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please explain the concept of virtual ground in opamps pls

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xyz2

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suppose a noninverting terminal is at ground then inverting terminal will also be at at ground because of large voltage gain. this is the concept of virtual ground. MY question is if a voltage is applied a non inverting terminal then same thing will appear at inverting terminal then it becomes a common mode signal.. Then in all cases the signal applied to one of the terminals will appear at another terminal making it common mode signals !!! which will be not amplified at all.. can anyone pls explain this
 

You are correct but if the output is fed straight back to the inverting input (and assuming it is unity gain stable) you do not get any again amplification, you have created a signal buffer.

The virtual ground is a point where the incoming signal and the feedback are equal but opposite polarities so they effectively cancel out. You will normally see this where the source and feedback are connected through resistors and meet at the vitual ground point. If you measure signal voltage from the virtual to the real ground you would see (almost) nothing but in reality there is current flowing into the point from both directions.

Brian.
 
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