Varactor diode Operation and Characteristics:
The varactor diode, also called a varicap,tuning or voltage variable capacitor diode,is also a junction diode with a small impurity dose at its junction,which has the useful property that its junction or transition capacitance is easily varied electronically.
When any diode is reverse biased , a depletion region is formed,as seen in fig 1.The larger the reverse bias applied across the diode ,the width of the depletion layer "W" becomes wider.Conversely,by decreasing the reverse bias voltage ,the depletion region becomes narrower.This depletion region is devoid of majority carriers and acts like insulator preventing conduction between the N and P regions of the diode ,just like a dielectric between two plates of a capacitor.
As the capacitance is inversely proportional to the distance between the plates ,the transition capacitance varies inversely with the reverse voltages .Consequently ,an increase in reverse bias voltage will result in an increase in the depletion region width and a subsequent decrease in transition capacitance.At zero volt ,the varactor depletion region is small and capacitance is large at approximately 600pF.When the reverse bias voltage across the varactor diode is 15V ,the capacitance is 30pF.
Varactor diode Applications :
- Varactor diode is used in FM radio and TV receivers .
- Varactor diode is used in AFC circuits,self adjusting bridge circuits and adjustable bandpass filters
- Varactor diode is used in tuning of LC resonant circuit in microwave frequency multipliers and in very low noise microwave parametric amplifiers.