Varactor
A varactor is a nonlinear device that provides a capacitance that varies responsive to an applied control voltage. A varactor is, essentially, a variable voltage capacitor. The capacitance of a varactor, when within its operating parameters, decreases as a voltage applied to the device increases. Variable reactors (varactors) are essential for the design of key radio frequency (RF) CMOS and BiCMOS circuits, and are specifically used as tuning elements in voltage controlled oscillators (VCOs), phase shifters, and frequency multipliers. Oscillator circuits, including voltage controlled oscillator circuits, are well known. Typically, an oscillator circuit includes at least one active device and a resonator circuit coupled to the active device. The resonator is coupled to the active device such that the active device is capable of oscillating only at a specific frequency determined by the resonator. A voltage controlled oscillator is an oscillator that generates an output signal with a frequency that varies in response to an input control voltage. In a VCO, the frequency of oscillation is controlled by electronically altering the point of resonance of the resonator circuit. Typically, a variable resonator circuit includes at least one variable capacitance diode (varactor). The frequency of operation is set by applying a bias voltage to the varactor to alter the capacitance of the varactor diode. This change in capacitance alters the net reactance of the resonator and alters the frequency of operation of the oscillator circuit. Varactors often comprise multiple voltage-tunable capacitor cells, with each cell having a capacitive range, wherein the net capacitive range of the varactor is substantially equal to a sum of the capacitive ranges of the individual capacitor cells. Varactor diodes are operated in a reverse-bias operating mode. When the diode is reverse-biased, the capacitance of the pn junction decreases with an increasing reverse voltage. An important characteristic of the varactor diode is that the diode presents a capacitance which varies as a result of applying a variable bias voltage to a depletion region of the diode.
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