Inductor-capacitor voltage-controlled oscillators (LC tank VCOs) generate signals at a particular frequency, or are able to pick out a signal at a particular frequency from a more complex signal. LC tank VCOs are employed in a broad range of integrated circuit applications including frequency tuning, voltage/current magnification, RF amplifiers, and Foster-Seeley discriminators. An LC tank VCO includes an inductor-capacitor tuned circuit (LC tank) which oscillates at a particular frequency (frequency of oscillation) determined by the characteristics of both the inductor and the capacitor. The LC tank generally includes two groups of capacitive elements (a bank of digitally selectable elements and a continuous frequency control element) so that, in operation, the approximate target frequency can be selected by the bank while the continuous frequency control element allows the LC tank to settle on a particular frequency.
However, the LC tank is not self-sustaining and loses energy over time, which results in the frequency of oscillation gradually decreasing in amplitude. To combat the loss in energy and the concomitant frequency of oscillation decrease, an LC tank VCO employs an amplifier, for example, a transconductor, which generates negative resistance and provides an amount of energy to the LC tank equal to that which is lost by the LC tank to maintain LC tank oscillation.