In this article, We will cover Varactor Diode also known as varicap diode, we will cover their VI Characteristics, Construction, and their practical applications step by step.
What is a Varactor Diode?
A P-N junction diode which acts as a variable capacitor by changing reverse bias is known as a Varactor Diode.
- Word Varactor is given to it due to its property of varying capacitance.
- Varactor diode can only be operated in reverse bias. It acts like variable capacitance in reverse bias operation.
- Varactor Diode is also known as Tuning Diode, Variable Reactance Diode, Varicap Diode or variable capacitance diode.
- The property of varying capacitance is utilized to achieve a change in the phase of an electrical circuit or in the frequency.
Symbol of Tunnel Diode:
The Varactor diode is a two terminal device one terminal is Cathode and another Anode. Its cathode has an extra capacitor, parallel two lines show the construction of capacitor and space between them shows a dielectric medium.
Working of Varactor Diode.
this is important
- When a P-N junction is formed, the depletion layer is created in the junction area. Since in the depletion region there are no charge carriers, hence the zone acts as an insulator.
- The P-type material with holes (considered as positive) as majority carriers and N-type material with electrons (considered as negative charge) as majority carriers act as charged plates in the PN junctions. Thus the diode may be considered as a capacitor, whose n and p-region are forming opposites charge plates and with depletion region between them is acting as a dielectric, as happens in the capacitor. This is illustrated in Fig. given below.
- A varactor diode is specially constructed to have high capacitance under reverse bias, it varies its capacitance when it is applied reverse biased voltages.
- As the reverse potential increases the width of the depletion region increases, which in turn reduces the capacitance.
V-I Characteristics of Varactor Diode.
By applying a reverse voltage across varactor diode, the width Wd of the depletion layer increases. Therefore, the capacitance of the junction decreases. On the other hand, if we reduce the reverse voltage across the varactor diode, the width Wd of the depletion layer decreases. Consequently, the total capacitance of the junction is increased. Given fig: below shows the curve between reverse bias voltage VR across varactor diode and total junction capacitance CT.
- Note that capacitance can be changed simply by varying the voltage VR. For this reason, a varactor diode is also called the voltage-controlled capacitor.
Let’s see how it is constructed 😉
Construction of Varactor Diode:
Consider the PN junction, The P-type is consisting of free carriers which are called holes, and N-type is consisting of free carriers which are called electrons. Since they are free carriers, when they are brought together, a junction is formed. Because they have a tendency to merge with each other; the hole has a tendency to merge with the electron and vanish.
When this happens, the region at the junction becomes free of carriers. Thereby, it creates a potential barrier; this is called the junction potential, and this potential barrier of a junction, happens to be around 0.5V to point 0.7 volts. It starts conducting only when a junction potential is about 0.5V to 0.7 volts is applied, Until that time, current is negligible. So, this region, where it is depleted of carriers, is called the depletion region. So, every P-N junction has a depletion region in between, and this depletion region width is dependent upon the voltage.
When the junction is forward biased, the barrier potential for these carriers decreases; so there is the notable amount of free carriers moving from this (electrons and holes), due to that moment it constitutes a current; hence large current can flow.
When the junction is reversed biased; the potential barrier is increasing; the reverse bias will help this potential barrier to get increased. That means, more area in this gets depleted of carriers and this region widens. So, we can conclude that the depletion layer width is dependent on the applied voltage.
When it is forward biased, the depletion layer width is decreased, and when it is reversed biased, the depletion layer width increases. Now, since the depletion region has depleted all the carriers, we can consider this region as equivalent to two parallel plates of a capacitor within which we have sandwiched an insulator (Di-electric), No carriers exist there; so, it is an insulator. So, we can say its equivalent to a capacitor.
Application of Varactor Diode:
Voltage-Controlled Tuning Circuit:
We have learned that we can decrease or increase the junction capacitance of varactor- diode by changing the reverse bias voltages on the diode. This makes a varactor diode ideal for being used in circuits that require voltage-controlled tuning. Given Fig: below shows the use of varactor diode in a tuned circuit.
Note that the inductor is parallel with the capacitance of varactor diode. The inductor and the varactor diode form a parallel LC circuit. For normal operation, a varactor diode is always operated in reverse bias as we studied.
In fact, this condition is shown in the picture above. The resistance RW in the circuit is the resistance of inductor’s winding. This winding resistance of the inductor is in series with the potentiometer (R1).
Thus RW and R1 form a voltage divider which is used to measure the amount of reverse bias voltage across the varactor diode D1 and therefore its capacitance. By adjusting the setting of a potentiometer (R1), we can vary the capacitance of diode easily. This, in turn, changes the resonant frequency of the Inductor Capacitor LC circuit.
The resonant frequency Fr of the Inductor Capacitor LC circuit is given by:
If the amount of reverse bias on varactor diode is increased, the value of Capacitance (C) of the varactor increases. The increase in Capacitance will cause the resonant frequency Fr of the circuit to decrease.
Thus, by decreasing reverse bias will decrease the resonant frequency and vice-versa.
Advantages of Varactor Diode:
The following are the few advantages of the varactor diode.
- It has small size and portable you can use it in circuits easily.
- It is reliable you can use it to vary capacitance.
- It is very low cost, thus it can be used in various electronic and electrical applications.
- It generates very less noise as compared to others P-N junction diodes.
- It has very low power loss because varactor diode produces very low noise.
Disadvantages of Varactor Diode
The following are the few Disadvantages of the varactor diodes.
- these type of diodes are specially designed to work in the reverse biased mode, Hence they are not useful when operating them in forward bias mode.
Applications of Varactor Diode
- Varactor diodes are mostly used as tuned capacitors and they have significantly replaced mechanically tuned capacitors in various electronic applications. Hence they can be used in televisions in the resonant LC tank circuit.
- A varactor is used in Radio Receivers for tuning the circuit.
- In terms of electronic application, it is also used as a frequency multiplier.
- A varactor is helpful in frequency modulation.
- A varactor is always operated in reverse bias and it produces capacitance effect, this is because its anode and cathode terminals act as the plate of the capacitor and the free space between them acts as a dielectric material. And because of a large range of capacitance variation these diodes are used in high-frequency areas like TV tunning and FM radio operations and frequency modulation.
- They are also used in frequency control circuits.
- They are used in bandpass filters.
- Voltage control oscillators are widely used in receiving and transmitting circuits in the field of communication. And a varactor diode plays a very significant role in the construction of voltage controlled oscillator.
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