The selection of output function for nonlinear feedback linearizing controller is an important task as the design of controller depends on the output function of the systems. This paper present a detailed analysis to select the output function to design nonlinear feedback linearizing excitation controller for power systems. Depending on the relative degree of the system which depends on the output function of the system, different approaches of feedback linearization needs to be used for the design of nonlinear control. In this paper, different approaches are discussed with suitable output functions and some of these may not be practical. Based on a practical output function, a nonlinear feedback linearizing excitation controller is designed and tested for single machine infinite bus (SMIB) system.