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This brief presents a K-band differential Vackar voltage-controlled oscillator (VCO) with gate inductive feedback which enhances negative impedance and thus simplifies the startup condition. Simple analysis and simulations examine the transistor loading effect and amplitude stability. Results indicate that the Vackar VCO has improved amplitude stability compared to the Colpitts VCO. The improved amplitude stability is favorable for suppressing amplitude-to-phase noise conversion. The Vackar VCO was implemented in a 0.13- RF CMOS process. The oscillation frequency ranged from 19 to 19.95 GHz. The measured phase noise at 1-MHz offset was 103 dBc/Hz at 19.5 GHz with a figure of merit of 182 dB.