Disclosed is a voltage-controlled oscillator (VCO) including at least an inductor-capacitor (LC) resonant circuit (including varactors that receive a variable input voltage), cross-coupled transistors connected to the LC resonant circuit, and an LC filter connected to a shared source node of the cross-coupled transistors. The cross-coupled transistors can have back gates connected to receive a variable back gate bias voltage (Vbg), which is dependent on Vin to ensure that an optimal relationship between the oscillating frequency (.sub.0) of the LC resonant circuit and the resonant frequency (.sub.1) of the LC filter is continuously maintained to minimize phase noise. For example, if Vin is increased to increase varactor capacitance and, thereby decrease .sub.0, then Vbg is also increased, thereby increasing the voltage (Vs-s) and the capacitance (Cs-s) on the shared source node connected to the LC filter, decreasing .sub.1, and maintaining an optimal relationship of .sub.0=.sub.1/2.

The present disclosure provides methods and apparatus for dynamically adjusting the common mode voltage at the LC tank node and/or the power supply voltage of a VCO with an LC resonator in order to force oscillation start-up by temporarily increasing gain. Methods according to certain preferred embodiments may reduce power consumption and/or overcome threshold voltage limitations and/or increase frequency and frequency tuning range during normal (steady-state) operation.

A voltage controlled oscillator includes a cross coupled pair circuit, a first capacitive circuit, a first inductor, a second inductor, a second capacitive circuit and a first switch circuit. The first capacitive circuit is arranged in parallel with the cross coupled pair circuit. The first inductor is arranged in parallel with the cross coupled pair circuit and has a center tapped node for receiving a power supply voltage. The second inductor is magnetically coupled with the first inductor and has a center tapped node for receiving a first control voltage. The second capacitive circuit is arranged in parallel with the second inductor. The first switch circuit is arranged in parallel with the second inductor and configured to be turned on or turned off by a second control voltage. The first control voltage and the second control voltage change synchronously.

A clock generation circuit has a voltage-controlled oscillator that includes a first transistor pair coupled in series between a power rail and ground, a replica transistor pair coupled in series between a reference node and ground, a current source having an output coupled to the reference node. The current source is coupled to a control signal that determines amplitude of current flowing through the replica transistor pair. A voltage regulator has an input coupled to the reference node and an output coupled to the power rail. The voltage regulator is configured to maintain the power rail at a voltage level defined by the voltage level of the reference node. Each transistor in the replica transistor pair is collocated on an integrated circuit with and has a same type as a corresponding transistor in the first transistor pair.

A voltage controlled oscillator includes a cross coupled pair circuit, a first capacitive circuit, a first inductor, a second inductor, a second capacitive circuit and a first switch circuit. The first capacitive circuit is arranged in parallel with the cross coupled pair circuit. The first inductor is arranged in parallel with the cross coupled pair circuit and has a center tapped node for receiving a power supply voltage. The second inductor is magnetically coupled with the first inductor and has a center tapped node for receiving a first control voltage. The second capacitive circuit is arranged in parallel with the second inductor. The first switch circuit is arranged in parallel with the second inductor and configured to be turned on or turned off by a second control voltage. The first control voltage and the second control voltage change synchronously.