The Varactor Diode

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 V_c 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