Varactor Diodes

       The varactor, also called a varicap, tuning or voltage variable capacitor diode, is 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 width “W” becomes narrower. This depletion region is devoid of majority carriers and acts like an insulator preventing conduction between the N and P regions of the diode, just like a dielectric, which separates the two plates o f a capacitor. 
Fig 1 Depletion region in a reverse biased PN junction

       The varactor diode with its symbol is shown in Fig. 2.
Fig. 2 circuit symbol of varactor diode

       As the capacitance is inversely proportional to the distance between the plates (CT α I/W), the transition capacitance CT varies inversely with the reverse voltage. Consequently, an increase in reverse bias voltage will result in an increase in the depletion region width and a subsequent decrease in transition capacitance CT. At zero volt, the varactor depletion region W is small and the capacitance is large at approximately 600 pF. When the reverse bias voltage across the varactor is 15 V, the capacitance is 30 pF.
       The varactor diodes are used in FM radio and TV receivers, AFC circuits, self adjusting bridge circuits and adjustable bandpass filters. With improvement in the type of materials used and construction, varactor diodes find application in tuning of LC resonant circuit in microwave frequency multipliers and in very low noise microwave parametric amplifier  


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