Stress in piezoelectric material can be controlled by imposing the electrical field. Thanks to a feedback, this electrical field can be a function of some strain related measurement so as to confer to the piezoelectric device a closed loop macroscopic behaviour. In this paper we address the modelling of such system by extending the modal decomposition methods to account for the closed loop. To do so the boundary conditions are modified to include the electrical feedback circuit, hence allowing a closed-loop modal analysis. A case study is used to illustrate the theory and to validate it. The main advantage of the method is that design issue such as coupling factor of the device and closed loop stability are simultaneously captured.