Varicap

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Varicap diode
Varactor.svg
Internal structure of a varicap
Type Passive
Pin configuration anode and cathode
Electronic symbol
Varicap symbol.svg

In electronics, a varicap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode is a type of diode designed to exploit the voltage-dependent capacitance of a reverse-biased p–n junction. [1]

Electronics physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter

Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. The identification of the electron in 1897, along with the invention of the vacuum tube, which could amplify and rectify small electrical signals, inaugurated the field of electronics and the electron age.

Diode electronic component

A diode is a two-terminal electronic component that conducts current primarily in one direction ; it has low resistance in one direction, and high resistance in the other. A diode vacuum tube or thermionic diode is a vacuum tube with two electrodes, a heated cathode and a plate, in which electrons can flow in only one direction, from cathode to plate. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals. Semiconductor diodes were the first semiconductor electronic devices. The discovery of asymmetric electrical conduction across the contact between a crystalline mineral and a metal was made by German physicist Ferdinand Braun in 1874. Today, most diodes are made of silicon, but other materials such as gallium arsenide and germanium are used.

Capacitance ability of a body to store an electrical charge

Capacitance is the ratio of the change in an electric charge in a system to the corresponding change in its electric potential. There are two closely related notions of capacitance: self capacitance and mutual capacitance. Any object that can be electrically charged exhibits self capacitance. A material with a large self capacitance holds more electric charge at a given voltage than one with low capacitance. The notion of mutual capacitance is particularly important for understanding the operations of the capacitor, one of the three elementary linear electronic components.

Contents

Applications

Varactors are used as voltage-controlled capacitors. They are commonly used in voltage-controlled oscillators, parametric amplifiers, and frequency multipliers. [2] Voltage-controlled oscillators have many applications such as frequency modulation for FM transmitters and phase-locked loops. Phase-locked loops are used for the frequency synthesizers that tune many radios, television sets, and cellular telephones.

Capacitor electrical component used to store energy for a short period of time

A capacitor is a passive two-terminal electronic component that stores electrical energy in an electric field. The effect of a capacitor is known as capacitance. While some capacitance exists between any two electrical conductors in proximity in a circuit, a capacitor is a component designed to add capacitance to a circuit. The capacitor was originally known as a condenser or condensator. The original name is still widely used in many languages, but not commonly in English.

Voltage-controlled oscillator

A voltage-controlled oscillator (VCO) is an electronic oscillator whose oscillation frequency is controlled by a voltage input. The applied input voltage determines the instantaneous oscillation frequency. Consequently, a VCO can be used for frequency modulation (FM) or phase modulation (PM) by applying a modulating signal to the control input. A VCO is also an integral part of a phase-locked loop.

In electronics, a frequency multiplier is an electronic circuit that generates an output signal whose output frequency is a harmonic (multiple) of its input frequency. Frequency multipliers consist of a nonlinear circuit that distorts the input signal and consequently generates harmonics of the input signal. A subsequent bandpass filter selects the desired harmonic frequency and removes the unwanted fundamental and other harmonics from the output.

The varicap was developed by the Pacific Semiconductor subsidiary of the Ramo Wooldridge Corporation who received a patent for the device in June 1961. [3] The device name was also trademarked as the "Varicap" by TRW Semiconductors, the successor to Pacific Semiconductors, in October 1967. This helps explain the different names for the device as it came into use.[ clarification needed ]

Operation

Operation of a varicap. Holes are blue, electrons are red, depletion zone is white. The electrodes are at the top and bottom. Varactor function.svg
Operation of a varicap. Holes are blue, electrons are red, depletion zone is white. The electrodes are at the top and bottom.

Varactors are operated in a reverse-biased state, so no DC current flows through the device. The amount of reverse bias controls the thickness of the depletion zone and therefore the varactor's junction capacitance. Generally, the depletion region thickness is proportional to the square root of the applied voltage, and capacitance is inversely proportional to the depletion region thickness. Thus, the capacitance is inversely proportional to the square root of applied voltage.

Square root inverse operation of square for finding the original base number

In mathematics, a square root of a number a is a number y such that y2 = a; in other words, a number y whose square (the result of multiplying the number by itself, or yy) is a. For example, 4 and −4 are square roots of 16 because 42 = (−4)2 = 16. Every nonnegative real number a has a unique nonnegative square root, called the principal square root, which is denoted by a, where √ is called the radical sign or radix. For example, the principal square root of 9 is 3, which is denoted by 9 = 3, because 32 = 3 · 3 = 9 and 3 is nonnegative. The term (or number) whose square root is being considered is known as the radicand. The radicand is the number or expression underneath the radical sign, in this example 9.

