Definition from

A semiconductor diode that exhibits change in capacitance with a change in applied voltage; used as a voltage-variable capacitor.

Varactor diodes are a specialized form of the common PN junction diode which exhibit a controllable capacitance. Varactor diodes are also referred to as Varicap diodes (Philips) and as Tuning Diodes. The schematic symbol for a varactor looks like a diode with a capacitor at the tip (cathode end).

All diodes (as well as most semiconductor devices) exhibit some level of capacitance. Usually this capacitance is not a desired property as in most cases it has a negative effect on switching speed especially in power MOSFETS. In the case of diodes the capacitance tends to vary relative to the applied reverse voltage. The increase is not linearly related to the applied voltage but does tend to increase as the applied voltage increases. While standard PN junction diodes can be used as a replacement for varactors they are not designed to operate in this manner, and the voltage/capacitance ratio is not always reliable. In addition, the range of capacitance values can be rather small. Better results can be had with a device designed to function as a variable capacitance diode. The capacitance of a varactor is usually in the picofarad range and can typically be varied by 20 picofarads or more.

The most common use for a varactor is in a the construction of a voltage controlled oscillator or VCO. One common configuration is to use two varactors in combination with an inductor to form a voltage variable tank circuit.

Figure 1 shows a tunable tank circuit constructed with varactor diodes and an inductor. The reference voltage is used to modulate the resonant frequency of the tank. If the tank is used as part of an oscillator then the reference voltage would also modulate the output frequency of the oscillator. This confuguration is often used as part of the circuitry for modulating and demodulating signals for FM radio.

    |                |
    |                |
    |     D1   D2    |
    |        |
   GND       |Reference Voltage

Figure 1: Voltage variable LC tank

Please note that in the configuration above two diodes are needed. If only one diode were used then it would become forward biased whenever the forward voltage across it increased above the minimum forward voltage drop of the part. This would remove energy from the tank and dampen oscillation quite rapidly.

Straw, R.D. Ed. The ARRL Handbook for Radio Amateurs. Newington, CT: ARRL, 1999

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