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Op amp circuit voltage ratios

  1. May 9, 2014 #1


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    ImageUploadedByPhysics Forums1399692594.248500.jpg

    I have to find both of the voltage ratios. This is just ugly algebra. Here is what I came up with, and was wondering if I was correct.

    ImageUploadedByPhysics Forums1399692649.153777.jpg

    ImageUploadedByPhysics Forums1399692665.489362.jpg
  2. jcsd
  3. May 9, 2014 #2
    If the opamps are ideal, they have infinite gain and their inputs draw no current.

    You correctly saw that this means R2 is not involved in the analysis, but you failed to notice that R5 is not involved for the same reasons.

    You'll need to redo your work with R5 replaced with a wire; no R5, in other words.
  4. May 10, 2014 #3


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    Is it really that easy?

    ImageUploadedByPhysics Forums1399705402.263499.jpg

    Edit: Wait a minute, I remember the professor saying to never do KCL at the output of an op-amp, I will redo this.
  5. May 10, 2014 #4


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    Ok here is my next attempt hopefully it's right

    ImageUploadedByPhysics Forums1399706185.468647.jpg

    ImageUploadedByPhysics Forums1399706199.284359.jpg
  6. May 10, 2014 #5


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    I found an algebra error in my Vout/Vin and fixed it

    ImageUploadedByPhysics Forums1399709957.638376.jpg
  7. May 10, 2014 #6
    You're good to go!
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