In general, we frequently use multiple number of electrical and electronics components to design electronic circuits. These basic components include resistors, capacitors, diodes, transistors, integrated circuits, thyristors, transformers, and so on. Let us consider diode which is a two-terminal semiconductor solid state electronic device. Diode exhibits nonlinear V-I characteristics and it allows the current to flow only in one direction – during forward bias in which it offers very low resistance. Similarly, in the other direction it blocks the flow of current during reverse bias in which it offers very high resistance (almost infinite resistance).
Based on the principle of working and the characteristics of diodes, they are classified into various types such as zener diodes, generic diodes, constant-current diodes, light emitting diodes, tunnel diodes, varactor diodes, peltier diodes, laser diodes, photodiodes, etc. Here, in this article let us discuss in detail about the varactor diode circuit working and characteristics.
What is a Varactor Diode?
The varactor diode was named because of the variable reactor or variable reactance or variable capacitor or variable capacitance property of these diodes. A varactor diode is considered as a special type of diode that is widely used in the electronics industry and is used in various electronics applications. Varactor diode is also a semiconductor microwave solid-state device, it is frequently used in applications where variable capacitance is desired which can be achieved by controlling voltage.
Varactor Diode Basics
Varactor diodes are also termed as varicap diodes, in fact, these days they are usually termed as varactor diodes. Even though the variable capacitance effect can be exhibited by the normal diodes (P-N junction diodes), but, varactor diodes are preferred for giving the desired capacitance changes as they are special types of diodes. These diodes are specially manufactured and optimized such that they enables a very high range of changes in capacitance. Varactor diodes are again classified into various types based on the varactor diode junction properties. And, these are termed as abrupt varactor diodes, gallium-arsenide varactor diodes, and hyperabrupt varactor diodes.
Varactor Diode Symbol
The symbol of varactor diode is shown in the figure. The varactor diode symbol consists of the capacitor symbol at one end of the diode that represents the variable capacitor characteristics of the varactor diodes.
In general, it looks like a normal PN- junction diode in which one terminal is termed as the cathode and the other terminal is termed as anode. Here, varactor diode consists of two lines at one end (cathode end of normal diode) that indicates the capacitor symbol.
Varactor Diode Working
To understand the working principle of the varactor diode, we must know what is a capacitor and how can we change the capacitance. Let us consider the capacitor that consists of two plates separated by an insulating dielectric as shown in the figure.
We know that the capacitance of an electrical capacitor is directly proportional to the area of the plates, as the area of the plates increases the capacitance of the capacitor increases. Consider the reverse biased mode of the diode, in which P-type region and N-type region are able to conduct and thus can be treated as two plates. The depletion region between the P-type and N-type regions can be considered as insulating dielectric. Thus, it is exactly similar to the capacitor shown above.
The size of the depletion region of diode changes with change in reverse bias. If the varactor diode reverse voltage is increased, then the depletion region size increases. Similarly, if the varactor diode reverse voltage is decreased, then the depletion region size decreases or narrows.
Hence, by varying the reverse bias of the varactor diode the capacitance can be varied.
Varactor Diode Characteristics
The varactor diodes have the following significant characteristics:
- Varactor diodes produces considerably less noise compared to other conventional diodes.
- These diodes are available at low costs.
- Varactor diodes are more reliable.
- The varactor diodes are small in size and hence, they are very light weight.
- There is no useful purpose of varactor diode operated when it is operated in forward bias.
- Increase in reverse bias of varactor diode increases the capacitance as shown in the figure below.
Varactor Diode Applications
Varactor diodes finds large number of applications within the RF design arena and it is difficult to discuss about all the applications of varactor diodes or varicap diodes. However, in this article, we are discussing about the worth providing (couple of applications) applications of varactor diodes, to show how the varactor diodes can be used in a practical circuit. The capacitor in a circuit can be replaced with the varactor diode, but it is essential to ensure the tune voltage, i.e. the voltage required to set the varactor diode capacitance. And ensure that varactor diode is not affected by the bias voltage or any other voltages in the circuit. By using voltage control technique in the varactor diode circuit, varying capacitance can be provided.
Voltage Controlled Oscillators (VCOs)
Consider the voltage controlled oscillator circuit designed using varactor diode D1 as shown in the figure. The oscillator can be enabled by tuning the diode D1. The capacitor C1 is used to prevent the reverse bias for the varactor diode, also avoids the diode getting short circuited through the inductor. The varactor diode can be tuned by applying tuning voltage or bias through an isolating series resistor R1.
The varactor diodes can be used to tune the filters. In the receive front end circuits, tracking filters may be essential. The varactor diodes enables the filters to track the incoming received signal frequency which can be controlled using a control voltage. Usually, this is provided by microprocessor control through the digital to analogue converter.
A few other important applications of varactor diodes can be listed as follows:
- Varactor diodes can be used as frequency modulators.
- In microwave receiver LO, varactor diodes can be used as frequency multipliers.
- Varactor diodes can be used as RF phase shifters.
- Varactor diodes are used to vary the capacitance in variable resonant tank LC circuits.
Do you know any other special diodes that are frequently used in real time electronics applications? Then, please share and improve the knowledge of other readers by posting your answers in the comments section below.