Varactors are operated in a reverse-biased state. 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; 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 varactor diodes are manufactured specifically to exploit this effect and increase the capacitance (and thus the range of variability), whereas most ordinary diode fabrication strives to minimize the capacitance.
A varactor diode is best explained as a variable capacitor. Think of the depletion region as a variable dielectric. The diode is placed in reverse bias. Junction capacitance is present in all reverse biased diodes because of the depletion region.
4.10 Depletion region in a reverse biased PN junction a) circuit symbol b)
characteristics of varactor diode
They are also called voltage-variable capacitance diodes. A Junction diode which acts as a variable capacitor under changing reverse bias is known as VARACTOR DIODE.A varactor diode is specially constructed to have high resistance under reverse bias. Capacitance for varactor dide are Pico farad. (10-12 ) range.
Curve between Reverse bias voltage Vr across varactor diode and total junction capacitance CT and CT can be changed by changing Vr.
· Junction capacitance is optimized in a varactor diode and is used fo r high frequencies and switching applications.
· Varactor diodes are often used for electronic tuning applications in FM radio s and televisions.
· Varactors are used as voltage-controlled capacitors. They are commmonly used in voltage-controlled oscillators, parametric amplifiers, and frequency multipliers.