Complex sentence: The sine wave cannot grow indefinitely; in all real oscillators some nonlinear process in the circuit limits its amplitude, reducing the gain as the amplitude increases, resulting in stable operation at some constant amplitude.
Split into simpler sentences: The sine wave cannot grow indefinitely; in all real oscillators some nonlinear process in the circuit limits its amplitude. The effect of this nonlinearity is to reduce the gain as the amplitude increases, resulting in stable operation at some constant amplitude.