NONLINEAR BEHAVIOR IN MICROWAVE GALLIUM-ARSENIDE MESFET AMPLIFIERS

GaAs MESFETs are increasingly finding application in power amplifiers, as solid-state replacements for TWT amplifiers, the output component in many microwave transmitters. Nonlinear behavior of the amplifier can result in intermodulation distortion, with distortion of existing frequency components within the transmission bandwidth, and the creation of spurious components outside it. Such behavior affects both the capacity and quality of communications links; its analysis is necessary to be able to control these parameters.; A large-signal, numerically efficient MESFET model together with a harmonic balance technique is used to examine the interaction between the MESFET and its circuit at different power levels. Under single-frequency excitation, it is found that the device can be represented in a quasilinear fashion through its large-signal S-parameters, which are functions of the incident power at both ports. These are found to give good characterization at the fundamental frequency for incident power levels up to the 1-dB compressed output power of the MESFET.; The bandpass sampling theorem is used to modify the harmonic balance approach to allow efficient analysis of the MESFET when driven by two closely spaced, nonharmonically related frequencies. Aliasing, introduced by sampling the signal waveform below the Nyquist rate, is accounted for and effectively removed by a repetitive frequency-shift technique. This enables the two-tone intermodulation response of the amplifier to be studied, as well as gain suppression effects in limiting amplifiers. Device-circuit interactions and the effects of bias and harmonic terminations are studied, and compared with experimental results.; A novel scheme for intermodulation distortion reduction is proposed. It is expected that large-signal S-parameters will prove useful in the design of oscillators and amplifiers, and that the modified harmonic balance technique will prove to be a necessary analytical tool in the design of highly linear MESFET amplifiers.