State of the Art for Differential Circuits in Wireless Transceivers: A New Wideband Active Balun in SiGe BiCMOS Technology

Authors: BALWANT GODARA, ALAIN FABRE

Abstract: The aim of this paper is 3-fold: firstly, to provide a comprehensive overview of the use of differential circuits for analogue signal processing in wireless transceivers; secondly, to describe, in detail, single-ended signal to differential conversion, and the corresponding theory of such devices, their characterisation, various methods of implementation, and comparative analyses of their performance; lastly, to propose a new transistor-based solution for wideband baluns. This novel solution is based on the current conveyor and has been modelled using the transistor parameters of a 0.35 \mu m SiGe BiCMOS technology. The salient features of the new implementation are: (a) stable 50-\Omega input port impedance and easily controllable output impedance (50 \Omega/75 \Omega/100 \Omega); (b) stable matching between the differential output ports, within 1 dB (3 dB) amplitude and 10° (20°) phase balance up to 2 GHz (3 GHz); (c) good signal quality with output signal harmonic distortion lower than 1% for peak-to-peak input signals up to 50 mV; (d) excellent S-parameter performance (0-3 GHz) with return losses lower than --10 dB, reverse signal rejection better than 20 dB, more than 25 dB isolation between the output ports, and 42 dB common-mode rejection; and (d) stable performance over a 100 °C operating temperature range. This performance advances the state of the art for single-ended to differential conversion circuits (evinced upon detailed comparisons to existent baluns).

Keywords: Active Balun, Current Conveyors, Differential Analogue Circuits, S-Parameters, SiGe BiCMOS

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