Methods of amplitude modulation can be put in the two categories namely Linear modulation methods and Square law modulation methods. Linear modulation method utilizes the linear region of the current voltage characteristics of the amplifying device that is transistor or electron tube. Square law modulation method utilizes the square law region of some current voltage characteristics of a diode or transistor or electron tube. A large number of linear modulation methods have been devised and have been used to varying degree. These methods are namely linear shunt plate modulation or anode choke modulation, linear series plate modulation, grid bias modulation, cathode modulation, suppressor grid modulation, screen grid modulation, collector modulation.
Square law modulation circuits make use of non linear current voltage characteristics of diodes or triodes and are in general suited for use at low voltages. Important square law modulation methods are square law diode modulation and balanced modulator.
In linear shunt plate modulation the modulation voltage is inserted in the plate circuit of a class C amplifier tuned to the carrier frequency. The plate currents of modulated amplifier tube and the modulating amplifier tube find parallel paths or shunt paths. The carrier voltage is applied at the input of the class C modulated amplifier. Linear series plate modulation makes use of a linear class C tunes amplifier as modulated amplifier. This tuned amplifier is so designed that a linear relation exists between the root mean square value of peak tank current and the plate supply voltage for constant grid excitation. The modulating voltage appears in series with the plate supply voltage of the tuned modulated amplifier.
The merits of linear series plate modulation are small distortion, high plate circuit efficiency and easy adjustments. Grid bias modulation uses a class C amplifier in which audio modulated voltage is placed in series with the fixed grid bias. In Cathode modulation the modulation voltage is introduced in the cathode circuit of the class C modulator amplifier. It has characteristics intermediate between those of grid bias modulation and plate modulation.
Suppressor grid modulation uses a pent code in a class C modulated amplifier wherein the modulating voltage is applied to the suppressor which is biased sufficiently negatively. It provides linear modulation up to eighty percent modulation depth. Screen grid modulation is applied at the screen grid. The distortion produced is low. In the collector modulation method the modulating voltage is applied in the collector circuit of tuned class C amplifier in push pull arrangement. It has the merits of high linearity of modulation, high collector circuit efficiency and high power output per transistor.
Square law modulation methods makes use of non linear current voltage characteristics of diodes or triodes and are suited only for use at low voltages. Square law diode modulation uses non linear region of the current voltage dynamic characteristics of a diode.
--
Tymon Hytem has worked in the electronics feild for the past 15 years. He enjoys helping people decide on electronic gadgets from telephones to XM Radio and choosing the perfect This article is free for republishing
Thank: http://www.articlealley.com/article_188472_48.html
The Two Methods of Amplitude Modulation and how They Work
Electrical Circuits
Electrical circuits are constructed with of a variety of electronic devices including resistors, capacitors, switches, voltage sources, and more. An electrical circuit consists of a network that loops, allowing for passage of electrical current.
Electronic circuit - An electronic circuit is an electrical circuit that is constructed with active electronic components including transistors and diodes. Transistors used to be far too large to use in a small circuit but are now much more practical in both size and function. There are many types of electronic circuits that are used in a variety of devices.
Printed Circuit Board - PCBs are a very rudimentary circuit but are extremely inexpensive and also reliable .Also referred to as printed wiring board or etched wiring board, the PCB supports and connects electronic components on a circuit by etching pathways into the board. When all electronic components are connected, the result is a Printed Circuit Assembly (PCA).
Integrated Circuit - Integrated circuits, ICs, are the likely the most important invention in our time. Microprocessors which are used in computers and cell phones are a type of integrated circuit. ICs grow smaller and smaller but are packed with more and more circuitry. They are used in everything from communication, manufacturing, computing, and transportation. Even the internet is possible because of integrated circuits.
Digital Circuit - Digital circuits, as opposed to analog circuits, are preferable because they can easily be used with computers and software. Digital circuits make it very easy to store and retrieve data without degradation of the data. Analog circuits create too much noise and therefore cannot recover or store data as well as digital circuits.
Series and Parallel Circuits - Series and parallel are the two basic ways for wiring electronic components on a circuit. Series is when components are lined up one after another, while parallel refers to placing components next to each other. A series circuit has one path for the current flow, while a parallel circuit has more than one path in order to reach all the elements.
