Voltage controlled oscillator

A voltage controlled oscillator which performs an oscillating operation at a frequency corresponding to a control voltage, comprising:

• a voltage-current converter circuit which converts the control voltage to a control current corresponding to a voltage value thereof;
• an oscillator circuit, having a plurality of logic gates, through which an operating current corresponding to the control current generated by the voltage-current converter circuit is caused to flow, and which oscillates at a frequency corresponding to a current value thereof;
• and
• a constant current adding circuit configured to add a constant bias current to the control current, wherein the constant current adding circuit is further configured to continuously provide the constant bias current to the control current so as to maintain the oscillating operation of the oscillator when the control voltage is below a threshold voltage;
• wherein the voltage-current converter circuit is a MOS transistor which, when receives the control voltage greater than or equal to the threshold voltage, causes the control current to flow through a drain thereof, the MOS transistor is current mirror connected to one of plurality of logic gates, and
• wherein the constant current adding circuit includes a current-control MOS transistor that is directly connected to the MOS transistor of the voltage-current converter circuit and directly to the oscillator circuit, and the MOS transistor of the voltage-current converter circuit effects an operating current corresponding to the control current to flow into each of the logic gates.