Varactor Diode or Varicap Diode Working and Applications

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Varactor Diode or Varicap Diode Working and
Applications
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Varactor Diode or Varicap Diode Working and
Applications
Introduction

• 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.

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Varactor Diode or Varicap Diode Working and
Applications
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.

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Varactor Diode or Varicap Diode Working and
Applications
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.

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Varactor Diode or Varicap Diode Working and
Applications
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.

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Varactor Diode or Varicap Diode Working and
Applications
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.

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Varactor Diode or Varicap Diode Working and
Applications
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.

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Varactor Diode or Varicap Diode Working and
Applications
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.

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Varactor Diode or Varicap Diode Working and
Applications
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.

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Varactor Diode or Varicap Diode Working and
Applications
Working of a Varactor Diode

• 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.

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Varactor Diode or Varicap Diode Working and
Applications
Working of a Varactor Diode

• 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.

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Varactor Diode or Varicap Diode Working and
Applications
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.

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Varactor Diode or Varicap Diode Working and
Applications
Characteristics of Varactor Diode

• In reverse bias mode, Varactor diode enhances the
  capacitance as shown in the graph below.

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Varactor Diode or Varicap Diode Working and
Applications
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.

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Varactor Diode or Varicap Diode Working and
Applications
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.

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Varactor Diode or Varicap Diode Working and
Applications
Voltage Controlled Oscillators

• Consider the circuit of VCO designed by using
  varactor diode D1.
• The oscillator can be allowed by changing the
  D1diode.
• 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).

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Varactor Diode or Varicap Diode Working and
Applications
Voltage Controlled Oscillators
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Varactor Diode or Varicap Diode Working and
Applications
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.

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Varactor Diode or Varicap Diode Working and
Applications
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, )

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Varactor Diode or Varicap Diode Working and
Applications
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.

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Varactor Diode or Varicap Diode Working and
Applications
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.

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Varactor Diode or Varicap Diode Working and
Applications
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.

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