Varactor diode is a reverse biased pn junction diode, whose capacitance can be varied electrically. The capacitance of the varicap diode between a and b increases as the voltage is reduced, using the variable resistor. Varactor diode working, characteristics and its applications. The disadvantage of this varactor diode modulator is that is uses a two terminal device. It always works in reverse bias condition and is a voltagedependent semiconductor device. It is well known that the operation of the pn junction depends. Macoms gaas and silicon varactor tuning diodes provide broadband performance ranging from 10 mhz to 70 ghz. Varactor diode modulator is a semiconductor diode having a variable junction capacitance. Index termsadaptive, broadband matching, diode varactor, dynamic load modulation, gan, high efficiency, high power, power amplifier pa, tunable matching. Microsemis prodiuct line inlcudes also step recovery diodes, harmonic generator varactors, impatt diodes and multiplier diodes. Introduction varactor diode is a special type of pn junction diode, in which pn junction capacitance is.
Varactor diode is known by several names as varicap, voltcap, voltage variable capacitance or tunning diode. Thus, the varactor diode modulator can only be used in limited. Varactor diodes are used as tuned capacitors and have replaced mechanically tuned capacitors in various applications. Varactor diodes quiz mcqs, varactor diodes quiz questions and answers pdf 143 to learn electronic devices courses online. The symbol of the varactor diode looks like a common pn junction diode that includes two terminals namely the cathode and the anode.
The l 1 and c 1 represent the tuned circuit of the carrier oscillator. Tunable hshaped microstrip antenna with dual feeding in these designs, the reactances x. The value of c 2 is made very large at the operating frequency so that its reactance is very low. Some of the more common specialpurpose diodes are i zener diode ii lightemitting diode led iii photo diode iv tunnel diode v varactor diode and vi shockley diode. Varactor diode doped with a very small quantity of impurities near the junction and impurity concentration increases moving away from the junction.
Varactor diode frequency multiplier and tuner application. Pdf application of varactor diodes for reflectarray phase. It may also be used to produce generation of frequency modulation. Varactor diode model figure 1 shows a commonly used lumped element varactor diode model. King varactor diode voltagecontrolled capacitor used in oscillators and detectors e. You need both of these to have a complete varactor diode. If positive potential is applied to the top terminal as shown in fig.
Jul 31, 2018 varactor diode is a reverse biased pn junction diode, whose capacitance can be varied electrically. Nov 14, 2015 page 1 1 lecture 21, slide 1 eecs40, fall 2003 prof. Lets look into the construction, working of diode, characteristics and applications of varactor diode authorstream presentation. It is used as frequency multiplier in microwave receiver lo.
It is possible to dvelop frequency doubler using microstrip elements. Each devices reflective load comprises two microstrip stubs with varactor diode, where the stubs are connected in a specific way for average losses and transmission. The variation is achieved by applying a reverse bias. The junction capacitance of varactor diode varies linearly with applied voltage.
Varactor diode free download as powerpoint presentation. In an fm modulation circuit, a variable capacitance circuit employs a transistor as a variable capacitance element. The variations in capacitance form part of the tuned circuit that is used to generate the fm signal to be transmitted. Varactor diodes are also referred to as varicaps or tuning diodes. The transfer function of amplitude modulation circuits using. The varactor phase shifter is another type of quadrature phase shifter, but in this case you are varying the imaginary impedance of the loads as opposed to the real impedance presented by the pin diode. The varactor diode symbol consists of the capacitor symbol at one end of the diode that represents the variable capacitor characteristics of the varactor diodes.
Some varactor diodes may be categorised as abrupt and hyperabrupt types these terms may sometimes be seen in the varcator data sheets the terms abrupt varactor diode and hyperabrupt varactor diode refer to the properties of the varactor diode junction. The normal operation, a varactor diode is always reverse biased. A varactor diode consists of ptype and ntype semiconductor layers sandwiched together, with the ntype layer attached to a mesa tableshaped structure. Pdf parametric amplificationmixing using the varactor diode.
Varactor diode construction, working, characteristics. A varactor diode is specially constructed to have high capacitance under reverse bias. Varactor diode definition, symbol, construction and working. The th7122 software, sw7122, is available for download from the melexis. Varactor diode d 1 is connected in series with capacitor c, across the tuned circuit. Varactor diodes are certainly employed frequently, together with a reactance modulator, to provide automatic.
As a result, when c 2 is connected in series with the lower capacitance of. Varactor diode nyamador enock seth university of mines and technology umat, tarkwa computer science and engineering 14 twitter. Semiconductor devices varactor diode tutorialspoint. Varactor diodes are specially designed to take advantage of this characteristic and are used as voltage controlled capacitors rather than traditional diodes. As a result these diodes are also referred to as varicaps, tuning diodes, voltage variable capacitor diodes, parametric diodes and variable capacitor diodes. May 12, 2014 varactor diode a microwave solidstate devices also called a parametric diode, tuning diodes or varicap diodes a nonlinear device provides a voltagedependent variable capacitance 3. Varactor diodes are also termed as varicap diodes, in fact, these days they are usually termed as varactor diodes. It is well known that the operation of the pn junction depends on the bias applied which can be either forward or reverse in characteristic. The capacitance of d 1, of course, is controlled by two factors. In electronics, a varicap diode, varactor diode, variable capacitance diode, variable reactance. The values for package inductance and capacitance,l p and c p shown are typical for an.
