The varactor also termed to as varicap, is a diode that behaves like a variable capacitor, with the PN junction working like the dielectric and plates of a common capacitor. Because of this, the symbol of the varactor is as shown below:
A varactor diode employs a PN junction in reverse bias and has a structure such that the capacitance of the diode varies with the reverse voltage. A voltage controlled capacitance is useful in tuning applications. The typical capacitance values are small, in the order of picofarads.
The figure below shows a typical tuning circuit where the DC voltage control Vc changes the capacitance of the varactor to form a resonant circuit.
Varactors are being used to replace the old variable capacitor tuning circuits as in television tuners.
Also read: Optoelectronics
