What is a Varactor?


A varactor is a diode whose junction capacitance can be altered with an applied reverse voltage.

For this reason, a varactor is also called a variable capacitance diode (also it's called a varicap).

A varactor's capacitance varies as the applied reverse voltage to it changes. As the applied reverse voltage increases, the width of its junction increases, which decreases its capacitance. Conversely, as the applied reverse voltage decreases, the width of its junction decreases, which increases capacitance. The typical capacitance range for varactors ranges from a few picofarads to over 100pF, with a maximum reverse voltage range from a few volts to close to a hundred volts, depending on the varactor. Check your manufacturer's datasheet for exact specifications for the varactor in use.

The low capacitance levels provided by a varactor usually limit its use to high-frequency RF circuits, where the applied voltage is used to change the capacitance of an oscillator circuit. The reverse voltage may be applied via a tuning potentiometer, which acts to change the overall frequency of an oscillator. Therefore, varactors have widespread use in tuning circuits where a change in capacitance tunes the circuit.

To see how varactors are used in tuning circuits, see Varactor Tuning Circuit. Tuning circuits are one of the most major applications that varactors are used for.

Related Resources

Types of Diodes

How to Test a Diode

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