Category: basic Electronics

Basic Electronics – Semiconductors ”; Previous Next A semiconductor is a substance whose resistivity lies between the conductors and insulators. The property of resistivity is not the only one that decides a material as a semiconductor, but it has few properties as follows. Semiconductors have the resistivity which is less than insulators and more than conductors. Semiconductors have negative temperature co-efficient. The resistance in semiconductors, increases with the decrease in temperature and vice versa. The Conducting properties of a Semiconductor changes, when a suitable metallic impurity is added to it, which is a very important property. Semiconductor devices are extensively used in the field of electronics. The transistor has replaced the bulky vacuum tubes, from which the size and cost of the devices got decreased and this revolution has kept on increasing its pace leading to the new inventions like integrated electronics. The following illustration shows the classification of semiconductors. Conduction in Semiconductors After having some knowledge on the electrons, we came to know that the outermost shell has the valence electrons which are loosely attached to the nucleus. Such an atom, having valence electrons when brought close to the other atom, the valence electrons of both these atoms combine to form “Electron pairs”. This bonding is not so very strong and hence it is a Covalent bond. For example, a germanium atom has 32 electrons. 2 electrons in first orbit, 8 in second orbit, 18 in third orbit, while 4 in last orbit. These 4 electrons are valence electrons of germanium atom. These electrons tend to combine with valence electrons of adjoining atoms, to form the electron pairs, as shown in the following figure. Creation of Hole Due to the thermal energy supplied to the crystal, some electrons tend to move out of their place and break the covalent bonds. These broken covalent bonds, result in free electrons which wander randomly. But the moved away electrons creates an empty space or valence behind, which is called as a hole. This hole which represents a missing electron can be considered as a unit positive charge while the electron is considered as a unit negative charge. The liberated electrons move randomly but when some external electric field is applied, these electrons move in opposite direction to the applied field. But the holes created due to absence of electrons, move in the direction of applied field. Hole Current It is already understood that when a covalent bond is broken, a hole is created. Actually, there is a strong tendency of semiconductor crystal to form a covalent bond. So, a hole doesn’t tend to exist in a crystal. This can be better understood by the following figure, showing a semiconductor crystal lattice. An electron, when gets shifted from a place A, a hole is formed. Due to the tendency for the formation of covalent bond, an electron from B gets shifted to A. Now, again to balance the covalent bond at B, an electron gets shifted from C to B. This continues to build a path. This movement of hole in the absence of an applied field is random. But when electric field is applied, the hole drifts along the applied field, which constitutes the hole current. This is called as hole current but not electron current because, the movement of holes contribute the current flow. Electrons and holes while in random motion, may encounter with each other, to form pairs. This recombination results in the release of heat, which breaks another covalent bond. When the temperature increases, the rate of generation of electrons and holes increase, thus rate of recombination increases, which results in the increase of densities of electrons and holes. As a result, conductivity of semiconductor increases and resistivity decreases, which means the negative temperature coefficient. Intrinsic Semiconductors A Semiconductor in its extremely pure form is said to be an intrinsic semiconductor. The properties of this pure semiconductor are as follows − The electrons and holes are solely created by thermal excitation. The number of free electrons is equal to the number of holes. The conduction capability is small at room temperature. In order to increase the conduction capability of intrinsic semiconductor, it is better to add some impurities. This process of adding impurities is called as Doping. Now, this doped intrinsic semiconductor is called as an Extrinsic Semiconductor. Doping The process of adding impurities to the semiconductor materials is termed as doping. The impurities added, are generally pentavalent and trivalent impurities. Pentavalent Impurities The pentavalent impurities are the ones which has five valence electrons in the outer most orbit. Example: Bismuth, Antimony, Arsenic, Phosphorus The pentavalent atom is called as a donor atom because it donates one electron to the conduction band of pure semiconductor atom. Trivalent Impurities The trivalent impurities are the ones which has three valence electrons in the outer most orbit. Example: Gallium, Indium, Aluminum, Boron The trivalent atom is called as an acceptor atom because it accepts one electron from the semiconductor