VARICAP

ANA PAULA QUADROS DE OLIVEIRA

MARRCELO MONTANHINNI DELOVA

Rondonópolis
MAY - 2012
 Ana Paula Quadros de Oliveira
Marcelo Montanhinni Delova

VARICAP

Work presented to the Professor

Carlos Beuter from the Electronics discipline

Basic of the class 2012/1, full time

of the Mechanical Engineering course.

Federal University of Mato Grosso

Rondonópolis - May 16, 2012
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SUMMARY

1 INTRODUCTIONO.......................................................................................... 4

2 VARICAP................................................................................................... 5
2.1 DEFINITION
2.2 POLARIZATIONO .
2.3 APPLICATIONS................................................. 11
2.3.1 Modulation .................................................................................. 12
3 FINAL CONSIDERATIONS ................................................................... 13
4 BIBLIOGRAPHIC REFERENCES ..................................................... 14
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1 INTRODUCTION

This work aims to present, succinctly, information

about the Varicap component. Also known as Varactor, diodes of
variable capacitance or even tuning diodes, its application became quite viable,
because variable capacitors can be replaced in electronic circuits.
As it is a semiconductor, its control can be done directly.
through external circuits, therefore, your variation system no longer depends on a
mechanical performance.

Thus, the development of the theme begins with the definition,

polarization, configuration and application of Varicap for better understanding of this
component.
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2 VARICAP

2.1 DEFINITION

Varicap is a word derived from the English words Voltage Variable.

Capacitance. As the very words of English origin say, Varicap is the
denomination given to a diode with variable capacitance by voltage, that is, as a function
of the tension to which it is subjected. This diode is also known as a varactor, varicap diode.

tuning, among others.

Braga [20--] also defines as:
Varicaps, varactors or variable capacitance diodes are
semiconductors whose capacitance depends on the applied reverse voltage.
These components are used in tuning circuits so as to
change the frequency by applying an external voltage.

Despite all diodes having a capacity that varies with

the applied voltage, the varactors are specially designed to have a
capacity is strongly dependent on voltage and are used in oscillators whose
frequency is controlled by voltage ).
The representation symbol of this component is given in figure 1, so
follow

Figure 1 - Varicap symbol

Source: Wikipedia, 2012. Available at: <http://pt.wikipedia.org/wiki/Varicap>

And varactors can be found commercially in the possible forms:

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Figure 2 - Varactor or Varicap

Source: Qariya, [20--]. Available at <http://www.qariya.com/electronics/varactor.htm>

2.2 POLARIZATION

This component, being a diode, its operation is explained

chemically as it happens in most semiconductors.
When directly polarized, the negative terminal of the source repels.
the free electrons from the N region towards the junction. These electrons meet the
holes becoming valence electrons and continue to move through the holes
no material P until they reach the other end of the crystal and flow to the terminal
positive from the source. Thus, it conducts and allows current to circulate when polarized.
reversely functions as an open switch, that is, preventing the flow of current.

Figure 3 Directly Forward Biased p-n Junction

Source: Boylestad and Nashelsky (1998, p.9)
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When reverse biased, the charge carriers move away from the
junction, forming a depletion region. The greater the polarization, the greater will be the
depletion layer. As illustrated in the figure below:

Figure 4 - Reverse-biased p-n junction

Source: Boylestad and Nashelsky (1998, p.8)

The ovulator presents a capacitance due to the carriers at its junction.

of charge separated by an insulating layer formed by the recombination of the
carriers and when subjected to a certain voltage there is a variation of these carriers
that function as a variable capacitor, that is, behave like
voltage-controlled variable capacitors.

Figure 5 - Reverse biased diode

Source: (BRAGA, [20--])
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Detailing, the charge carriers accumulated in the material and separated by the
insulating region similar to the structure of a common capacitor, Braga [20--] describes
this similarity:
[...] the location where the accumulated loads are located corresponds to the
capacitor shields and the region where we have no conduction in
the junction return corresponds to the dielectric. In a common capacitor,
the obtained capacitance depends on 3 factors: a) size of the plates,
that is, its effective surface. b) distance of separation between the
armors c) material from which the dielectric is made (dielectric constant).

