The first method can be accomplished by the use of a reverse biased diode, since the capacitance of such a diode varies with applied voltage. There are two types of fm modulators direct and indirect. Generation of frequency modulation varactor diode modulator. The capacitance of d 1, of course, is controlled by two factors. The free electrons and holes always try to move from a higher concentration region to a lower concentration region. Varactor theory free download as powerpoint presentation. A varactor is a diode for which its capacitance depends nonlinearly on the voltage applied to its terminals, and thus it is a nonlinear capacitor. The varactor diode modulator circuit is shown in fig5. Varactor modulator radio technology electronic engineering. This concept is modified to create a digital modulator as shown in fig. The different types of varactor diodes are available in the market such as hyper abrupt, abrupt and galliumarsenide varactor diodes. 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 is a pn junction diode whose capacitance is varied by varying the reverse voltage.
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. Pdf distortionfree varactor diode topologies for rf adaptivity. Before going to varactor diode, lets first take a look at the capacitor. The l1 and c1 represent the tuned circuit of the carrier oscillator.
Pdf varactor diodebased circuit topologies, which can act as highq. The symbol of the varactor diode is shown in the above figure that includes a capacitor symbol at one end of the diode that signifies the characteristics of the variable capacitor of the varactor. For free electrons, nregion is the higher concentration region and pregion is the lower. A varactor diode is specifically designed for this.
Varactor diode working, characteristics and its applications. The varactor diode, sometimes called tuning diode, is the diode whose. Varactor diodes are used along with reactance modulator to provide automatic frequency correction for an fm transmitter. Semiconductor devices varactor diode this is a special pn junction diode with an inconsistent concentration of impurities in its pn materials. 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. As a result these diodes are also referred to as varicaps, tuning diodes, voltage variable capacitor diodes, parametric diodes and variable capacitor diodes. Figure 1 shows a direct frequency modulator which uses a varactor diode. For comprehensive details of different types of diodes, visit this page. Varactor diode construction, working, characteristics. Varactor diodes are also termed as varicap diodes, in fact, these days they are usually termed as varactor diodes. Varactor diode is a reverse biased pn junction diode, whose capacitance can be varied electrically. In frequency modulators they can be placed across the resonant element within the generator and the audio applied to the diode.
Linear gain voltage controlled oscillator with modulation compensation. Semiconductor devices varactor diode tutorialspoint. In an fm modulation circuit, a variable capacitance circuit employs a transistor as a variable capacitance element. In a normal pn junction diode, doping impurities are. Usually either r 1 or r 2 is made variable so that the center carrier frequency can be adjusted over a narrow range. Varactor diode is also sometimes referred to as varicap diode, tuning diode, variable reactance diode, or variable capacitance diode. As an alternative, it is possible to realize dielectricfree actuation electrodes. View and download powerpoint presentations on varactor diode ppt. Varactor diodes ideal for vco, vcxo, voltage controlled. A varactor diode also known with the names varicap diode, varactor diode, tuning diode is a pn junction diode which acts as a variable capacitor under varying reverse bias voltage across its terminals. A varactor diode is a semiconductor diode whose junction capacitance varies linearly with the applied voltage when the diode is reversebiased. The voice signal is applied from an input terminal 1 to the varactor diode 6 through a capacitor 2 and a. Varactor diodes are certainly employed frequently, together with a reactance modulator, to provide automatic.
A varactor diode is a semiconductor diode whose junction capacitance varies linearly with applied voltage when the diode is reverse biased. Fig b shows the basic concept of a varactor frequency modulator. Us4303894a fm modulator using variable transistor capacitance. It may also be used to produce generation of frequency modulation. Varactor diode definition, symbol, construction and. Infineon has more than 30 years development expertise for the rf small signal discretes.
Distortionfree varactor diode topologies for rf adaptivity. 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. Find powerpoint presentations and slides using the power of, find free. The vco has an input voltage of 2v and is running at its free running.
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