Telecommunication Circuit - A telecommunication circuit is any circuit on which information is transmitted. Telecommunication circuits can provide one-way or two-way communication, multiple channels.
Angela Oliver is an author for HRent.com. H and R Enterprises specializes in the wholesale distribution of electronic components, including various types of circuits such as digital circuits and integrated circuits.
Thank: http://www.articlealley.com/article_122318_45.html
Understanding The High Voltage Transistor In Computer Monitor
High voltage transistors are transistors with higher specification than the normal transistor. It is usually located in the area of power transformer such as the switch mode power transformer and flyback transformer. In television and monitor that uses the flyback transformer to generate high voltage, a high voltage transistor is required to perform the job. It's location mainly beside the flyback transformer and attach to a heat sink in order to transfer heat faster. Otherwise it may blow in a very short time due to the hard work of this transistor in switching. Typical part number of high voltage transistor that used in monitor and TV is BU2508DF, C3998, C5148, C5047, C5589, C5803 and etc. Usually their rating of volt, amp and watt is in the 1500 volt and above, 10 amps, 50 watt and more. Their sizes are much bigger than the common signal transistor. If this high voltage transistor is shorted, you have to find the exact replacement part number in order for the equipment to last longer. Substitution part number usually won’t last long-perhaps in the few weeks or few months time and it will blow again unless you know the storage and the fall time of the transistor.
Higher voltage, ampere and wattage will not guarantee that your transistor will last long. Many times a bigger specification will blow in a split of seconds the moment you switch on the equipment. You have to go to that particular manufacturer to get the transistor datasheet and find out the spec of the transistor. Look for the fall time and storage time value. The original and the replacement value should have tolerance of not more than 20%. If not the replacement transistor will run hot and eventually blow the transistor. This will waste your precious time.As to why sometimes even the nearest specification transistor also won't work well is because the particular equipment such as monitor-the yoke coil and the flyback transformer is designed based on the specification of the transistor.
Thus, only original part number transistor that can be install in that particular equipment. I have seen even the original number with different batch of transistor when install in the monitor causing the display to curve in (pincushion out) and the transistor runs very hot. I've checked all the circuit and components that connected to the transistor to be okay. Even replacing the flyback transformer also won’t help. The transistor's temperature keeps rising and if I don't turn off the monitor, I believe it will eventually blow the transistor. I took out the transistor and compared with the original one and discovered that the replacement logo is a little bit different from the original transistor logo. When I called my electronic supplier about the logo, they said that the transistor that I bought is of a newer version, thought from the same factory. After long hour of searching for the original transistor, I managed to find one. Unbelievable, the original transistor that i replaced runs cool and the picture is perfectly fine. Guess what is the high voltage transistor part number? It was C5148!
In another case, there was one Monitor came in with no power symptom. Upon carefully check the high voltage area; I confirmed that only HOT shorted causing no power. After the replacement (the part number was BU2508DF), when the monitor is switch on, the power blink. I thought that there was some other bad components are causing the power to blink. I check again the power and high voltage section but just could not locate the fault. I took out the HOT that I had just replaced and realize that it was a little bit different from the original one. I took out another HOT that is exactly the same from my spare parts compartment and voila the monitor works perfectly okay. I begin to compare both HOT and you can clearly see the different from the picture shown below.
The lesson that I learned from this mistake is always must replace with an original part number (if possible) and don’t buy any components that is cheaper than the normal market price.
Jestine Yong is a electronic repairer and a writer, for more electronic repair information and tips, please visit his website site map at http://www.electronicrepairguide.com/Site-Map.html
6 Tips On Choosing Online File Storage Systems
6 Tips On Choosing Online File Storage Systems
by: Timothy Rudon
Storage systems have been in use from ancient times. People have used boxes, trunks, filing cabinets, safety deposit boxes, computer serves, floppy disks, external drives and so on to store important personal and business data. Safe storage now has new avatar online storage facilities that will securely store important information and protect the data from natural, physical, or mechanical disasters. Since the information in online storage systems is stored away from the home or business premises the security is multifold.
