This letter presents the design and analysis of phase noise optimization of a 4-GHz differential Colpitts voltage-controlled-oscillator (VCO). A low phase noise is achieved by a Colpitts oscillator and a VCO bias optimization using an amplitude control method. The measured phase noise is −134.8dBc/Hz at 1.25MHz offset frequency from 4GHz operating frequency. The VCO is implemented using 0.24µm SiGe BiCMOS process with integrated copper inductors. The wide VCO frequency range covers both PCS and IMT bands and draws about 15.9mA from a 2.7V power supply.