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# Op-Amp Question

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I'm an Electrical Eng. Tech. Student and have a small question...

Its a basic problem, all the information I have is,

An Op-Amp has a CMMR of 76dB and an open-loop gain of 81,000.

What is its common gain.. now here is my little problem.. I have the formula..

Acm=(Aol/CMMR)

This is I believe not correct? Since I've been getting some weird values, does this have anything to do with log20 that would get me the CMMR'?

Im pretty certain thats where I've went wrong does anyone have any ideas how I can make Aol/Acm=CMMR work to give me Acm.. when I have CMMR'?

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80dB is a voltage gain of 10,000. 74dB is a voltage gain of 5000. So 76dB is a voltage gain of about 6000. So the common gain is 13.5.

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Hi Electrical_tech_Student!

When you use the formula Acm=(Aol/CMMR), all the quantities have to be pure numbers without dimensions (not dB), so you cannot use CMMR=76dB and Aol=81000 at the same time! You have to convert 76dB into 10^(76/20)=6300, since CMMR(in dB)=20log10CMMR. Then you can use this number with the above formula!

Another solution is to convert Aol in dB and use the formula Acm(in dB)=Aol(in dB)-CMMR(in dB) and then you find Acm in dB.

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