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972 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 51, NO. 3, MARCH 2003 Linearity Analysis of CMOS for RF Application Sanghoon Kang, Byounggi Choi, and Bumman Kim , Senior Member, IEEE Abstract— The linearity of CMOS has been analyzed using the Taylor series. Transconductance and output conductance are two dominant nonlinear sources of CMOS. At a low frequency, the transconductance is a dominant nonlinear source for a low load impedance, but for a usual operation level impedance the output conductance is a dominant nonlinear source. Capacitances and the substrate network do not generate any significant nonlinearity, but they suppress output-conductance nonlinearity at a high frequency because output impedance is reduced by the capacitive shunts, and output voltage swing is also reduced. Therefore, above 2–3 GHz, the transconductance becomes a dominant nonlinear source for a usual load impedance. If these capacitive elements are tuned out for a power match, the behavior becomes similar to the low-frequency case. As gate length is reduced, the transconductance becomes more linear, but the output conductance becomes more nonlinear. At a low frequency, CMOS linearity is degraded as the gate length becomes shorter, but at a higher frequency (above 2–3 GHz), linearity can be improved. Index Terms— CMOS, linearity. I. INTRODUCTION T HERE IS A strong interest in using CMOS technology for RF and microwave circuits. To support the growing needs of RF circuits, the various characteristics of CMOS have been evaluated using various device models. However, they are usually small-signal behaviors, such as small-signal gain, noise figure, etc. More recently, the large signal characteristics of CMOS, such as power gain, power generation, efficiency, and mixer operation have been intensively studied. However, linearity is always one of the most important issues for RF circuit design. Some research considers only the transconductance in CMOS linearity analysis, while other effects are neglected [1]. For a short gate CMOS, however, the output conductance becomes an important nonlinear source and must be included in CMOS analysis. Capacitances and substrate effects are also far from negligible at high frequency. We have studied the nonlinear characteristics of CMOS using Taylor-series analysis and a BSIM3–based model [2]. We have analyzed the output-conductance nonlinearity of CMOS and have compared it with the transconductance nonlinearity. The linearity trend with a down-scaled device is also described in Section II. The capacitance contribution for linearity and substrate network behavior are also analyzed, and their effects are discussed in Section III. Manuscript received May 13, 2002; revised September 18, 2002. This work
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This note was uploaded on 04/02/2011 for the course ECE 264 taught by Professor Song during the Spring '11 term at UCSB.

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