A novel vacuum field-emission differential amplifier (diff-amp) based on carbon-nanotube (CNT) emitters has been developed, modeled, and its ac performance characterized. A dual-mask microfabrication process was employed to achieve a single-chip diff-amp by integrating matched CNT field-emission triodes with built-in split gates and integrated anodes. The identical pair of triode amplifiers was well matched in device characteristics. The measured ac common-mode-rejection ratio (CMRR) of the diff-amp was ~ 50 dB. The proposed CMRR semianalytical model was validated with the experimental data. The successful implementation of the CNT diff-amp demonstrates a new way to achieve high-temperature and radiation-tolerant vacuum integrated circuits.