In this article, an output feedback control scheme is proposed for trajectory tracking of differentiallyflat systems. After applying the exact feedforward linearization, the tracking error dynamics is linearized as alinear time-varying (LTV) system, where only the state matrix is time-varying while the input and output matricesare time-invariant. Using linear parameter-varying (LPV) techniques based on polytopes, for the LTV system thecontroller matrices, which are set to be affine on time-varying parameters, are calculated by solving a set of linearmatrix inequalities (LMIs). An example for trajectory tracking of a two-wheeled mobile robot is given to show theeffectiveness of the proposed method.