Gm-enhanced differential colpitts VCO

This paper presents a novel gm-enhanced differential Colpitts voltage-control oscillator (VCO) that requires a lower start-up current. The central frequency is 1.8-GHz and the tuning range is about 15%. The tank inductors were designed with a quality factor of 9. The proposed gm-enhanced differential Colpitts VCO was compared with conventional cross-coupled VCO as well as differential Colpitts VCO, and demonstrated 3.5 dBc/Hz and 1.9 dBc/Hz better phase noise performances over the two. Comparison of the three VCOs was conducted at 2.5-V supply voltage and 3-mA biasing current, using 0.18-μm standard CMOS process. The proposed VCO showed a phase noise of -128.9 dBc/Hz at 1-MHz offset.