A valve in a medical ventilator opens and closes as is needed to permit venting of respiratory gases from a patient to the atmosphere. Changes in position of the valve are caused by a stepper motor, which is joined to the valve by a mechanical linkage. The stepper motor, in turn, is powered by electronic signals from a microcomputer controller. The position of the valve is controlled on a closed loop basis by the controller, which receives feedback regarding the pressure of the proximal gases from a pressure transducer. The valve is uniquely designed to enable the controller to cause the valve to open quickly once the patient begins to exhale, and to permit accurate control over the proximal pressure as the valve is approaching the closed position. In particular, the valve comprises a poppet and a valve seat which have curved mating surfaces, with the curved surface on the poppet being truncated so that pressure gain variance is minimized over the range of positions of the poppet relative to the valve seat. To enhance this improved pressure gain characteristic, a cam in the linkage between the poppet and stepper motor has a variable pitch. All of the components of the valve which are exposed to respiratory gases are easily removed for sterilization and replacement. Further, the poppet is movable to ensure complete closure of the valve despite variability in the tolerances of the replaceable components.