The invention generally relates to an analog varactor.
A varactor is an electronic device whose capacitance is a function of a control voltage that is applied to the device. The varactor typically is used, for example, in an oscillator to control its oscillation frequency.
As a more specific example, the oscillator may be a voltage controlled oscillator (VCO) of a phase locked loop (PLL). The VCO typically generates an oscillating output signal for the PLL, and the PLL controls the oscillation frequency of the VCO to lock the output signal onto a reference signal. In this regard, the PLL typically regulates a frequency control signal that is provided to a varactor of the VCO in a feedback loop to achieve the lock. The frequency control signal is a function of the phase difference between the reference signal and the PLL's output signal. Thus, by controlling the capacitance of the varactor with the frequency control signal, the PLL finely tunes the oscillation frequency of the VCO to achieve the lock.