Derivative Superposition - a linearisation technique for ultra broadband systems

D.R. Webster
1996 IEE Colloquium on Wideband Circuits, Modelling and Techniques   unpublished
A bs tra ct This paper summarises progress on a distortion reduction and nonlinear function synthesis technique called Derivative Superposition. The paper starts by reviewing the idea of 1)erivative Superposition. We show how low distortion amplifiers and function circuits such as frequency triplers can be designed. We review the results of a 4 HEMT low 3rd order distortion amplifier demonstrator. We consider the effect of the strong 2-port nonlinearity of a MESlFET and show its effect on a 2
more » ... SFET demonstrator. We delscribe progress on a software tcml d e d "Super Deriv" that provides CAD support. We describe some altemative implementations of Derivative Superposition. -- Inrfroducriion The linearisatioii of a nonlinear amplifier has been traditionally approached as a solution for a black box. Such techniques have included bdancing, feedfoxward, feedback and preclistortion. Feedforward [l] requires the use of phase shifting networks to match amplifier deka , which renders it relatively narrowband (<lo!&. The use of resonant matching networks within the amplifier can make phase matching difficult. F'redistortion hats the potential to be broadband, if an almost exact inverse of the amplifier nonlinearity tm be synthesised The implenientation of predistortion based on correcting amplitude and phase shift changes arising from tiistortion [2] is genexiilly only pract~cd for single carrier situations.. Direct applicalion of negative feedback [3] to microwave circuits is generally avoided, as the levelk of open loop gain are usually too small (circa 10dB) to amminodate the reduction in overall gain that is required to give an improvement in linearity. Cartesian loop feedback [4] applies negative feedback to the whole syrtem, making use of high gain in the low power part of the system to provide a high suppression of distortion. However in a practical system, the bandwidth is limited typically to 4Hgz to maintain loop stability. Balancing [5] c m be broadband if realised with transformer Ihybridis, however, this only reduces even order disitortion components in amplifier circuits. In general, it is the 3rd orlder distortion that is of the greatest concern to the designer, as it leads to the generation of interfering signals that are close to the parent tones. Specifications for 3rd order intermodulation distortion Isuppression of up to -70dBc are encountered for some power amplifiers. 2nd order is gemerally less important as it is usually outside of the band of interest and is more amenable to filtering. (Except for zero IF frequency systems) In this paper we review a design technique called derivative superposition ):61,7]. This technique allows the designs of amqlifiers with very low 3rd order intermodulation distortion, and also function circuits such as fircquency triplers. This techniques tackles distorbtcm reduction inside an amplifier, leading to broad band distortion reduction. We describe our experience with two demonstration circuits. We discuss the effect of load dependant distortioin in MESFETs on Derivative Superposition l[8,9, IO]. Finally we describe some alternative: implementations of derivative superposition, including a version for Class AB amplifiers and a novel Distributed Amplifier version. Extensive use is made of the Parker Skellem HEMT& MESFETModeI [11,12, 13, 14, 15] . This model is the most realistic of the MESFET models available in coniinon circuit simulator packages such as SPICE, and iprovides valuable insight into circuit behaviour [lo]. The model --"liege London,
doi:10.1049/ic:19960703 fatcat:2yqobx4rprdghoafq3lpn46myq