Application of Volterra series to intermodulation distortion analysis of transistor feedback amplifiers

This paper analyzes the intermodulation distortion of a solid-state feedback amplifier. Each transistor of the amplifier has been represented by a frequency-dependent model incorporating four nonlinearities. The Volterra series is used as the analysis tool. The primary motivation for this study is that intermodulation distortion is a critical problem in high-frequency long-haul solidstate systems. A computer program that calculates the amplifier intermodulation distortion has been developed. This program can be used to select the optimum bias point of each stage, optimum interstage and feedback networks, and optimum load and source impedances. Good correlation between calculated and measured results was obtained. The closed-form expressions derived show that the feedback is fully effective in reducing the open-loop second-order distortion and that the feedback may not significantly reduce the open-loop third-order distortion, if the "second-order interaction" phenomenon is dominant. The second-order interaction is explained in detail in this paper.