A 5.4-GHz high-Q tunable active-inductor bandpass filter in standard digital CMOS technology

A transistor-only CMOS active-inductor with an all-NMOS signal path is presented. By tuning the varactor-augmented parasitic capacitance at the only internal node the circuit losses from submicron MOSFETs can be partially or fully compensated to permit realizing unlimited values of Q, with little frequency and no power-consumption penalties. Transistor-only second-order bandpass filters using the active inductor were built in the TSMC 0.18-μm CMOS process, and high filter Q was obtained by tuning the varactor. The highest center frequency measured was f0 = 5.7 GHz for 0.2-μm gate lengths and the maximum repeatably measured Q was 665. Lower Qs can be obtained by reducing the capacitive compensation or by adjusting the circuit biasing. f0 and Q are tunable via separate varactors. IIP3 and input 1-dB compression point were simulated as 0.523 VPP and 0.128 VPP (−1.65 and −13.9 dBm from a 50-Ω source) at 5.7 GHz with Q = 100 and midband gain equal 4.7 dB. For the same conditions, the output noise and noise figure (RS = 50 kΩ) were simulated to be 0.8 μV/Hz1/2 and 25.6 dB, respectively. The filter core occupies an area of 26.6 μm × 30 μm and dissipates 4.4 mW at 5.4 GHz from a 1.8-V power supply. As the circuits use only MOSFETs they are fully compatible with standard digital CMOS processes. f0 statistics were obtained by measuring 40 chips at identical biasing condition.

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