Re: Virtual Ground
The above posts are correct but the concept of virtual ground is better understood by the investigating the notion of feedback.
Now listen to me...
If u use an op amp without a feedback, then the rules U stated are by no means correct. That is, the two inputs of op amp are treated as a regular high-valued resister whose potential difference can be any amount! But when u use feedback, it is the Element in the feedback path which couses the two potentials to be equal. For instance, consider a simple inverting op amp. As u provide the input with some voltage, the output goes to a value so as to provide a cuurent which is sufficient to make the two input pins of the op amp equipotential.
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Regards.