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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