This question already has an answer here:
In studying the OpAmp amplifier, the derivations always take the voltage difference between the inverting- and non-inverting input to be approximately zero. But I can't find any explanation "why". This consideration is always taken for both the inverting- and non-inverting amplifier.
Why is this voltage difference always approximately zero? Any explanation?
I found a lecture of my senior professor, where he said that "as the ideal op-amp's input impedance is infinite, its positive- and negative terminal are virtually short. I mean very little voltage drop which can be neglected."
I can't understand why a very high (practical case) input impedance may cause a very little voltage drop.