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Old 03-24-2008, 07:30 PM
zbst zbst is offline
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Cool Common Mode Output/OpAmp

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)

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

Last edited by zbst; 03-24-2008 at 07:35 PM. Reason: 10 raised to 3.5 power was unclear!
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Old 03-24-2008, 11:59 PM
Distort10n Distort10n is offline
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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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Old 03-25-2008, 12:10 AM
zbst zbst is offline
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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!
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