atom. Extrinsic Semiconductor An impure semiconductor, which is formed by doping a pure semiconductor is called as an extrinsic semiconductor. There are two types of extrinsic semiconductors depending upon the type of impurity added. They are N-type extrinsic semiconductor and P-Type extrinsic semiconductor. N-Type Extrinsic Semiconductor A small amount of pentavalent impurity is added to a pure semiconductor to result in Ntype extrinsic semiconductor. The added impurity has 5 valence electrons. For example, if Arsenic atom is added to the germanium atom, four of the valence electrons get attached with the Ge atoms while one electron remains as a free electron. This is as shown in the following figure. All of these free electrons constitute electron current. Hence, the impurity when added to pure semiconductor, provides electrons for conduction. In N-type extrinsic semiconductor, as the conduction takes place through electrons, the electrons are majority carriers and the holes are minority carriers. As there is no addition of positive or negative charges, the electrons are electrically neutral. When an electric

Basic Electronics – Hall Effect ”; Previous Next Hall Effect was named after Edwin Hall, its discoverer. This is somewhat similar to Fleming’s right hand rule. When a current carrying conductor I is placed in a transverse magnetic field B, an electric field E is induced in the conductor perpendicular to both I and B. This phenomenon is called as Hall Effect. Explanation When a current carrying conductor is placed in a transverse magnetic field, then this magnetic field exerts some pressure on the electrons which take a curved path to continue their journey. The conductor with energy applied is shown in the following figure. The magnetic field is also indicated. As electrons travel through the conductor that lies in a magnetic field B, the electrons will experience a magnetic force. This magnetic force will cause the electrons to travel close to one side than the other. This creates a negative charge on one side and positive charge on the other, as shown in the following figure. This separation of charge will create a voltage difference which is known as Hall Voltage or Hall EMF. The voltage builds up until the electric field produces an electric force on the charge that is equal and opposite of the magnetic force. This effect is known as Hall Effect. $$overrightarrow{F_{magnetic}}::=::overrightarrow{F_{Electric}}::=::q::overrightarrow{V_{D}}::overrightarrow{B}::=::q::overrightarrow{E_{H}}$$ VD is the velocity that every electron is experiencing $overrightarrow{E_{H}}::=::overrightarrow{V_{D}}::overrightarrow{B}::$ Since V = Ed Where q = quantity of charge $overrightarrow{B}$ = the magnetic field $overrightarrow{V_{D}}$ = the drift velocity $overrightarrow{E_{H}}$ = the Hall electric effect d = distance between the planes in a conductor (width of the conductor) $$V_{H}::=::varepsilon_{H}::=::overrightarrow{E_{H}}::d::=::overrightarrow{V_{D}}::overrightarrow{B}::d$$ $$varepsilon_{H}::=::overrightarrow{V_{D}}::overrightarrow{B}::d$$ This is the Hall EMF Uses The Hall Effect is used for obtaining information regarding the semiconductor type, the sign of charge carriers, to measure electron or hole concentration and the mobility. There by, we can also know whether the material is a conductor, insulator or a semiconductor. It is also used to measure magnetic flux density and power in an electromagnetic wave. Types of Currents Coming to the types of currents in semiconductors, there are two terms need to be discussed. They are Diffusion Current and Drift Current. Diffusion current When doping is done, there occurs a difference in the concentration of electrons and holes. These electrons and holes tend to diffuse from higher concentration of charge density, to lower concentration level. As these are charge carriers, they constitute a current called diffusion current. To know about this in detail, let us consider an N-type material and a P-type material. N-type material has electrons as majority carriers and few holes as minority carriers. P-type material has holes as majority carriers and few electrons as minority carriers. If these two materials are brought too close to each other to join, then few electrons from valence band of N-type material, tend to move towards P-type material and few holes from valence band of P-type material, tend to move towards N-type material. The region between these two materials where this diffusion takes place, is called as Depletion region. Hence, the current formed due to the diffusion of these electrons and holes, without the application of any kind of external energy, can be termed as Diffusion Current. Drift Current The current formed due to the drift (movement) of charged particles (electrons or holes) due to the applied electric field, is called as Drift Current. The following figure explains the drift current, whether how the applied electric field, makes the difference. The amount of current flow depends upon the charge applied. The width of depletion region also gets affected, by this drift current. To make a component function in an active circuit, this drift current plays an important role. Print Page Previous Next Advertisements ”;