Online storage systems are both practical and cost effective ways of storing data. The files are stored after SSL encryption and this ensure protection of privacy. Files stored online can only be retrieved using a password and log in. Reputed storage is offered by Acpana, NovaStar Online Backup Service, connected Data Protector and Data Deposit Box. When selecting an online storage system consider: 1. How much space you need and the frequency with which you will be accessing the stored information. 2. Find out what system the online storage facility uses. Check reliability of the system. 3. Find out whether they charge a fixed amount for storage or charge depending on the amount of space you actually use. Fixed costs are ideal for large storage but if your storage needs are 500MB or less then a pay per storage plan is more feasible. 4. Always comparison shop for an online storage solution. Compare prices as well as features. 5. Read through the terms and conditions carefully and always conduct a reliability check. 6. In case the online storage offers free storage for a limited period take up the offer as this will help you understand the system as well as its pros and cons. All computer systems big or small should have a data protection plan in action. Ensure regular local back ups and a second back up in the form of an online storage facility. Log on to the World Wide Web and read unbiased reviews as well as expert tips on online storage facilities and its pros and cons. Always be an informed user of any system. According to IT experts online storage solutions are superior to traditional back ups. The main advantage of an online back up system is that files can be accessed from anywhere. However to be effective the online back up system has to be understood and implemented properly. Whether or not online back up is cost effective for every individual depends on the amount of space needed. An important concern is of course security of the data files and whether the encryption process if efficient. Before selecting an online storage system ask about: encryption methods and frequency; security measures and guarantees; whether only files that have been changed will be stored in back up or all files; how will data transfer take place and its regularity. Choose an online storage facility that is most suited to your needs and cost effective.
Bipolar Junction Transistors
Introduction
A Bipolar Transistor essentially consists of a pair of PN Junction Diodes that are joined back-to-back. This forms a sort of a sandwich where one kind of semiconductor is placed in between two others. There are therefore two kinds of Bipolar sandwich, the NPN and PNP varieties. The three layers of the sandwich are conventionally called the Collector, Base, and Emitter. The reasons for these names will become clear later once we see how the transistor works. 
Bipolar transistors, having 2 junctions, are 3 terminal semiconductor devices. The three terminals are emitter, collector, and base. A transistor can be either NPN or PNP. See the schematic representations below:
As with all semiconductors, breakdown voltage is a design limitation. There are breakdown voltages that must be taken into account for each combination of terminals. i.e. Vce, Vbe,and Vcb. However, Vce(collector-emitter voltage) with open base, designated as Vceo, is usually of most concern and defines the maximum circuit voltage.
Also as with all semiconductors there are undesireable leakage currents, notably Icbo ,collector junction leakage; and Iebo, emitter junction leakage. A typical collector characteristic curve is shown below:
The spectacular rise of the MOSFET market share during the last decade has completely removed the bipolar transistor from center stage. Almost all logic circuits, microprocessor and memory chips contain exclusively MOSFETs.
Nevertheless, bipolar transistors remain important devices for ultra-high-speed discrete logic circuits such as emitter coupled logic (ECL), power-switching applications and in microwave power amplifiers.
In this chapter we first present the structure of the bipolar transistor and show how a three-layer structure with alternating n-type and p-type regions can provide current and voltage amplification. We then present the ideal transistor model and derive an expression for the current gain in the forward active mode of operation. Next, we discuss the non-ideal effects, the modulation of the base width and recombination in the depletion region of the base-emitter junction.
Structure and principle of operation
A bipolar junction transistor consists of two back-to-back p-n junctions, who share a thin common region with width, wB. Contacts are made to all three regions, the two outer regions called the emitter and collector and the middle region called the base. The structure of an NPN bipolar transistor is shown in Figure 1 (a). The device is called "bipolar" since its operation involves both types of mobile carriers, electrons and holes.
(a) Structure and sign convention of a NPN bipolar junction transistor. (b) Electron and hole flow under forward active bias, VBE > 0 and VBC = 0.
Since the device consists of two back-to-back diodes, there are depletion regions between the quasi-neutral regions w.
The sign convention of the currents and voltage is indicated on Fig 1(a). The base and collector current are positive if a positive current goes into the base or collector contact. The emitter current is positive for a current coming out of the emitter contact. This also implies the emitter current, IE, equals the sum of the base current, IB, and the collector current, IC:
The base-emitter voltage and the base-collector voltage are positive if a positive voltage is applied to the base contact relative to the emitter and collector respectively.
