Transfer Characteristics
Op-amps are designed to sense the difference between the voltage signals applied at its two input terminals, (that is, the quantity VIN+ - VIN-), multiply this by a number A, and cause the resulting voltage A(VIN+ - VIN-) to appear at output terminal 3.
VOUT = A(VIN+ - VIN-)
According to this formula, we can make a graph, a straight line, which shows the transfer characteristic of a linear amplifier. This means the output voltage is proportional to the input voltage difference.