6.5 THE PRINCIPLES OF ‘VIRTUAL SHORT’ AND ‘ZERO INPUT CURRENT’
We continue with the analysis of the circuit in Fig. 6.4-1(b). We have ascertained that the feedback involved in this circuit is negative in sense. Hence, we expect the output to be zero when the input is zero. Now, we set out to find the output in terms of input, when the input is non-zero. Assume that the Opamp is ideal. Hence, its input resistance is an open-circuit, output resistance is a short-circuit and its gain is infinity. The gain is taken as A first and is moved to infinity at the end of the circuit solution. The circuit is redrawn with the Opamp replaced by its ideal equivalent circuit as in the circuit in Fig. 6.5-1(b). We derive the equation for the output as below: ...
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