A portable mechanical ventilator having a Roots blower is configured to providea desired gas flow and pressure to a patient circuit. The mechanical ventilatorincludes a flow meter operative to measure gas flow produced by the Roots blowerand an exhalation control module configured to operate an exhalation valve connectedto the patient circuit. A bias valve connected between the Roots blower and thepatient circuit is specifically configured to generate a bias pressure relativeto the patient circuit pressure at the exhalation control module. The bias valveis further configured to attenuate pulsating gas flow produced by the Roots blowersuch that gas flowing to the mass flow meter exhibits a substantially constantpressure characteristic. The bias pressure facilitates closing of the exhalationvalve at the start of inspiration, regulates positive end expiratory pressureduring exhalation, and purges sense lines via a pressure transducer module.
