Definition: The Varactor diode is a type of diode whose internal capacitance varies with the reverse voltage. The varactor diode operates in reverse bias and it is voltage-dependent semiconductor device used for storing charge. The voltage-dependent device refers to a diode whose output is dependent on its input voltage.
The varactor diode is a device used in situations where variable capacitance is required, which is controlled by applying voltage. This type of diode is also known as a varicap, voltcap, voltage variable capacitance, or tuning diode.
Symbol of Varactor Diode
The symbol of a varactor diode looks similar to that of a PN-junction diode. It has two terminals – an anode and a cathode. One end of the symbol represents the diode, while the other end has two parallel lines that depict the conductive plates of the capacitor. The gap between these plates indicates their dielectric. The symbol of the varactor diode is shown in the image below.
Working of Varactor Diode
A Varactor diode is composed of two types of semiconductor material – n-type and p-type. In an n-type semiconductor material, electrons are the majority charge carriers, while in a p-type material, holes are the primary carriers. When these two materials are combined, a p-n junction is formed, and a depletion region is created at the PN junction. The depletion region is composed of positive and negative ions and blocks current from entering the PN region.
The primary purpose of a varactor diode is to store charges; hence, it is always operated in reverse bias conditions. If a forward bias voltage is applied, the electric current begins to flow, and the depletion region becomes much smaller. This is undesirable because the smaller depletion region prevents ions from moving from one place to another, and the diode does not store the charge.
The formula below provides the capacitance value of the diode.
The capacitance of a p-n junction diode is inversely proportional to the width of its depletion layer. This means that if the depletion layer is narrow, the capacitance will be high and vice versa. The reverse bias voltage should be reduced to increase the capacitance of a varactor diode. This will decrease the width of the depletion layer, resulting in a higher capacitance. Conversely, increasing the reverse bias voltage will decrease the capacitance. The main advantage of a varactor diode over a normal variable capacitor is its ability to provide different capacitance values by changing the applied voltage.
Characteristic of Varactor Diode
The graph below shows the characteristic curve of the varactor diode. It indicates that increasing reverse bias voltage results in an increased depletion region and decreased diode capacitance.
Advantages of Varactor Diode
The advantages are listed below.
- It generates less noise than the other diode.
- Cheaper and more reliable.
- It is characterized by its small size and lightweight.
Varactor diode in tuning Circuit
The parallel resonance circuit consists of Varactor diodes D1 and D2, which provide variable capacitance. Vc controls the diodes’ reverse voltage. The circuit diagram is given below.
The varactor diodes D1 and D2 are connected in the series. The total capacitance of these diodes is.
The tuning frequency is,
The circuit’s inductance (L) is measured in Henry, while the resonant frequency is expressed as C1, and C2 represents the maximum voltage-dependent capacitance of the diode.