Operational Amplifier Linearity

When amplifying a signal, linearity means the information contained in the signal is not changed and no new information is introduced except the amplitude of signal.

When feeding a sine wave signal of amplitude V and frequency f shown in the figure to an op-amp, the signal measured at the op-amp output also is a sine wave with a different amplitude A*V, but exactly the same frequency f (sine wave phase may change). Otherwise, it is considered to be distortion and is obviously undesirable.