Active and Passive Component Notes For ECE of SSC Scientific Assistant Exam 2017

1)Active Components:An active device is any type of circuit component with the ability to electrically control electron flow (electricity controlling electricity). In order for a circuit to be properly called electronic, it must contain at least one active device.Active devices include, but are not limited to, vacuum tubes, transistors,diodes,silicon-controlled rectifiers (SCRs), and TRIACs.

a)Vacuum Tubes: The vacuum tube is a glass tube that has its gas removed, creating a vacuum. Vacuum tubes contain electrodes for controlling electron flow and were used in early computers as a switch or an amplifier. The picture shows a collection of different vacuum tubes used with different devices.Vacuum tubes were also used in radios, televisions, radar equipment, and telephone systems during the first half of the 1900s.

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b)Transistors:Transistors are active components and are found everywhere in electronic circuits. They are used as amplifiers and switching devices. As amplifiers, they are used in high and low frequency stages, oscillators, modulators, detectors and in any circuit needing to perform a function. In digital circuits they are used as switches.There is a large number of manufacturers around the world who produce semiconductors (transistors are members of this family of components), so there are literally thousands of different types. There are low, medium and high power transistors, for working with high and low frequencies, for working with very high current and/or high voltages. The most common type of transistor is called bipolar and these are divided into NPN and PNP types.Their construction-material is most commonly silicon (their marking has the letter B) or germanium (their marking has the letter A). Original transistor were made from germanium, but they were very temperature-sensitive. Silicon transistors are much more temperature-tolerant and much cheaper to manufacture.

Today’s computers use circuitry made with complementary metal oxide semiconductor (CMOS) technology. CMOS uses two complementary transistors per gate (one with N-type material; the other with P-type material). When one transistor is maintaining a logic state, it requires almost no power.

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c)Diodes:Diodes are one of the simplest, but most useful of all semiconductor devices. Many types of diode are used for a wide range of applications. Rectifier diodes are a vital component in power supplies where they are used to convert AC mains (line) voltage to DC. Zener diodes are used for voltage stabilisation, preventing unwanted variations in DC supplies within a circuit, and to supply accurate reference voltages for many circuits. Diodes can also be used to prevent disastrous damage to battery powered equipment when batteries are connected in the wrong polarity.Signal diodes also have many uses in processing signals in electronic equipment; they are used to obtain the audio and video signals from transmitted radio frequency signals (demodulation) and can also be used to shape and modify AC signal waveforms (clipping, limiting and DC restoration).

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A diode is a specialized electronic component with two electrodes called the anode and the cathode. The fundamental property of a diode is its tendency to conduct electric current in only one direction. When the cathode is negatively charged relative to the anode at a voltage greater than a certain minimum called forward breakover, then current flows through the diode. If the cathode is positive with respect to the anode, is at the same voltage as the anode, or is negative by an amount less than the forward breakover voltage, then the diode does not conduct current. This is a simplistic view, but is true for diodes operating as rectifiers, switches, and limiters. The forward breakover voltage is approximately six tenths of a volt (0.6 V) for silicon devices, 0.3 V for germanium devices, and 1 V for selenium devices.The above general rule notwithstanding, if the cathode voltage is positive relative to the anode voltage by a great enough amount, the diode will conduct current. The voltage required to produce this phenomenon, known as the avalanche voltage, varies greatly depending on the nature of the semiconductor material from which the device is fabricated. The avalanche voltage can range from a few volts up to several hundred volts.When an analog signal passes through a diode operating at or near its forward breakover point, the signal waveform is distorted. This nonlinearity allows for modulation, demodulation, and signal mixing. In addition, signals are generated at harmonics, or integral multiples of the input frequency. Some diodes also have a characteristic that is imprecisely termed negative resistance. Diodes of this type, with the application of a voltage at the correct level and the polarity, generate analog signals at microwave radio frequencies.

Diode PN Junction:When P and N type silicon are brought together during manufacture, a junction is created where the P type and N type materials meet, and holes close to the junction in the P type silicon are attracted into negatively charged N type material at the other side of the junction. Also, electrons close to the junction in the N type silicon are attracted into the positively charged P type silicon. Therefore along the junction between the P and N type silicon, a small natural potential is set up between the P and N semiconductor material with negatively charged electrons now on the P type side of the junction, and positively charged holes on the N side of the junction. This layer of opposite polarity charge carriers builds up until it is just sufficient to prevent the free movement of any further holes or electrons. Because of this natural electrical potential across the junction, a very thin layer has been formed between the P and N layers at the PN junction that is now depleted of charge carriers and so is called the Depletion Layer. When a diode is connected into a circuit therefore, no current can flow between anode and cathode until the anode is made more positive than the cathode by a forward potential or voltage(VF) at least sufficient to overcome the natural reverse potential of the junction. This value depends mainly on the materials the P and N layers of the diode are made from and the amount of doping used. Different types of diode have natural reverse potentials ranging from approximately 0.1V to 2 or 3V. Silicon PN junction diodes have a junction potential of about 0.6V to 0.7V

