
     
  2. jcsd
  3. Sep 1, 2012 #2

    Redbelly98

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    The currents through R2 and RL are not the same.

    Question for you: what must Vout be, to get the necessary power in RL?
     
  4. Sep 1, 2012 #3
    Wow, how did I not see the currents were not the same?

    Anyway, I figured it out after finding Vout (15.81V). It makes complete sense to me now, thank you so much for your help!
     
  5. Sep 2, 2012 #4

    Redbelly98

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    You're welcome, glad it worked out.

    Understanding op-amps can take some getting used to, but they are incredibly useful once you get the hang of them.
     
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