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Non-ideal Op Amp voltage

  1. Nov 21, 2012 #1
    1. The problem statement, all variables and given/known data

    I have to use the basic equivalent model of an op-amp to develop an expression for Vo from the given op-amp. I also have to simplify the expression by treating the given op-amp as an ideal op-amp.

    2. Relevant equations

    Vo=A(Vp-Vn)
    Vo=G*Vs

    3. The attempt at a solution

    Vp=6v
    Vn=6-Vs

    Vo = A(6-6-Vs) = -A*Vs. Am I on the right track? I don't even know how to start on the other half of the problem. I've only had half a lecture on op-amps.
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     

    Attached Files:

  2. jcsd
  3. Nov 21, 2012 #2

    CWatters

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    Looks right so far.

    For an ideal op-amp you can essentially ignore the two 6v sources. For a real world op-amp you might have to check out the common mode rejection ratio.
     
  4. Nov 21, 2012 #3
    So for an ideal op-amp, would Vo just end up being Vs?
     
  5. Nov 21, 2012 #4

    berkeman

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    I think you have a couple small sign errors in your equations, but you are on the right track.

    The 6V source and Vs look to add going into the - opamp input, but you have written that equation as a subtraction.

    And then When you calculate Vo, you do not account for the fact that Vs is going into the - opamp input. Your answer is almost correct, you just need to fix those two sign errors.
     
  6. Nov 21, 2012 #5

    berkeman

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    Unless, by this equation:

    Vo = A(6-6-Vs) = -A*Vs

    you mean Vo = A[6 - (6+Vs)] = -A*Vs

    If so, then you don't have a sign error.
     
  7. Nov 21, 2012 #6

    rude man

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    The attached circuit shows an op amp in open-loop operation.

    That means that, ignoring offsets, output = AVs.

    It also means the output is infinty times Vs if the op amp is ideal.

    Very unsuitable circuit!
     
  8. Nov 22, 2012 #7

    CWatters

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    For an ideal opamp the output would be -A*Vs where A => ∞ as others have said.

    Is there more text to go with the diagram? The title of the thread says "Non-ideal Op Amp voltage". To answer your question fully we would need to know what was Non-ideal about it.
     
  9. Nov 23, 2012 #8
    well you might start to call it a comparator instead of an amplifier.
     
  10. Nov 23, 2012 #9

    CWatters

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    as long as you don't mind some spikes on the output as the input transitions through Vs=0V.
     
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