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 μm 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.