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Homework Help: Designing an op-amp given the output formula

  1. Mar 28, 2013 #1
    1. The problem statement, all variables and given/known data
    Design an op-amp based circuit to produce the function
    Vout = 5*V1 - 4*V2

    2. Relevant equations

    3. The attempt at a solution
    I have no idea how to approach this. I know there are two voltages on the two inputs, and that the feedback resistors should be going to the negative input, but how do I pick the values for the resistors?
  2. jcsd
  3. Mar 28, 2013 #2


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    Staff: Mentor

    What opamp configurations are you familiar with? Have you worked with the summing opamp configuration? Can you think of a way to do this subtraction all with one opamp? How about with two opamps?
  4. Mar 29, 2013 #3
    I think I got it. I can use one op amp and there is a voltage source on each input. So using feedback to the negative side, I can use KCL on the node there and once rearranging that equation, I tried values for the two resistors to make them equal the equation.
  5. Mar 29, 2013 #4

    rude man

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    Homework Helper
    Gold Member

    How many resistors in total you plan to use?
  6. Mar 29, 2013 #5
    I used 2: one was 20k and the other was 5k
  7. Mar 29, 2013 #6

    rude man

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    Homework Helper
    Gold Member

    If you can do that with only 2 resistors you are a genius!
    So where are your two inputs V1 and V2 connected?
    (Hint: try 4).
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