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A novel output-feedback sliding-mode control for linear plants is proposed. An observer-based chattering avoidance technique (OBCAT) is incorporated to counteract the effect of parasitics (unmodeled dynamics). A variable-gain unit control is designed to ensure global stability even in the presence of unmodeled dynamics. A necessary and sufficient Nyquist-like criterion for global stability is obtained, encompassing systems with time-delays, without prior assumptions about the observer gain and the characteristic parameter of the unmodeled dynamics which is not required to be sufficiently small. Numerical simulations and experimental results with a commercial robot manipulator illustrate the performance and effectiveness in chattering avoidance of the proposed control scheme and confirm its increased robustness with higher observer gain. • A novel observer-based sliding-mode controller for chattering avoidance is proposed. • Global regulation or tracking can be guaranteed despite the parasitics. • A Nyquist-like criterion for stability and chattering avoidance was developed. • Performance recovery can result increasing the observer gain up to the Nyquist limit. • A variable-gain unit control is designed for output feedback with global stability. • The results apply for time-delay systems, plant uncertainties and external disturbances. [ABSTRACT FROM AUTHOR]