Therefore, in the opposite sense, the capacitance presented will depend then
of the size of the semiconductor material used (armatures), of the separation between the regions
in which the charges accumulate and the dielectric constant of the semiconductor material used
(silicon).
In conventional capacitors, the three factors mentioned by Braga remain.
fixed, in variable capacitors it is possible to vary the distance between the plates or their
effective surface, as shown in figure 6 below:

Figure 6 - Illustration of a Variable Capacitor

Source: (BRAGA, [20--])

Novaractor, these armors are formed by charge carriers that

they can move within the material, being able to separate or bring them closer by action
by an electric field, that is, by the application of an external voltage.
If it is off (zero voltage between the anode and cathode), the carriers of
the charges of the armor attract each other, but do not fully recombine because there is a
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potential barrier at the junction, so the distance between the electrodes is at a minimum, therefore
its capacitance is maximum. Figures 7 and 8 show the above description:

Figure 7 - Illustration of the minimum and maximum capacitance of a diode

Source: (BRAGA, [20--])

In the varicap, there is a significant initial decline in capacitance ( ) with the

increase in reverse polarization. This polarization is limited to 20V, as per the

Graph in the figure below:

Figure 8 - Varicap characteristic: C (pF) versus

Source: Boylestad and Nashelsky (2006, p.589)

The maximum reverse voltage that the diode allows determines the minimum

capacitance that we can achieve in the diode:

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Figure 9 - - reverse voltage - volts

Source: Boylestad and Nashelsky (2006, p.589)

An important characteristic of this device is the frequency response.

elevated as a function of the capacitance between the semiconductor regions.
The relationship between the maximum and minimum capacitance determines the width of

range that the component can vary.

Referring to the applied reverse voltage, the transition capacitance is given
due to the division of the constant—determined by the semiconductor material and by the
manufacturing technique—(K) by the sum of the knee stress ( ) and from the tension of

reverse polarization applied ) raised to the respective index of the junction (n).

( )

Boylestad and Nashelsky (2006, p.590) say: "Capacitance as a function

of ) can be expressed in terms of C(0) (capacitance when there is no polarization) by:
()
( ) ( )
.”

We must take into account the temperature coefficient, where it occurs

capacitance variations due to a variation in temperature.
The temperature coefficient can be calculated through:

( )
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2.3 APPLICATIONS

Its use is not as simple as it seems. One cannot replace the

component of the variable by a varicap, as it needs to be biased with a certain voltage, and
having a coil or inductor in its traditional configuration, which forms the circuit
resonant, has a low resistance, causing a short circuit. The voltage would be
short-circuited by the coil losing the function of the varicap.

Figure 10 - Theoretical circuit

Source: (BRAGA, [20--])

The practical usage circuit is:

Figure 11 - Practical circuit of the Varicap

Source: (BRAGA, [20--])

Although it is little known, this device appears in many

electronic equipment, always in the RF stage, both in transmission and in
Reception is also widely used in linear modulators in general. In addition
of these applications, it appears in a myriad of others, among which:
• Parametric amplifiers - low-level microwave amplifier
of noise that uses variable reactance to amplify microwave signals;
• Voltage-controlled oscillators (VCO) - the signal information is
"transported" in the form of a frequency deviation, this modulation method
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provides better immunity to noise, allowing for optical isolation or

galvanic, between the transmitter and the receiver, without loss of information;

• Harmonic frequency generators - are class C amplifiers

specials whose voltage is 3 to 10 times the normal cut voltage. They are used for
generate a frequency that is a multiple or harmonic of a lower frequency.

2.3.1 Modulation

Modulation of an RF signal in frequency to obtain an FM signal.

Figure 12 - Application of a Varicap in modulation

Source: (BRAGA, [20--])

An audio signal is applied by varying the capacitance of the Varicap in it.

rhythm that the low-frequency signal. Thus, there is a shift in the frequency of
tuned circuit passing the audio signal frequency.

Figure 13 - Frequency Modulation

Source: (BRAGA, [20--])
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3 FINAL CONSIDERATIONS

As mentioned, the operating principle of the Varicap is quite

similar to a capacitor, but without the need for adjustments or intervention
mechanics.
Its capacitance variation occurs based on the technology of the diodes, by
transition or movement of electrons, only with the application of a reverse voltage
even from an external circuit.
Being manufactured to replace more complex components, its use
it is of paramount importance in tuning circuits.
We know that technology in the electronic field has been developing a lot.
quickly and every day new components and circuits emerge to simplify and
streamline processes.
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4 BIBLIOGRAPHIC REFERENCES

BOYLESTAD, Robert and NASHELSKY, Louis. Electronic Devices and Theory of

Circuits. Translation by Alberto Gaspar Guimarães and Luis Alves de Oliveira. 8th ed. Rio
of January: JC, 2006.

BOYLESTAD, Robert and NASHELSKY, Louis. Electronic Devices and Theory of

Circuits. Translation by Alberto Gaspar Guimarães and Luis Alves de Oliveira. 6th ed. Rio
January: JC, 1998.

BRAGA, Newton C. Varicaps. S.l: s.n. [20--]. Available I am

http://www.newtoncbraga.com.br/index.php/how-it-works/891-varicaps-art126.html
Accessed on: May 11, 2012.

BRAGA, Newton C. Calculating Varicaps. n.p.: n.p., [20--]. Available at

http://www.newtoncbraga.com.br/index.php/matematica-para-eletronica/1400-
m062.html>. Accessed on: May 11, 2012.