A transformer-based LC tank with differential-turned structure and the differential-tuned voltage controlled oscillator using the LC tank are provided to improve the quality factor of the resonant cavity by applying the transformer to the LC tank resonant cavity. A differential tuning voltage controlled oscillator comprises a current source portion(110), the first active portion(121), the second active portion(122), and the LC tank resonant cavity. The LC tank resonant cavity generates the resonant frequency and is located between the first active portion and the second active portion. The LC tank resonant cavity comprises the transformer and the first to fourth varactors(Cv11,Cv12,Cv21,Cv22). The first and second varactors are serially connected and the capacitance is controlled into the positive direction. The capacitance of the third and fourth varactors is controlled into the negative direction.
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