This paper presents a new set of exact analytical expressions for the small signal analysis of the non-quasi-static operation of the MOS transistor. This model is derived from a standard charge-based description, with the help of the EKV compact model [Enz C, Krummenacher F, Vittoz E. Analog Integ Circuits Signal Process 1995;8:83-114.] formalism. For the first time, it gives simple expressions for all AC parameters which are valid in all operating modes, from weak to strong inversion and conduction to saturation. The model is derived from physics and only relies on the very few basic assumptions needed for a charge-based compact model. The results are written in the form of a normalized transadmittance matrix which is expressed in terms of normalized variables (currents and frequency), so that they are independent of the process parameters. From this exact approach, simpler first- and second-order approximations, dedicated to circuit simulation tools, have been obtained. Finally, the theoretical results have been compared with measurements showing a very good agreement with measurements.