# 3 – Op Amp: Characteristic Equation And Curve

The mathematical model that describes the behavior of any Operational Amplifier is expressed by the following equation:

$V_{OUT}=A\cdot \left( V_{+}-V_{-} \right)$

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:

$v_{D}=V_{+}-V_{-}$

This value is also referred to as differential voltage input.

So the characteristic equation of operational amplifiers can also be written as follows:

$V_{OUT}=A\cdot v_{D}$

From the above it follows that different inputs may correspond to the same output.