Ataei, Gholamreza; (2000) Design of linear and nonlinear circuits using derivative superposition. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Modern communication systems require a wide variety of linear and nonlinear circuits such as power amplifiers and frequency multipliers. The derivative superposition (DS) technique has been proposed for the design of linear and nonlinear circuits. A DS circuit uses a number of field effect transistors (FETs) in parallel with each other and having different gate widths and operating points. This allows the designer to control the shape of the transfer characteristic to meet a particular linear or nonlinear specification. This technique has been chosen to be the subject of the work in this thesis. It is shown that a MMIC DS power amplifier using a novel phase reversal form of DS achieves a two tone carrier to interference ratio (c/i) of 45 dBc with an efficiency of 22.5 % close to 1 dB compression around 0.5 GHz providing a better compromise between linearity and efficiency than conventional single amplifier classes. A novel composite Doherty-DS structure is proposed which is capable of providing a wider dynamic range for better c/i performance close to 1 dB compression and a good compromise in terms of efficiency compared to those of either the Doherty or the DS amplifiers alone. The DS approach is used to design a nonlinear function circuit for a quasi- optical tracking system and a MMIC frequency tripler design which achieves a considerable suppression of unwanted frequency products. A refined version of the DS frequency tripler is proposed which employs a rat race coupler for operation well into the microwave region. During this work some improvements were made to the set-up for the intermodulation distortion measurements of FETs, a study of factors affecting distortion of various FETs was carried out and a visualisation technique was developed for mapping the distortion of a FET allowing identification of an optimum bias and loading condition.

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