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How to set the common mode feedback loop ?

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desperado1

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In a differential operational amplifier, you have two terminals, the positive one, Vp, and the negative one, Vn. Therefore, you have a differential signal Vd=Vp-Vn, and a common mode signal, Vcm=(Vp+Vn)/2.
I have read that he amplifier works properly if the common mode voltage is set accurately by a common mode feedback loop. If the common mode DC voltage is not set correctly, the amplifier saturates to its rails (either VDD, either VSS) and it not able to amplifier the differential AC signal.
My question is how do we set the common mode feeback loop in a differential amplifier ?
 

lwjbh

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you should set the voltage to the point which can make your circuit works normally.
 

desperado1

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lwjbh said:
you should set the voltage to the point which can make your circuit works normally.
thanks a lot for your answer. can you please explain in more detail to this question ?
 

airboss

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simply speaking, you need to create an output CM voltage detector. the detected output CM voltage then needs to go to a "comparator" because you want to know "is it pretty close to the desired CM voltage or not." then, you need to make a negative feedback loop so that when the output CM voltage is changed, there is a mechanism to bring the CM voltage back to what it's suppoed to be.

also, when it comes to CMFB, you're concerned about the output common mode voltage, not the input common mode voltage. where there's a undetermined drain voltage is where you need the feedback so as to gain the control. maybe you understand this correctly but just from your wording, it seemed to me you had some misunderstanding.
 

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