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What is a Tuning / Varactor Diode?

What is a Tuning / Varactor Diode?
A tuning diode, also known as a varactor diode, variable capacitance diode, varicap diode or variable reactance diode, is a diode that has a variable capacitance which is a function of the voltage that is impressed on its terminals. Tuning / varactor diodes are operated reverse-biased, and therefore no current flows. However, since the thickness of the depletion zone varies with the applied bias voltage, the capacitance of the diode can be made to vary. Usually, the capacitance is inversely proportional to the depletion region thickness and the depletion region thickness is proportional to the square root of the applied voltage. Therefore, the capacitance is inversely proportional to the square root of the voltage applied to the diode.

Applications for Tuning / Varactor Diodes:
Tuning / varactor diodes are often used in RF design. They provide a way of varying the capacitance within a circuit by applying a control voltage. Due to this capability, tuning / varactor diodes are widely used in the RF industry. They can be found in voltage controlled oscillators such as in a phase locked loop and are used in almost all radio, cellular and wireless receivers. They can also be found in RF filters for tuning purposes (for example, as tuners of television sets to electronically tune the receiver to different stations), parametric amplifiers, parametric oscillators and frequency synthesizers.

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