All diodes exhibit this variable junction capacitance, but varactors are manufactured to exploit the effect and increase the capacitance variation.

The figure shows an example of a cross section of a varactor with the depletion layer formed of a p–n junction. This depletion layer can also be made of a MOS or a Schottky diode. This is important in CMOS and MMIC technology.

MOSFET transistor used for amplifying or switching electronic signals

The metal-oxide-semiconductor field-effect transistor is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, whose voltage determines the conductivity of the device. This ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals. A metal-insulator-semiconductor field-effect transistor or MISFET is a term almost synonymous with MOSFET. Another synonym is IGFET for insulated-gate field-effect transistor.

Schottky diode semiconductor diode formed by the junction of a semiconductor with a metal, semiconductor diode with a low forward voltage drop

The Schottky diode, also known as Schottky barrier diode or hot-carrier diode, is a semiconductor diode formed by the junction of a semiconductor with a metal. It has a low forward voltage drop and a very fast switching action. The cat's-whisker detectors used in the early days of wireless and metal rectifiers used in early power applications can be considered primitive Schottky diodes.

CMOS technology for constructing integrated circuits

Complementary metal–oxide–semiconductor (CMOS) is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits. CMOS technology is also used for several analog circuits such as image sensors, data converters, and highly integrated transceivers for many types of communication. Frank Wanlass patented CMOS in 1967 while working for Fairchild Semiconductor.

Use in a circuit

Tuning circuits

Generally the use of a varicap diode in a circuit requires connecting it to a tuned circuit, usually in parallel with any existing capacitance or inductance. [4] A DC voltage is applied as reverse bias across the varicap to alter its capacitance. The DC bias voltage must be blocked from entering the tuned circuit. This can be accomplished by placing a DC blocking capacitor with a capacitance about 100 times greater than the maximum capacitance of the varicap diode in series with it and by applying DC from a high impedance source to the node between the varicap cathode and the blocking capacitor as shown in the upper left circuit in the accompanying diagram.

Example circuits using varicaps Varicap Ccts.jpg
Example circuits using varicaps

Since no significant DC current flows in the varicap, the value of the resistor connecting its cathode back to the DC control voltage resistor can be somewhere in the range of 22 kΩ to 150 kΩ and the blocking capacitor somewhere in the range of 5100 nF. Sometimes, with very high-Q tuned circuits, an inductor is placed in series with the resistor to increase the source impedance of the control voltage so as not to load the tuned circuit and decrease its Q.

Another common configuration uses two back-to-back (anode to anode) varicap diodes. (See lower left circuit in diagram.) The second varicap effectively replaces the blocking capacitor in the first circuit. This reduces the overall capacitance and the capacitance range by half, but has the advantage of reducing the AC component of voltage across each device and has symmetrical distortion should the AC component possess enough amplitude to bias the varicaps into forward conduction.

When designing tuning circuits with varicaps it is usually good practice to maintain the AC component of voltage across the varicap at a minimal level, usually less than 100 mV peak to peak, to prevent changing the diode capacitance too much, which would distort the signal and add harmonics.

A third circuit, at top right in diagram, uses two series-connected varicaps and separate DC and AC signal ground connections. The DC ground is shown as a traditional ground symbol, and the AC ground as an open triangle. Separation of grounds is often done to (i) prevent high-frequency radiation from the low-frequency ground node, and (ii) prevent DC currents in the AC ground node changing bias and operating points of active devices such as varicaps and transistors.

These circuit configurations are quite common in television tuners and electronically tuned broadcast AM and FM receivers, as well as other communications equipment and industrial equipment. Early varicap diodes usually required a reverse voltage range of 033 V to obtain their full capacitance ranges, which were still quite small, approximately 110 pF. These types were – and still are – extensively used in television tuners, whose high carrier frequencies require only small changes in capacitance.

In time, varicap diodes were developed which exhibited large capacitance ranges, 100500 pF, with relatively small changes in reverse bias: 05 V or 012 V. These newer devices allow electronically tuned AM broadcast receivers to be realized as well as a multitude of other functions requiring large capacitance changes at lower frequencies, generally below 10 MHz. Some designs of electronic security tag readers used in retail outlets require these high capacitance varicaps in their voltage-controlled oscillators.