Due to their remarkable properties, varactor diodes now find extensi,e use everywhere from the audio frequencies to the outer reaches of the extreme microwave frequencies. Because of large range of capacitance variation these diodes are used in some of the high frequency areas like frequency modulation tv tunning and fm radio 2. Specialpurpose diodes 129 actually contain two pn junctions that are connected in reverseparallel i. In fig b, the bias on d 1 is set by the voltage divider which is made up of r 1 and r 2. Varactor diode definition, symbol, construction and. Varactor diode is a type of diode whose internal capacitance varies with respect to the reverse voltage. Usually either r 1 or r 2 is made variable so that the center carrier frequency can be adjusted over a narrow range. The varactor diode modulator circuit is shown in fig5.
Fm transmitter with varactor diode tuning presented here is a lowpower fm transmitter with varactor diode tuning using surfacemount devices smd that will be received with a standard fm radio. Even though i put the 1n4001 back in its drawer after i finished testing it last week i was able to retrieve the same diode for retesting. Design of frequency reconfigurable multiband meander antenna. Pdf varactor diodebased circuit topologies, which can act as highq. Varactor is used as frequency modulator in radios and television sets. Varactor diode is also sometimes referred to as varicap diode, tuning diode, variable reactance diode, or variable capacitance diode. The varactor diode has a parasitic resistance in series, which dissipates power. Pdf this paper deals with the largesignal analysis of parametric. If the internal varactor diode of the th7122x is used for tuning the vco at vcc 3.
There are two types of fm modulators direct and indirect. Varactor diode or varicap diode working and applications. Solutlons d017 2 july 1998 features direct fm fsk uhf modulator adjustable modulator output, description the cmx017 is a single chip uhf fm fsk transmitter that combines both the rf vco modulator and rf. The capacitor repeatedly charges and discharges under the control of the current sourcesink. Varactor spice models for rf vco applications varactor equivalent. Varactor diode symbol in general, it looks like a normal pn junction diode in which one terminal is termed as the cathode and the other terminal is termed as anode. Varactor diode is the one which works on the principle of variation in capacitance by changing the width of the depletion region of pn junction. The l1 and c1 represent the tuned circuit of the carrier oscillator. Jun 28, 2015 here c part is varied using varactor diode. The second method of direct fm involves the use of a voltage controlled oscillator, which is depicted in figure 2. Generation of frequency modulation varactor diode modulator.
Jul 09, 2016 the junction capacitance of diodes varies with the amount of reverse bias. Even though the variable capacitance effect can be exhibited by the normal diodes pn junction diodes, but, varactor diodes are preferred for giving the desired capacitance changes as they are special types of diodes. It has very fast response electrical tunning applications 1. The varactor is simply a diode, or pn junction, that is designed to have a certain amount of capacitance between junctions.
A varactor frequency multiplier is capable of higher efficiency and higher power than a resistive type frequency multiplier. Types of varactor diodes abrupt and hyper abrupt type. Typically you can achieve a range where the capacitance doubles from one voltage extreme to the. And at one end this diode is inbuilt with two lines that specify the capacitor symbol. Fig b shows the basic concept of a varactor frequency modulator. The varactor loaded dualband design is simulated where each varactor diode is simply modeled as a variable capacitance c. A number of specific types of diodes are manufactured for specific applications in this fast developing world. In view of the fact that the varactor diode is a semiconductor diode, many parameters are the same as those of other diodes. Varactor diode modulator is the direct method of fm generation wherein the carrier frequency is directly varied by the modulating signal. Another fm modulator which is widely used in transistorized circuitry uses a voltagevariable capacitor varactor. Pdf distortionfree varactor diode topologies for rf adaptivity. Jan 07, 2008 in ads you place the model parameters in a model card e. How can you use a varactor diode in the generation of fm wave.
How can you use a varactor diode in the generation of fm. A gold plated molybdenum stud is connected to ntype layer via the mesa structure and it acts as cathode terminal. By varying the applied reverse voltage, the diodes capacitance can. The form of junction in the diodes has a major impact on the properties and performance. Parametric amplificationmixing using the varactor diode. When choosing a varactor diode from the information on the datasheet, the varactor specifications need to be carefully assessed to see whether its performance meets the needs for the circuit in which it is intended for uses. Soldering surface mounted devices is not so hard and actually is quite easy. This video describes in detail, what is a varactor diode, structure of varactor diode and working of varactor diode. Application of varactor diodes for reflectarray phase control. A varactor diode is a semiconductor diode whose junction capacitance varies linearly with applied voltage when the diode is reverse biased. The values of capacitance of varactor diodes are in the picofarad 10.
Varactor modulator radio technology electronic engineering. Figure3 depicts lumped element based frequency doubler and tripler. Varactor diode in a pn junction a very thin depletion region acts as an insulator of the o rder of 5 x 10 exists on either side of the p and n regions and it separate the p and n regions are acts as conducting plates, as a result this capacitor is formed. These devices are commonly used in communication systems. A varactor diode is a semiconductor diode whose junction capacitance varies linearly with the applied voltage when the diode is reversebiased. These are specially designed to work in the reverse biased mode, it possesses the least significance when operated in forward biasing. To know the varactor diode working principle, we must know the function of capacitor and.
Us4303894a fm modulator using variable transistor capacitance. Varactor diode is a capacitor of which capacitance varies according to the change of reverse bias voltage and has advantages of small size. Varactor diode modulator electronics forum circuits. Varactor diode article about varactor diode by the free. Generally the use of a varicap diode in a circuit requires connecting it to a tuned circuit. Afc automatic frequency control where in varactor diode is used to set lo signal. In conventional junction diode, the depletion region is an area which separates the p and n material. Varactor diode working, structure and applications. When the changeover pn junction is abrupt then it is called abrupt type. In 12, an electrically small antenna esa based on the meander antenna structure is proposed. Varactor diodes are used along with reactance modulator to provide automatic frequency correction for an fm transmitter. View a of figure 215 shows the varactor schematic symbol. The varactor diode is manufactured in such as way that it shows better transition capacitance property than the ordinary diodes.
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