Virtual short theory of op-amos

the virtual short theory for op-amps which states that whatever is the voltage on any one input the same will appear on other input holds good when properly understood. First the source should not be directly connected to the input, there should be a input resistor in series. Feedback should be there; either positive or negative. And the most interesting one is that the voltage input on which feedback is not given dominates the input node voltage on which feedback is given. What I still have to understand is that, why after certain point increasing the voltage on dominating node doesn't change the input on the other node. i.e. it saturates. for example in the positive feedback configuration of this circuit the feedback node voltage follows the voltage on non feedback node until 7.5v then it doesn't increase even if I increase it any more. I think the answer for my above doubt is that the voltage at the feedback node gets adjusted by output voltage divided by the voltage divisor which forms the feedback network, but at a certain input on the non feedback node such that output hits saturation the voltage can't increase beyond that point as a result the feedback node stops following and this is especially important to take care when we are running the op-amp single rail supply (even more care required for positive feedback, however there isn't much application of positive feedback other than osc and schmitt trigger) and saturation can be high or low any saturation will stop the tracking.

published 10 years ago