Patent ID: 7639093

Claim:
A voltage-controlled oscillator having an output signal whose frequency is responsive to a tuning signal, comprising: a plurality of differential inverters coupled to form a loop, each differential inverter having a differential pair of transistors configured to steer a tail current from a current source transistor, each transistor in the differential pair coupling to a corresponding triode-mode transistor having a gate controlled by the tuning signal to control a frequency for the output signal, the current source transistor sourcing the tail current responsive to a feedback signal; and a control circuit comprising a first transistor matched to each of the triode-mode transistors and a second transistor matched to each of the current source transistors, wherein a first terminal of the first transistor couples to a node that also couples to a first terminal of the second transistor such that a voltage of the node equals a voltage of the first terminals, the first transistor having a gate driven by the tuning signal, the control circuit further comprising a differential amplifier configured to drive a gate of the second transistor with the feedback signal responsive to a difference between a reference signal voltage and the voltage of the node such that a current through the first and second transistor is a function of the difference between a supply voltage and the reference voltage and such that an amplitude of the output signal is independent of the tuning signal and depends on the reference signal.