The varactor diode is reverse biased P-N junction whose capacitance is change in accordance with the applied reverse voltage across its terminal. The varactor diode is an electronics small component in which having two terminals one is anode and other is cathode. The diode can only work in one direction so it is called unidirectional component. In this tutorial we will discuss about variant of the diode.
“It is a variable capacitor type diodes whose capacitance value is changes with respect to voltage in a circuit and it is also known as varicap. It is a voltage dependent capacitor.”
What is Varactor Diode?
Varactor diode is the one of the types of diode whose internal capacitance or junction capacitance is varied in accordance with the applied voltage. This diode is always works in reverse bias direction. The varactor diode is also known as Voltcap, Varicap, Tunning diode or Voltage variable capacitance.
This diode is use when fixed capacitor is replaced by variable capacitors. The main advantage is that the capacitance is changes with change in voltage across its terminals. We will learn more about its operation in the following sections.
Symbol of Varactor Diode
The symbol of varactor diode is shown below. It has two terminals one is anode terminal and other is cathode terminal. In the figure the one end show the diode and other end shows the two parallel lines that means conductive plates of the capacitor and the gap is dielectric material.
Construction
The varactor diode is a two layer of P type and N type of material. The P type is attached with gold plated molybdenum stud (which acts as anode) via a gold wire and N type is attaching another gold plated molybdenum stud. This is enclosed by a ceramic layer. This structure is shown in diagram below.
The silicon material is employed in P and N layer. The type of material is depends on which type of application are uses. For low frequency application the silicon is use and for high frequency the gallium arsenide is use.
Working of Varactor Diode
This diode is made of P and N type material. The N has an electron majority carrier and P has a holes majority carrier. When these two layers are joining together the PN junction is created. The depletion region is from by positive and negative ions. Depletion region blocks current to enter from the PN-region.
This diode is operated in reverse region only, the region behind it, the current does not flow. Hence form depletion region. If it is connected in forward bias the current flow from anode to cathode and the width of depletion region are decreases. The ions cannot moves from one region to another region because depletion layer does not allow it. This diode is use for storing the charge.
The capacitance of varactor diode is given below,
Where,
ε – Permittivity of semiconductor material,
A – Area of PN-junction,
W – Width of depletion layer,
The capacitance of varactor is depending on the width of n and the p-type region. When capacitance is increase the more charge is store in diode. When the more charge is required we decrease the width of depletion region.
Characteristic of Varactor Diode
The characteristic of varactor diode is shown in the figure below. This show the how capacitor value change according to voltage, when reverse bias voltage increases depletion region increases and capacitance of the diode reduces. It is a voltage controlled capacitor. Sometime it is also known as varicap diode.
Varactor Diode Formulae
Capacitance of Varactor
Cj = CK/(Vb−V) m
Quality factor of the Varactor
Q=F/f
- Cj → Diode capacitance.
- C → Diode capacitance when no voltage across it.
- V→ The applied voltage.
- Vb → Barrier voltage at the junction.
- m → Constant depending upon the material.
- K → Constant equal to 1.
- F → Maximum operating frequency.
- f → Operating frequency.
Equivalent Circuit
For designing the equivalent circuit of varactor diode, firstly we need to break down in electrical parameters like capacitor, resistor and inductor. The figure shows below, the approximate equivalent circuit of varactor or varicap diode under the reverse bias condition.
- RR – Reverse bias resistance
- RG– Geometric resistance of varactor.
- CJ– Junction capacitance, CJ is dependent on the reverse bias voltage.
- LF– Effective inductance and depends on varactor operated frequency.
Advantages of Varactor
The advantage of varactor is given below-
- It generates less noise as compared to the other diode.
- It is economical and reliable.
- The varactor is small in size and light in weight.
Applications Varactor
Varactor is a import in our daily life. Some applications of varactor are given below.
- It is used in RF design arena and provides a method of varying the capacitance.
- It provides them with special capability due to which varactor diodes are used in the RF industry.
- It is possible to tune filters by using varactor diodes. Tracking the frequency of incoming received signals.
- Voltage-controlled oscillators are used for many applications and oscillators within a phase-locked loop are the major region. Voltage-controlled oscillators are present in almost all wireless receivers and radio.
People also Asked on Varactor
What is abrupt varactor diode?
Abrupt varactor is a inverse square of the capacitor voltage function and it has constant doping concentration.
What is a hyper abrupt varactor diode?
Hyper-abrupt varactor diode is used as high capacitance for the change of voltage.
Which layer can be used for the detection of bad frames?
The detection of bad frames link layer is used.
What is the major use of varactor diode?
The major use of varactor is in tuning circuit.
What happens to plate separation when the depletion region in the varactor diode widens?
As the plate separation increases the depletion region in the varactor widens.
Which type of diode is commonly used in electronic tuners in TVs?
Varactor diode is mostly used diode in electronic tuners in TVs.
Also read- Zener diode, photodiode, Schmitt trigger, full wave rectifier, Schottky diode.
