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Varactor Diode

A specialized diode that changes its level of capacitance depending on the level of reverse bias applied to the diode. Also known as a varicap diode.

Varactor-Diode-Structure.png

A microwave solid-state device.

Also called a parametric diode, tuning diodes or varicap diodes.

A nonlinear device

Provides a voltage-dependent variable capacitance

Features

The varactor , or variable capacitance diode is a junction diode.

1.It has the useful property that its junction capacitance is easily varied electronically by changing reverse bias on the diode.

2.It is also used for frequency modulation of oscillators.

3.The resistive cutoff frequency is often used as figure of merit

4.The capacitance is naturally inversely proportional to the width of this layer and thus be varied with changes in the bias

Types Of Varactor Diode

A hyper abrupt type :

    When the changeover p-n junction is abrupt then it is called abrupt type. When change is very abrupt, they are called hyper abrupt type. They are used in oscillators to sweep for different frequencies.

Gallium-Arsenide varactor diodes :

    The semiconductor material used is gallium arsenide. They are used for frequencies from 18 GHz up to and beyond 600 GHz.

Important Criteria

Capacitance : Capacitance of the device. Capacitance from a few picoFarads to hundreds of picoFarads is provided.

Capacitance range :

Range of capacitance produced when voltage is varied.

Voltage range :

The minimum and maximum voltage that can be applied to the device.

Bias current :

The bias is always reverse. This means that the varactor diode does not conduct electricity. If the bias is turned positive then the device will start conducting.

Other criteria to be considered include : reverse and breakdown voltage, leakage current, junction temperature.

Voltage and other transients must be avoided.  Transients can occur if the DC voltage is put off.

Characteristics

Low-noise characteristic : produce much less noise than most conventional amplifiers.

Low cost

High reliability

Light weight

Small size

Varactor-Diode-V-I-characteristics.png

Notice the nonlinear increase in capacitance as the reverse voltage is decreased

Operation

It has a p-n junction of semi-conducting material and is always reverse biased.

construction-of-varactor-diode.png

electrical-equivalent-of-varactor-diode.png

varactor-diode1.png

The depletion zone depends on the applied voltage and this makes the capacitance vary with the applied voltage.

Voltage variable capacitance of reverse biased junction.

They have nonlinearity of capacitance which is just fast enough to follow microwaves.

Losses in the nonlinear element is almost negligible.

The junction capacitance depends on the applied voltage and junction design.

In some cases a junction with fixed reverse bias may be used as a capacitance of a fixed value.

With a reverse bias, the junction is depleted by mobile carriers resulting in capacitance.

Width of the depletion region increases with reverse bias & capacitance decreases as reverse bias increases.

Avalanche region is never used as it is likely to destroy the device.

Diffused junction MESA are widely used as capable of handling large powers and large reverse breakdown voltage and have low noise.

Freq limit of SI diodes is up to 25GHz.

Varactors made up of GaAs(over 90GHz) & better functioning at lowest temperatures.

Manufacturing techniques are easier for Si.

•Variation in Cj is most important characteristic.

•There are parasitic resistances, capacitances & conductances associated with every practical encapsulated diode.

•The diode encapsulation contains electrical leads attached to wafer and low loss ceramic cases as a mechanical support to a wafer.

•The parasitics should be kept as low as possible.

For many applications there should be a large capacitance variation and small value of minimum capacitance & series resistance

Application

FREQUENCY MULTIPLIERS -

  • used in applications where it’s difficult to generate microwave signals.
  • Producing relatively high power outputs at frequencies up to 100GHz.
  • Does not have gain ; in fact, it produces a signal power loss,Output can be as high as 80% of the input

PARAMETRIC/ REACTANCE AMPLIFIERS. -

  • Named for the time-varying parameter, or value of capacitance or reactance, associated with the operation.

TUNING -

  • Since the frequency can be made to vary they are used as electronic tuning devices in tuners for television, mobiles.

Other Applications:

  •   PLL, AM radios
  •    voltage controlled oscillators
  •    harmonic generation
  •   Electronic tuning devices in tuners for television, mobiles, parametric amplification,, voltage-variable tuning, frequency multipliers, etc.

References

  • WikiNote Foundation
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Created by Sujit Wagh on 2017/10/26 18:33
Translated into en by Sujit Wagh on 2017/10/26 18:47