Australian market band I-III-U television tuner with varicaps highlighted TVTunerM 01.jpg
Australian market band I-III-U television tuner with varicaps highlighted
Consumer AM-FM broadcast tuner with varicaps highlighted AMFMM 01.jpg
Consumer AM-FM broadcast tuner with varicaps highlighted

The three leaded devices depicted at the top of the page are generally two common cathode connected varicaps in a single package. In the consumer AM/FM tuner depicted at the right, a single dual-package varicap diode adjusts both the passband of the tank circuit (the main station selector), and the local oscillator with a single varicap for each. This is done to keep costs down two dual packages could have been used, one for the tank and one for the oscillator, four diodes in all, and this is what was depicted in the application data for the LA1851N AM radio chip. Two lower-capacitance dual varactors used in the FM section (which operates at a frequency about one hundred times greater) are highlighted by red arrows. In this case four diodes are used, via a dual package for the tank / bandpass filter and a dual package for the local oscillator.

Switching

Special types of varicap diode exhibiting an abrupt change in capacitance can often be found in consumer equipment such as television tuners, which are used to switch radio frequency signal paths. When in the high capacitance state, usually with low or no bias, they present a low impedance path to RF, whereas when reverse biased their capacitance abruptly decreases and their RF impedance increases. Although they are still slightly conductive to the RF path, the attenuation they introduce decreases the unwanted signal to an acceptably low level. They are often used in pairs to switch between two different RF sources such as the VHF and UHF bands in a television tuner by supplying them with complementary bias voltages.

Harmonic multiplication

In some applications, such as harmonic multiplication, a large signal amplitude alternating voltage is applied across a varicap to deliberately vary the capacitance at signal rate to generate higher harmonics, which are extracted through filtering. If a sine wave current of sufficient amplitude is applied driven through a varicap, the resultant voltage gets "peaked" into a more triangular shape, and odd harmonics are generated.

This was one early method used to generate microwave frequencies of moderate power, 12 GHz at 15 watts, from about 20 watts at a frequency of 3400 MHz before adequate transistors had been developed to operate at this higher frequency. This technique is still used to generate much higher frequencies, in the 100 GHz 1 THz range, where even the fastest GaAs transistors are still inadequate.

Substitutes for varicap diodes

All semiconductor junction devices exhibit the effect, so they can be used as varicaps, but their characteristics will not be controlled and can vary widely between batches.

Popular makeshift varicaps include LEDs, [5] 1N400X series rectifier diodes, [6] Schottky rectifiers and various transistors used with their collector-base junctions reverse biased, [7] particularly the 2N2222 and BC547.[ clarification needed ] Reverse biasing the emitter-base junctions of transistors also is quite effective as long as the AC amplitude remains small. Maximum reverse bias voltage is usually between 5 and 7 Volts, before the avalanche process starts conducting. Higher-current devices with greater junction area tend to possess higher capacitance. The Philips BA 102 varicap and a common zener diode, the 1N5408, exhibit similar changes in junction capacitance, with the exception that the BA 102 possesses a specified set of characteristics in relation to junction capacitance (whereas the 1N5408 does not) and the "Q" of the 1N5408 is less.

Before the development of the varicap, motor driven variable capacitors or saturable-core reactors were used as electrically controllable reactances in the VCOs and filters of equipment like World War II German spectrum analyzers.

See also

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Heterostructure barrier varactor

The heterostructure barrier varactor (HBV) is a semiconductor device which shows a variable capacitance with voltage bias, similar to a varactor diode. Unlike a diode, it has an anti-symmetric current-voltage relationship and a symmetric capacitance-voltage relationship, as shown in the graph to the right. The device was invented by Erik Kollberg together with Anders Rydberg in 1989 at Chalmers University of Technology.

This article provides a more detailed explanation of p–n diode behavior than that found in the articles p–n junction or diode.

The following outline is provided as an overview of and topical guide to electronics:

References

  1. Sedra, Adel; Smith, Kenneth (2010). Microelectronic circuits (6th ed.). New York: Oxford University Press. p. 214. ISBN   9780195323030.
  2. Calvert, James (15 February 2002). "Varactors". Dr Tuttle's Home Page. Retrieved 23 January 2017.
  3. ‹See Tfd› US 2989671, ‹See Tfd› Barnes, Sanford H.&John E. Mann,"Voltage sensitive semiconductor capacitor",published 23 May 1958,issued 20 June 1961, assigned to Pacific Semiconductors, Inc.
  4. Varactor Circuits http://www.radio-electronics.com/info/data/semicond/varactor-varicap-diodes/circuits.php
  5. LEDs as Varicaps http://www.hanssummers.com/varicap/varicapled.html
  6. Rectifier Diodes As Varicaps http://www.hanssummers.com/varicap/varicapdiode.html
  7. John Linsley Hood (1993). The Art of Linear Electronics. Elsevier. p. 210. ISBN   978-1-4831-0516-1.

Further reading