The operation of the device is illustrated with Fig 1 (b). We consider here only the forward active bias mode of operation, obtained by forward biasing the base-emitter junction and reverse biasing the base-collector junction. To simplify the discussion further, we also set VCE = 0. The corresponding energy band diagram is shown in Fig 2. Electrons diffuse from the emitter into the base and holes diffuse from the base into the emitter. This carrier diffusion is identical to that in a p-n junction. However, what is different is that the electrons can diffuse as minority carriers through the quasi-neutral region in the base. Once the electrons arrive at the base-collector depletion region, they are swept through the depletion layer due to the electric field. These electrons contribute to the collector current. In addition, there are two more currents, the base recombination current, indicated on Fig 2 by the vertical arrow, and the base-emitter depletion layer recombination current (not shown).
This explains how a bipolar junction transistor can provide current amplification. If the collector current is almost equal to the emitter current, the transport factor, a, approaches one. The current gain, b, can therefore become much larger than one.
To facilitate further analysis, we now rewrite the transport factor, a, as the product of the emitter efficiency, gE, the base transport factor, aT, and the depletion layer recombination factor, dr.
Minority-carrier distribution in the quasi-neutral regions of a bipolar transistor (a) Forward active bias mode. (b) Saturation mode.
The values of the minority carrier densities at the edges of the depletion regions are indicated on the Fig 3. The carrier densities vary linearly between the boundary values as expected when using the assumption that no significant recombination takes place in the quasi-neutral regions. The minority carrier densities on both sides of the base-collector depletion region equal the thermal equilibrium values since VBC was set to zero. While this boundary condition is mathematically equivalent to that of an ideal contact, there is an important difference. The minority carriers arriving at x = wB - xp,C do not recombine. Instead, they drift through the base-collector depletion region and end up as majority carriers in the collector region.
The emitter current due to electrons and holes are obtained using the "short" diode expressions yielding:
where tr is the average time the minority carriers spend in the base layer, i.e. the transit time. The emitter current therefore equals the excess minority carrier charge present in the base region, divided by the time this charge spends in the base.
A combination of equations (11), (14) and (15) yields the transit time as a function of the quasi-neutral layer width, wB', and the electron diffusion constant in the base, Dn,B.
From this equation, we conclude that the current gain can be larger than one if the emitter doping is much larger than the base doping. A typical current gain for a silicon bipolar transistor is 50 - 150.
The base transport factor, as defined in equation (18), equals:
Variation of the minority-carrier distribution in the base quasi-neutral region due to a variation of the base-collector voltage.
A variation of the base-collector voltage results in a variation of the quasi-neutral width in the base. The gradient of the minority-carrier density in the base therefore changes, yielding an increased collector current as the collector-base current is increased. This effect is referred to as the Early effect. The Early effect is observed as an increase in the collector current with increasing collector-emitter voltage as illustrated with Figure 5. The Early voltage, VA, is obtained by drawing a line tangential to the transistor I-V characteristic at the point of interest. The Early voltage equals the horizontal distance between the point chosen on the I-V characteristics and the intersection between the tangential line and the horizontal axis. It is indicated on the figure by the horizontal arrow.
Operating in the Middle The transistor will operate very nicely if one could insure that no input voltage, i.e., signal voltage--would cause the collector current to ever operate beyond either end of the linear portion of the operating curve.
When (negative) feedback is introduced, most of these problems diminish or disappear, resulting in improved performance and reliability. There are several ways to introduce feedback to this simple amplifier, the easiest and most reliable of which is accomplished by introducing a small value resistor in the emitter circuit. The amount of feedback is dependent on the relative signal level dropped across this resistor, e.g., if the resistor value approached that of the collector load resistor, the gain would approach unity (Gv ~ 1).
From the explanation of how a Bipolar Transistor works, we can expect the main characteristic of a Bipolar Transistor to be its Current Gain value. In practice this value isn't a 'universal constant' but depends on various factors: e.g. the transistor's temperature, the size and shape of its Base region, the way it's various parts were doped to make them into semiconductors, etc.
Unijunction Transistor
The unijunction transistor (UJT) is a three terminal device with characteristics very different from the conventional 2 junction, bipolar transistor. It is a pulse generator with the trigger or control signal applied at the emitter . This trigger voltage is a fraction (n) of interbase voltage, Vbb.The UJT circuit symbol, junction schematic, and characteristic curve are shown below.
