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 v_{D}:

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.