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We propose a novel buffer scheme for cross-coupled differential Voltage-Controlled-Oscillator (VCO) design. Transformers are used for the new buffer configuration. The primary coil of the transformer is the inductor of the LC resonator, and the oscillation signal is extracted through the secondary. We also implement differential VCOs using a conventional buffer, and compare the performance with that of VCOs with a transformer buffer. With transformer buffers, we achieve a fivefold efficiency enhancement and 9 dBc more harmonic suppression than the conventional structure. The conventional type VCO shows an oscillation frequency of 4.4/spl sim/5.4 GHz with approximately 0 dBm output power using a total current of 15 mA. The transformer buffer VCO shows 4.7/spl sim/5.1 GHz with 6.5/spl plusmn/1 dBm output power using only 10 mA for the cross-coupled core. The 2/sup nd/ harmonic suppression of each VCO shows -10 dBc and -19 dBc, respectively. Both show a phase noise of about -111 dBc/Hz at 1 MHz offset. To the best of our knowledge, this is the first demonstration of enhancing efficiency and linearity in a VCO using transformers as a buffer stage.