Nonlinear Output-Feedback Tracking Using High-gain Observer and Variable Structure Control**An earlier version of this paper was presented at the 1995 IFAC Nonlinear Control Systems Design Symposium, held in Lake Tahoe, U.S.A. in June 1995.,11A globally bounded output-feedback variable structure controller with a high-gain observer is designed for a feedback-linearizable minimum-phase nonlinear system in the presence of unknown disturbance