CMOS Integrated LC RF Bandpass Filter with Transformer-Coupled Q-Enhancement and Optimized Linearity

A loss-compensated second-order radio frequency (RF) bandpass filter with tunable quality factor, Q, is implemented in a 0.18 mum standard digital CMOS process. This filter utilizes an integrated resonant tank comprised of an on-chip transformer along with parasitic and designed capacitors. The design incorporates a novel method of controlled positive feedback via the integrated transformer and a single transistor, providing an adjustable quality factor for the inherently lossy on-chip resonator. The circuit transformer feedback and the unique Q-enhancement technique facilitate increased voltage swing across the resonator and subsequent linearity improvement. The filter has a measured center frequency, f_o, of 2.14 GHz, a maximum gain of 5 dB (~10dB unbuffered), and a tunable Q of 3 to 29. With the filter adjusted for the maximum quality factor and gain, the input compression point, P_1dB, is measured at -3.5 dBm with a power dissipation of 143.1 mW and a measured input referred noise power of -144.3 dBm at a spot bandwidth of 1 Hz. As a prospective Bluetooth receiver input stage, dynamic range is measured at 59.6 dB and 80.8 dB over the filter operational and channel bandwidths of 132 MHz (84timespi/2 MHz) and 1 MHz.