A CMOS monolithic image-reject filter

A CMOS inductorless image-reject filter based on active RLC circuitry is discussed and designed with the emphasis on low-noise, low-power, and gigahertz-range circuits. Two Q-enhancement techniques are utilized to circumvent the low Q characteristics inherent in the simple feedback circuit. The frequency tuning is almost independent of Q tuning, facilitating the design of the automatic tuning circuitry. The stability and the tuning scheme of the filter are also discussed. Simulations using 0.6 mu m CMOS technology demonstrate the feasibility of the tunable image-reject filter for GSM wireless applications. Simulation results show 4.75 dB voltage gain, 9.5 dB noise figure, and -20 dBm IIP3 at a passband centered at 947 MHz. The image signal suppression is 60 dB at 1089 MHz and the power consumption is 27 mW.