signal distortion have several reasons including THD.IMD.Nonlinearity and so on.For examples, the relationship between current and voltage of transistors are not linearity,especially in case of large signal acts,which cause THD. Meanwhile,the common mode voltage,that determine dc opration point, would introduce Nonlinear
errors when set up inappropriate.
Fundamentally, it will at least have sth to do with the inherient nonlinear
I-V relations either for CMOS, BJT, GaAs etc. Feedback is a very effective
way to reduce it, recall, for example, source degeneration.