Varactor diode or varicap diode working and applications

Varactor Diode or Varicap
Diode Working and
Applications

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Introduction:
Varactor Diode or Varicap Diode
Working and Applications
 A Varactor diode is a P-N junction diode that changes its capacitance
and the series resistance as the bias applied to the diode is varied. The
property of capacitance change is utilized to achieve a change in the
frequency and/or the phase of an electrical circuit. Lets we have an
overview of a varactor diode, that includes working, construction,
applications and characteristics.

What is a Varactor Diode?
 Varactor diode is a one kind of semiconductor microwave solid-state device.
 Varactor diodes are also named as Varicap diodes.
 The outcome of the variable capacitance can be showed by the normal P-N
junction diodes.
 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
permits a high range of changes in capacitance.

What is a Varactor Diode?
 The symbol of the Varactor diode looks like a common PN- junction diode.
 It includes two terminals namely the cathode and the anode.
 At one end this diode is inbuilt with two lines that specifies the capacitor
symbol.

Different Types of Varactor Diodes
 The different types of Varactor diodes are available in the market such as
• Hyperabrupt Varactor diode.
• Abrupt Varactor diode.
• Gallium-arsenide Varactor diode.

Abrupt Varactor Diodes
 Abrupt varactor diodes are the more commonly used for of diode.
 Abruptness of the junction is governed by the doping concentration and
also the profile.
 For an abrupt varactor diode the doping concentration is held constant,
i.e constant doping level as far as reasonably possible.
 The abrupt varactor exhibits an inverse square law C-V function.

Abrupt Varactor Diodes
 This provides for an inverse fourth law frequency dependence.
 In applications where a linear dependence is required, a lineariser is
needed.
 It additional circuitry that may be an additional burden for some
applications, not only in terms of circuitry.
 It slower response speed caused by the lineariser.

Hyperabrupt Varactor Diodes
 Hyperabrupt junctions provide a C-V curve that has an inverse square
law curve over at least some of the characteristic.
 This provides a narrow band linear frequency variation.
 The hyperabrupt junction gives a much greater capacitance change for
the given voltage change.
 The advantages of the hyperabrupt varactor come at a cost as there is
a substantial reduction in Q when compared to abrupt varactor diode.
 The hyperabrupt diodes are generally only used at lower microwave
frequencies - up to a few GHz at most.

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.

 The capacitance of a capacitor is directly proportional to the region of the
terminals.
 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 insulating dielectric.
 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.
 If the reverse voltage of the Varactor diode is decreased, then the size
of the depletion region decreases.
 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 light weight.
• There is no useful when it is operated in forward bias.

Characteristics of Varactor Diode
 In reverse bias mode, Varactor diode enhances the capacitance
as shown in the graph below.

Applications of Varactor Diode
 The applications of Varactor Diode mainly involve within the RF design
arena.
 In this article, we are discussing about the 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.

Applications of Varactor Diode
 It is necessary to make sure the tune voltage that is, the voltage
necessary to set the diode capacitance.
 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’.
 The oscillator can be allowed by changing the’ D1’diode.
 The capacitor C1 is used to stop the reverse bias for the varactor diode.
 Neglects the diode getting short circuited through the inductor.
 The diode can be adjusted by applying bias through an R1 resistor
(isolating series resistor).

Voltage Controlled Oscillators

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.
 Varactor 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.

Choosing the Right Tuning / Varactor Diode
 Check its Power Dissipation (225 mW, 300 mW, 330 mW),
 Check its Minimum Breakdown Voltage (12 V, 14 V, 25 V, 30 V)
 Check its Nominal Diode Capacitance (2.8 pF, 22 pF, 33 pF, …)

Advantage of Using Varactor Diode
 Tuning circuits to replace the old style variable capacitor tuning of FM
radio.
 Small remote control circuits.
 Tank circuits of receiver or transmitter for auto-tuning as in case of TV.
 Signal modulation and demodulation.
 Adjusting bridge circuits.
 Adjustable bandpass filters.

Applications Of Varactor Diodes
 These diodes can be used as frequency modulators and RF phase
shifters.
 These diodes can be used as frequency multipliers in microwave
receiver.
 These diodes are used to change the capacitance in tank LC circuits.

Conclusion
 The varactor diode is also called a varicap, tuning or voltage variable
capacitor diode, is also a junction diode. In this presentation we
have seen its working, construction, applications and characteristics.

Varactor diode or varicap diode working and applications

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