Op-amps are (theoretically) just linear differential amplifiers with extremely high gain (like 10,000x or greater). They appear nonlinear in open-loop configura

Op-amps are (theoretically) just linear differential amplifiers with extremely high gain (like 10,000x or greater). They appear nonlinear in open-loop configuration because their gain is so high that any realistic input signal immediately pegs the output against the rails. But with negative feedback they start to appear linear again because you can set the gain to something more reasonable. They're marvelous devices.