A voltage controlled oscillator generating an oscillation signal according to a first control signal without a silent region. The voltage controlled oscillator includes a control signal adjuster and a plurality of delay cells. The control signal adjuster receives the first control signal and generates a second and a third control signal according to the first control signal. The voltage level of the third control signal is higher than that of the second control signal and the voltage level of the second control signal is higher than that of the first control signal. The plurality of delay cells are ring-connected and controlled by the first, the second, and the third control signals to generate the oscillation signal. Each delay cell includes three sets of current generation transistors. The three sets of current generation transistors are separately controlled by the three different control signals.