Design of linear transconductance elements in CMOS technology

Abstract

Transconductance elements are useful active building blocks in fully-monolothic analog signal-processing systems. Efforts of the present research have been to develop, fabricate, and experimentally evaluate some novel circuit implementations of linearized CMOS transconductance elements, and to obtain, thereby, a better understanding of the characteristics, potential capabilities, and limitations of these circuits in continuous-time applications. The focus has been on general methods that cancel out the dominant nonlinearities inherent in MOS device characteristics, resulting in transconductors exhibiting good linearity over a wide input voltage range. It is shown that many reported schemes, as well as the proposed circuits, can be derived within a general systematic framework. Simple MOS device models are used to arrive at and analyze initial circuit configurations. Second-order effects due to deviations from the simple model are studied both analytically and with extensive circuit simulation. Practical tradeoffs between important specifications such as linearity and frequency response are examined. The effects of and limitations due to modelling inaccuracies, device mismatches and parasitics on the above performance measures are studied. To demonstrate the practical utility of transconductance elements, test circuits such as filters and amplifiers using these elements as basic building blocks have been fabricated in a standard CMOS process and tested. The test circuits are designed to operate at frequencies beyond 100 KHz and into the MHz range. The results demonstrate the viability of realizing transconductance-based analog signal-processing circuits at high frequencies where an alternative to sampled-data techniques is most needed.