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Varicap

In electronics, a varicap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode is a type of diode which has a variable capacitance that is a function of the voltage impressed on its terminals.


Applications

Varactors are used as voltage-controlled capacitors, rather than as rectifiers. They are commonly used in parametric amplifiers, parametric oscillators and voltage-controlled oscillators as part of phase-locked loops and frequency synthesizers. For example, varactors are used in the tuners of television sets to electronically tune the receiver to different stations.

Operation
Internal structure of a varicap
Operation of a varicap

Varactors are operated reverse-biased so no current flows, but since the thickness of the depletion zone varies with the applied bias voltage, the capacitance of the diode can be made to vary. Generally, the depletion region thickness is proportional to the square root of the applied voltage; and capacitance is inversely proportional to the depletion region thickness. Thus, the capacitance is inversely proportional to the square root of applied voltage.

All diodes exhibit this phenomenon to some degree, but specially made varactor diodes exploit the effect to boost the capacitance and variability range achieved - most diode fabrication attempts to achieve the opposite.

In the figure we can see an example of a crossection of a varactor with the depletion layer formed of a p-n-junction. But the depletion layer can also be made of a MOS-diode or a Schottky diode. This is very important in CMOS and MMIC technology.

See also

Heterostructure barrier varactor diode
Ferroelectric capacitor

Further reading

Mortenson, Kenneth E. (1974). Variable capacitance diodes: the operation and characterization of varactor, charge storage and PIN diodes for RF and microwave applications. Dedham, Mass.: Artech House.
Penfield, Paul and Rafuse, Robert P. (1962). Varactor applications. Cambridge, M.I.T. Press.

External links

Media related to Varicaps at Wikimedia Commons
Learning by Simulations Calculation of the characteristics of a varactor diode for various doping profiles
[1] Trimless IF VCO: Part 1: Design Considerations from Maxim.
Basics of varactor diode with design tips

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