virtual short in opamp




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When negative feedback is employed in opamp the concept of virtual short at the input terminals of opamp arises as follows: Assume we gave input signal to noninverting terminal of an opamp and through inverting terminal we gave a feedback signal, Now we measure a finite signal at the output (because a negative feedback system is characterised by transfer function

A/(1+Aβ)

 where A is open loop gain β is feedback factor as A tend to infinity overall gain tends to 1/β which is finite) of opamp. Opamp gain=output/input (should not be confused with system gain as a whole) where gain is infinite for an ideal opamp so for finite output, input should be zero. Here input is difference of signals at non inverting terminal and inverting terminal, which is zero which means a virtual short exists at the input terminals of an opamp. This concept very much simplifies solving of opamp circuits.

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