Varactor diode or varicap diode working and applications

Varactor Diode or Varicap Diode Working

and Applications

http://www.elprocus.com/

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.

https://plus.google.com/u/0/+Elprocus/posts

https://www.facebook.com/elprocus

https://twitter.com/elprocus

https://www.youtube.com/user/elprocus



ApplicationsWhat 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.







ApplicationsWhat 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.







ApplicationsDifferent 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.







ApplicationsAbrupt 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.







ApplicationsAbrupt 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.







ApplicationsHyperabrupt 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.







ApplicationsWorking 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.







ApplicationsWorking 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.







ApplicationsWorking 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.







ApplicationsCharacteristics 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.







ApplicationsCharacteristics of Varactor Diode In reverse bias mode, Varactor diode enhances the capacitance

as shown in the graph below.







ApplicationsApplications 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.







ApplicationsApplications 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.







ApplicationsVoltage 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).







ApplicationsVoltage Controlled Oscillators







ApplicationsRF 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.







ApplicationsChoosing 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, …)







ApplicationsAdvantage 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.







ApplicationsApplications 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.

https://www.elprocus.com/series-and-parallel-lc-circuit-resonance/







ApplicationsConclusion

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.

https://www.elprocus.com/varactor-diode-working-and-applications/












Engineering

Varactor diode or varicap diode working and applications