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This paper proposes a gm-boosted differential drain-to-source feedback Colpitts voltage-controlled oscillator (VCO) that allows larger output oscillation amplitude while keeping the switching transistor from triode-mode operation such that lower optimal close-in phase noise can be obtained by delaying the AM-to-FM conversion by the switching transistors. The phase-noise behavior of the proposed VCO is analyzed and compared with previously reported gm-boosted Colpitts and conventional NMOS-only LC-VCOs, and it is demonstrated in comparison that the proposed VCO can provide 2-and 4-dB lower optimum figure of merit, respectively. The feedback capacitor ratio effect of the proposed VCO is also explored and shown the directions of the capacitor ratio for better phase noise. The pro posed VCO is implemented in 0.18-μm CMOS for 1.76-1.93-GHz operation. Measurement shows, at 1.84 GHz, the phase noise of -104 and -126 dBc/Hz (FOM = 190 dBc/Hz) at 100-kHz and 1-MHz offset, respectively, while dissipating 1.5 mA from 0.9-V supply.