A high-frequency nonlinear model of FET's is developed in which all sources of nonlinearities, including the output conductance, are accounted for. The distortion resulting from this model is represented using the Volterra series approach, and a high degree of accuracy between the theory and the measured results is obtained. Although the theory holds only for circuits that exhibit a slight deviation from linearity, it is found that fairly good agreement exists at larger signal levels. Harmonic, intermodulation, and cross-modulation distortion (CMD) that occur in a typical high-frequency single-stage FET amplifier are all analyzed and experimentally verified. It is observed that the CMD that occurs in the FET under consideration is of the AM type.