Study of nonlinear distortion in microwave amplifiers using Schottky gate field effect transistors

The use of transistor S-parameters, measured at various input signal levels is outlined. The application of this method to the design of a MESFET narrowband amplifier is described. The nonlinear S parameters served as the initial data for designing the topologies of two narrowband amplifiers. After the geometry of the matching gamma section networks using microstrip lines was determined, a transistor was matched in one instance for the small signal case, where its S-parameters corresponding to an input power of 0.1 mW were used, while in the other case, the matching was accomplished for an input power of 1.5 mV. The amplifier characteristics were calculated in the input power range at 6 GHz. Curves are plotted showing the amplifier gain and phase of the gain as a function of the input signal power for the case when the amplifier is tuned for a maximum small signal gain, and also for a maximum large signal gain. The third order intermodulation distortion was calculated for these two amplifiers, when two frequencies are fed to the input. The intermodulation distortion was also calculated for a balanced amplifier, in which two amplifier stages tuned for a small signal were used. This circuit provides a significant reduction in the intermodulation signal level.