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Reverse Biased Diode:When the diode is reverse biased (the anode connected to a negative voltage and the cathode to a positive voltage), as shown in Fig. 2.0.6, positive holes are attracted towards the negative voltage on the anode and away from the junction. Likewise the negative electrons are attracted away from the junction towards the positive voltage applied to the cathode. This action leaves a greater area at the junction without any charge carriers (either positive holes or negative electrons) as the depletion layer widens. Because the junction area is now depleted of charge carriers it acts as an insulator, and as higher voltages are applied in reverse polarity, the depletion layer becomes wider still as more charge carriers away from the junction. The diode will not conduct with a reverse voltage (a reverse bias) applied, apart from a very small ‘Reverse Leakage Current’ (IR), which in silicon diodes is typically less than 25nA. However if the applied voltage reaches a value called the ‘Reverse Breakdown Voltage’ (VRRM) current in the reverse direction increases dramatically to a point where, if the current is not limited in some way, the diode will be destroyed.

2)Passive Components:Components incapable of controlling current by means of another electrical signal are called passive devices. Resistors, capacitors, inductors, transformers are all considered passive devices.

Resistors:A resistor is an electrical component that limits or regulates the flow of electrical current in an electronic circuit. Resistors can also be used to provide a specific voltage for an active device such as a transistor.All other factors being equal, in a direct-current (DC) circuit, the current through a resistor is inversely proportional to its resistance, and directly proportional to the voltage across it. This is the well-known Ohm’s Law. In alternating-current (AC) circuits, this rule also applies as long as the resistor does not contain inductance or capacitance.

Resistor, a basic component of electric circuits. Resistors are used to produce heat, as in an electric toaster or furnace; to produce light, as in an incandescent light bulb; to provide an electrical bypass, as in the shunt in an ammeter; to regulate the electric power entering a device, as in a light dimmer or radio volume control; and to set voltages within an electric circuit.

The resistors used in heating and lighting applications are almost exclusively metallic. Such materials as platinum, tungsten, and Nichrome are commonly used. A wire-wound resistor consists of a coil of wire made of Nichrome or a similar material wound on a ceramic core and covered with a protective ceramic material.There are several types of resistors used in electronic circuits. A carbon resistor is made of carbon mixed with a binding material such as clay and molded into a cylinder. A film-type resistor is made of a thin film of carbon, metal, or metal oxide on a ceramic base. Resistors in integrated circuits typically consist of a thin layer of semiconductor material or a thin metallic film.Resistors may be either fixed or variable. Variable resistors having two terminals are called rheostats; those having three terminals are called potentiometers.

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Conductor,Semiconductor and Insulator Notes For ECE of SSC Scientific Assistant Exam 2017

Conductors are generally substances which have the property to pass different types of energy. In the following, the conductivity of electricity is the value of interest.

Metals: The conductivity of metals is based on the free electrons (so-called Fermi gas) due to the metal bonding. Already with low energy electrons become sufficiently detached from the atoms and a conductivity is achieved.

The conductivity depends, inter alia, on the temperature. If the temperature rises, the metal atoms swing ever stronger, so that the electrons are constrained in their movements. Consequence, the resistance increases. The best conductors, gold and silver, are used relatively rare because of the high costs (gold e.g. for the contacting of the finished chips). The alternatives in the semiconductor technology for the wiring of the individual components of microchips are aluminum and copper.

Salts: In addition to metals, salts can also conduct electricity. There are no free electrons, so the conductivity depends on ions which can be solved when a salt is melting or dissolving, so that the ions are free to move

Insulators: Glass, most polymers (plastics), rubber and wood are all examples of insulators. These are materials which will refuse to carry an electric current. They are useful for jobs like coating electric wires to prevent them from ‘shorting together’ or giving you a shock. Silk and cotton are also good insulators (when they’re dry!!) and some of the mains wiring in very old houses once used them – but by modern standards this was pretty dangerous since you could get a shock when wet or a spark would set them alight when dry. Modern insulators like PVC (PolyVinylChloride) are much better and safer.Insulators are also very useful to fill the ‘gap’ in between the metal plates of a capacitor.

Semiconductors are solids whose conductivity lies between the conductivity of conductors and insulators. Due to exchange of electrons – to achieve the noble gas configuration – semiconductors arrange as lattice structure. Unlike metals, the conductivity increases with increasing temperature. Increasing temperatures leads to broken bonds and free electrons are generated. At the location at which the electron was placed, a so-called defect electron (“hole”) remains.

 

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