A unified study of the noise and modulation characteristics of Fabry-Perot and DFB semiconductor lasers with arbitrary external optical feedback and arbitrary external cavity length is presented. By performing a small-signal analysis and taking into account the nonlinear gain saturation effect, generalisations of previous analytical results and novel features are presented in the following topics: intensity and frequency noise, intensity and frequency modulation responses, chirp reduction, frequency and damping rate of relaxation oscillations.