A voltage-controlled oscillator comprises a variable inductor, a negative impedance circuit, an operating voltage source and a ground point. The variable inductor comprises a transformer and a transistor switch, the transformer comprising a primary side coil and a secondary side coil, the primary side coil comprising a first coil and a second coil, and the secondary side coil comprising a third coil and a fourth coil. The transistor switch is connected in parallel with the primary side coil to adjust an inductance value of the variable inductor based on a gate voltage. The negative impedance circuit is connected in parallel with the secondary side coil to compensate the power consumption of the voltage-controlled oscillator during oscillation. The operating voltage source is electrically connected between the third coil and the fourth coil, and the ground point is electrically connected between the first coil and the second coil.