A droplet delivery device with a small volume drug ampoule and related methods for delivering precise and repeatable dosages to a subject for pulmonary use is disclosed. The droplet delivery device is configured to facilitate the ejection of small, e.g., single use, volumes of a therapeutic agent. The droplet delivery device includes a housing, a mouthpiece, a small volume drug ampoule, an ejector mechanism, and at least one differential pressure sensor. The delivery device is automatically breath actuated by the user when the differential pressure sensor senses a predetermined pressure change within housing. The droplet delivery device is then actuated to generate a plume of droplets having an average ejected particle diameter within the respirable size range, e.g, less than about 5-6 μm, so as to target the pulmonary system of the user. The small volume drug ampoule may include a reservoir which comprises an internal flexible membrane separating two internal volumes, a first background pressure fluid volume and a second drug volume.