easa-part-66-module-4 Varactor Diode

The varactor diode is a semiconductor, voltage-dependent variable capacitor alternatively known as varicap or voltacap or voltage-variable capacitor (VVC) diode. Basically, it is just a reversebiased junction diode whose mode of operation depends on its transition capacitance (CT). Reverse-biased junctions behave like capacitors whose capacitance is ∝ 1/VRn where n varies from 1/3 to 1/2. As reverse voltage VR is increased, depletion layer widens thereby decreasing the junction capacitance. Hence, we can change diode capacitance by simply changing VR. Silicon diodes which are optimised for this variable capacitance effect are called varactors.

Figure 4.1.1.22

Applications

Since the junction capacitance of a varactor is in the pF range, it is suitable for use in highfrequency circuits. Its main applications are as

1. automatic frequency control device, 2. FM modulator,

3. adjustable band-pass filter, 4. Parametric amplifier.