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Homework Help: Op Amp Problem

  1. Sep 15, 2009 #1
    1. The problem statement, all variables and given/known data
    Calculate the output of the op-amp as shown in the given circuit.
    http://img9.imageshack.us/img9/7554/184v.png [Broken]

    2. Relevant equations
    Kirchhoff's Current Rule

    3. The attempt at a solution

    By applying KCL,
    V1/R1 + V2/R2 = Vo/R3
    Substituing the values and rearranging the equation we get,
    Vo= (5/10,000 + 2/4,000)*20,000
    Vo= 20
    But then I realized that if i use a minus sign between (5/10,000 + 2/4,000), I get Vo=0...
    So does that minus sign come into the equation because we're trying to find out the output of an inverting amplifier, or is it simply because V2="-"2
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Sep 16, 2009 #2
    This type of circuit is known as adder or summer


    The solution is given by:

    vo = - RF * [ V1/R1 + V2/R2 + ... + Rn/Rn]


    Vo = - 20k * [ 5/10k + -2/4k] ; I think you forgot to use negative signs
    Vo = 0
  4. Sep 16, 2009 #3
    Why did you put a minus sign before 20k?
  5. Sep 16, 2009 #4
    Since the input is on the inverting input a negative sign should be used.
    On yourpost you didn't specify the type of inputs of the op-amp, but it seems that the input is on the inverting input.
  6. Sep 16, 2009 #5
    Yeah you're right...thanks
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