Common Mode Output/OpAmp

Discussion in 'Homework Help' started by zbst, Mar 24, 2008.

  1. zbst

    zbst Thread Starter New Member

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    I was given the following question on a homework assignment dealing with op-amps.


    "Given an input of 0.1V DC with a gain of 10 and a common mode voltage of 0.009, what is the common mode output with a CMRR of 70dB? What is the total output voltage?"

    So far, the only thing I have been able to calculate is the common mode gain, which has gotten me nowhere. I'm not even sure that part is correct, as I assume the gain of 10 the instructor gave us is a closed loop gain, and I was under the impression that open-loop gain was used in the following equation:


    CMRR=20log(differential V gain/CM gain)
    70 =20log(10/CM gain)
    3.5 =log(10/CM gain)
    10(to 3.5 power) =(10/CM gain)
    CMgain=10/10(to3.5)
    =0.0032



    I have spent hours researching this question and am obviously missing something. :confused: Can anyone please help me figure out where I went wrong? Thanks!!
     
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  2. Distort10n

    Distort10n Active Member

    Joined:
    Dec 25, 2006
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    Well...

    Homework questions...professors and books really need to get it together. No circuit, no mention of supplies (single, dual), or configuration. However, since the gain is stated to be positive then I think it would be safe to say that the configuation is either a non-inverting or difference configuration. I vote for non-inverting.

    A non-inverting op-amp with a gain of 10 and an input of 100mV will produce a 1V output ideally. You have a common mode voltage of 9mV and a CMRR of 75 dB.

    So all you need to do is convert 75dB into an absolute number: 10^(75/20) and divide 9mV by the result.

    Your final answer will be 1V + x, where x is the additional voltage offset from the common mode voltage.
     
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  3. zbst

    zbst Thread Starter New Member

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    Mar 24, 2008
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    Thank you so much! I really appreciate your time on this one-our professor makes me feel completely stupid if I ask him anything, although I don't believe he means to do so. The assignment was hand-written by the professor and photocopied, and my class has had to make a lot of assumptions on various parts of it! Thanks again!:)
     
    #3
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