Varicap
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A varicap diode, varactor diode or tuning diode is a type of diode used in electronic circuits.
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[edit] Applications
Varactors are commonly used in parametric amplifiers, parametric oscillators and voltage-controlled oscillators as part of phase-locked loops and frequency synthesisers.It is principally used as a voltage-controlled capacitor, and its diode function is secondary.
[edit] Operation
Varactors are operated reverse-biased so no current flows , but since the width of the depletion zone varies with the applied bias voltage, the capacitance of the diode can be made to vary. Generally, the depletion region width is proportional to the square root of the applied voltage; and capacitance is inversely proportional to the depletion region width. 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.
[edit] Non diode varicaps
Not all varactors are formed by diodes. In CMOS processes, varactors can be formed by placing a heavily positively-doped region (called an P+ implant) inside a lightly positively-doped region (called an PWELL.) The capacitance of these junctions behaves similarly to that of an NMOS transistor, which has an N+ implant inside a lightly negatively-doped region (called a NWELL) - which also forms a P-N junction device.
[edit] External links
- 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.
