A CMOS VGA with DC offset cancellation for direct-conversion receivers

Authors:
Yuanjin Zheng;Jiangnan Yan;Yong Ping Xu
Affiliations:
Institute of Microelectronics, Agency for Science, Technology and Research, Singapore;Department of Electrical and Computer Engineering, National University of Singapore, Singapore;Department of Electrical and Computer Engineering, National University of Singapore, Singapore
Venue:
IEEE Transactions on Circuits and Systems Part I: Regular Papers
Year:
2009

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Abstract

A CMOS dB-linear variable gain amplifier (VGA) with a novel I/Q tuning loop for dc-offset cancellation is presented. The CMOS dB-linear VGA provides a variable gain of 60 dB while maintaining its 3-dB bandwidth greater than 2.5 MHz. A novel exponential circuit is proposed to obtain the dB-linear gain control characteristics. Nonideal effects on dB linearity are analyzed and the methods for improvement are suggested. A varying-bandwidth LPF is employed to achieve fast settling. The chip is fabricated in a 0.35-µm CMOS technology and the measurement results demonstrate the good dB linearity of the proposed VGA and show that the tuning loop can effectively remove dc offset and suppress I/Q mismatch effects simultaneously.