The mathematical model that describes the behavior of any Operational Amplifier is expressed by the following equation:
Its meaning is simple: output voltage is equal to the difference between input voltages amplified by a positive factor A (called open-loop gain).
Usually the voltage difference between inputs is a value that is referred to as vD:
This value is also referred to as differential voltage input.
So the characteristic equation of operational amplifiers can also be written as follows:
From the above it follows that different inputs may correspond to the same output.