Varactor Diode or Varicap Diode Working and Applications

In general, electronic circuits can be built with various electrical and electronic components like resistors, capacitors, diodes, transistors, integrated circuits, transformers, Thyristors, etc. Let us discuss the diode which is a two-terminal electrical device. The V-I characteristics of the diode are non-linear and it permits the flow of current in only one direction In forward bias mode, the diode allows the flow of current and offers very low resistance. Likewise, in the reverse bias mode, the diode blocks the current flow and offers very high resistance. There are different types of diodes are available in the market based on the working principle and characteristics such as tunnel diodes, Zener diodes, constant-current diodes, Varactor diodes, photodiodes, laser diodes, etc. Here, this article discusses an overview of a varactor diode, that includes working, construction, applications, and characteristics.

What is a Varactor Diode?

Varactor diode is one kind of semiconductor microwave solid-state device and the applications of this diode mainly involve in where variable capacitance is preferred which can be accomplished by controlling voltage. These diodes are also named as varicap diodes. Even though the outcome of the variable capacitance can be shown by the normal P-N junction diodes, but these diodes are chosen for giving the desired capacitance changes as they are special types of diodes. Varactor diodes are specifically fabricated and optimized such that they permit a high range of changes in capacitance.

Varactor Diode
Varactor Diode

The different types of Varactor diodes are available in the market such as hyper abrupt, abrupt and gallium-arsenide Varactor diodes. The symbol of the Varactor diode is shown in the above figure that includes a capacitor symbol at one end of the diode that signifies the characteristics of the variable capacitor of the Varactor diodes.

The symbol of the Varactor diode looks like a common PN- junction diode that includes two terminals namely the cathode and the anode. And at one end this diode is inbuilt with two lines that specify the capacitor symbol.

Working of a Varactor Diode

To know the Varactor diode working principle, we must know the function of capacitor and capacitance. Let us consider the capacitor that comprises of two plates alienated by an insulator as shown in the figure.

We know that the capacitance of a capacitor is directly proportional to the region of the terminals, as the region of the terminals increases the capacitance of the capacitor increases. When the diode is in the reverse biased mode, where the two regions of P-type and N-type are able to conduct and thus can be treated as two terminals. The depletion area between the P-type & N-type regions can be considered as the insulating dielectric. Therefore, it is similar to the capacitor shown above.

Working of a Varactor Diode
Working of a Varactor Diode

The volume of the depletion region of the diode varies with change in reverse bias. If the reverse voltage of the diode is increased, then the size of the depletion region increases. Likewise, if the reverse voltage of the Varactor diode is decreased, then the size of the depletion region decreases. Hence, by changing the reverse bias of the diode the capacitance can be changed.

Characteristics of Varactor Diode

The characteristics of Varactor diode have the following:

  • These diodes significantly generate less noise compared to other diodes.
  • The cost of these diodes is available at lower and more reliable also.
  • These diodes are very small in size and very lightweight.
  • There is no use when it is operated in forwarding bias.
  • In reverse bias mode, the Varactor diode enhances the capacitance as shown in the graph below.
Characteristics of Varactor Diode
Characteristics of Varactor Diode

Applications of Varactor Diode

The applications of Varactor Diode mainly involve within the RF design arena, However, in this article, we are discussing a couple of applications of Varactor diodes, to illustrate how these diodes can be used in a practical. The capacitor in a practical circuit can be changed with the Varactor diode, but it is necessary to make sure the tuning voltage that is, the voltage necessary to set the diode capacitance. And to ensure that this diode is not influenced by the bias voltage in the circuit. By using voltage control technique in the diode circuit, changing capacitance can be offered.

Voltage Controlled Oscillators

Consider the circuit of VCO designed by using varactor diode ‘D1’ as symbolized in the figure. The oscillator can be allowed by changing the’ D1’ diode. The capacitor C1 is used to stop the reverse bias for the varactor diode, also neglects the diode getting short-circuited through the inductor. The diode can be adjusted by applying bias through an R1 resistor (isolating series resistor).

Varactor Diode in VCOs
Varactor Diode in VCOs

RF Filters

The varactor diodes can be used in the RF filters to tune. In the receive front end circuits, tracking filters may be crucial. These diodes allow the filters to follow the frequency of the incoming received signal which can be restricted using a control voltage. Usually, this is offered by microprocessor control through the DAC. A few of the main applications of Varactor diodes can be listed below:

  • These diodes can be used as frequency modulators and RF phase shifters.
  • These diodes can be used as frequency multipliers in microwave receivers.
  • These diodes are used to change the capacitance in-tank LC circuits.

Do you know any other diodes that are regularly used in real-time electrical and electronics projects Then, please give your feedback by commenting in the comments section below. Here is a question for you, what is the function of the Varactor diode?

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