CMOS integrated LC Q-enhanced RF filters for wireless receivers
Gee, Wesley Albert
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In wireless transceiver circuits some of the most prevalent required off-chip components are discrete filters. These components are generally implemented with surface acoustic wave (SAW) or ceramic components. These devices are used in the receiver section for discrimination of incoming radio frequency (RF) signals as well as downconverted intermediate frequency (IF) signals. Presently, with the growing demand for multi-functional wireless consumer devices, the need for full integration of RF and logic circuits in wireless communications systems is becoming increasingly evident. If integrated RF filters with acceptable electrical characteristics could be realized, this might reduce or eliminate the currently required off-chip filters, prospectively decreasing the complexity, size, and cost of future wireless transceiver circuits and systems. The objective of the present research effort is to implement an integrated Q-enhanced LC bandpass filter in a prospective receiver front-end RF amplifier using the passive and active components available in a standard digital complementary metal-oxide semiconductor (CMOS) process. CMOS is the standard design medium for digital circuitry, and with the increased unity gain or transit frequency (fT) values that accompany steadily shrinking CMOS device sizes, the implementation of gigahertz frequency communications circuits in this medium is increasingly feasible. The circuit design specifically investigated in this work introduces a loss-compensated second-order gigahertz range bandpass filter implemented in a 0.18 쭠digital CMOS process provided by National Semiconductor. This filter incorporates a unique design technique that provides improvements in filter linearity through an independently variable bias level shifting method, while also facilitating prospective single-to-differential signal conversion. One distinctive characteristic of the investigated circuit, in comparison to other RF integrated filter work, is the implementation of a novel integrated transformer feedback method that facilitates magnetically coupled loss-restoration and subsequent filter Q-enhancement. Additionally, this loss restoration method is achieved using a single transistor, in contrast to the multi-transistor cross-coupled transconductor Q-enhancement technique commonly implemented in other previous and current integrated RF filter research.