1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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.


    2. Relevant equations

    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:


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


    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


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    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


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook