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Homework Help: Voltage gain Vo/Vi

  1. Mar 20, 2016 #1
    1. The problem statement, all variables and given/known data
    Determine the voltage gain Vo/Vi of the op-amp circuit
    2. Relevant equations
    inverting amplifer
    vout=-(Rf/Rin) x vin

    differential amplifer
    vout= (R2/R1) x (vo - vin)

    3. The attempt at a solution
    im really struggling to even get started with this one sorry for the limited information i just need a starting block.

    Attached Files:

    Last edited by a moderator: Mar 21, 2016
  2. jcsd
  3. Mar 21, 2016 #2


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    Science Advisor

    The file you attached is not applicable to the circuit in your post. Notice that in the attached file there is a path from output to the "+" input of the same opamp. That configuration is not in your posted circuit.

    Try redrawing your circuit by moving the lower opamp upward and to the left of the upper opamp, and make it face in the same direction. That should help in seeing the overall function. If you're still having trouble, start by considering the two opamp circuits individually.

    Where/what are R2, R1?
  4. Mar 21, 2016 #3
    sorry about the attached file i dont know why it did that, it has nothing to do with my circuit.o_O

    this is what i have so far.

    inverting amplifier

    vout= -(RF/RIN) X VIN
    Vout = -(68/12) x Vin
    Vout = -5.667Vin

    Differential amplifier
    vout = -(R2/R1) x (VX x VIN)
    Vout = -(220/12) x (VX x VIN)
    Vout = -18.334 x ((-5.667Vin) - Vin)
    Vout = -18.334 x (-6.667Vin)
    vo/vi = 122.233
  5. Mar 21, 2016 #4


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    Science Advisor

    VX and VIN are Multipied here...
    ...but Added here.

    R1 & R2 are still undefined.
  6. Mar 21, 2016 #5
    Where/what are R2, R1?[/QUOTE]

    R2 in the formula is the feedback resistor, R1 the series one.
  7. Mar 22, 2016 #6


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    This result is correct.
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