Circuit Connections in Inductors ”; Previous Next An Inductor when connected in a circuit, that connection can be either series or parallel. Let us now know what will happen to the total current, voltage and resistance values if they are connected in series as well, when connected in parallel. Inductors in Series Let us observe what happens, when few inductors are connected in Series. Let us consider three resistors with different values, as shown in the figure below. Inductance The total inductance of a circuit having series inductors is equal to the sum of the individual inductances. Total inductance value of the network given above is $$L_{T}::=::L_{1}::+::L_{2}::+::L_{3}$$ Where L1 is the inductance of 1st resistor, L2 is the inductance of 2nd resistor and L3 is the inductance of 3rd resistor in the above network. Voltage The total voltage that appears across a series inductors network is the addition of voltage drops at each individual inductances. Total voltage that appears across the circuit $$V::=::V_{1}::+::V_{2}::+::V_{3}$$ Where V1 is the voltage drop across 1st inductor, V2 is the voltage drop across 2nd inductor and V3 is the voltage drop across 3rd inductor in the above network. Current The total amount of Current that flows through a set of inductors connected in series is the same at all the points throughout the network. The Current through the network $$I::=::I_{1}::=::I_{2}::=::I_{3}$$ Where I1 is the current through the 1st inductor, I2 is the current through the 2nd inductor and I3 is the current through the 3rd inductor in the above network. Inductors in Parallel Let us observe what happens, when few resistors are connected in Parallel. Let us consider three resistors with different values, as shown in the figure below. Inductance The total inductance of a circuit having Parallel resistors is calculated differently from the series inductor network method. Here, the reciprocal (1/R) value of individual inductances are added with the inverse of algebraic sum to get the total inductance value. Total inductance value of the network is $$frac{1}{L_{T}}::=::frac{1}{L_{1}}::+::frac{1}{L_{2}}::+::frac{1}{L_{3}}$$ Where L1 is the inductance of 1st inductor, L2 is the inductance of 2nd inductor and L3 is the inductance of 3rd inductor in the above network. From the method we have for calculating parallel inductance, we can derive a simple equation for two-inductor parallel network. It is $$L_{T}::=::frac{L_{1}::times:: L_{2}}{L_{1}::+:: L_{2}}$$ Voltage The total voltage that appears across a Parallel inductors network is same as the voltage drops at each individual inductances. The Voltage that appears across the circuit $$V::=::V_{1}::=::V_{2}::=::V_{3}$$ Where V1 is the voltage drop across 1st inductor, V2 is the voltage drop across 2nd inductor and V3 is the voltage drop across 3rd inductor in the above network. Hence the voltage is same at all the points of a parallel inductor network. Current The total amount of current entering a Parallel inductive network is the sum of all individual currents flowing in all the Parallel branches. The inductance value of each branch determines the value of current that flows through it. The total Current through the network is $$I::=::I_{1}::+::I_{2}::+::I_{3}$$ Where I1 is the current through the 1st inductor, I2 is the current through the 2nd inductor and I3 is the current through the 3rd inductor in the above network. Hence the sum of individual currents in different branches obtain the total current in a parallel network. Inductive Reactance Inductive Reactance is the opposition offered by an inductor to the alternating current flow, or simply AC current. An inductor has the property of resisting the change in the flow of current and hence it shows some opposition which can be termed as reactance, as the frequency of the input current should also be considered along with the resistance it offers. Indication − XL Units − Ohms Symbol − Ω In a purely inductive circuit, the current IL lags the applied voltage by 90°. Inductive reactance is calculated by, $$X_{L}::=::2pi fL$$ Where f is the frequency of the signal. Hence inductive reactance is a function of frequency and inductance. Print Page Previous Next Advertisements ”;

Circuit Connections in Resistors ”; Previous Next A Resistor when connected in a circuit, that connection can be either series or parallel. Let us now know what will happen to the total current, voltage and resistance values if they are connected in series as well, when connected in parallel. Resistors in Series Let us observe what happens, when few resistors are connected in Series. Let us consider three resistors with different values, as shown in the figure below. Resistance The total resistance of a circuit having series resistors is equal to the sum of the individual resistances. That means, in the above figure there are three resistors having the values 1KΩ, 5KΩ and 9KΩ respectively. Total resistance value of the resistor network is − $$R::=::R_{1}:+:R_{2}:+:R_{3}$$ Which means 1 + 5 + 9 = 15KΩ is the total resistance. Where R1 is the resistance of 1st resistor, R2 is the resistance of 2nd resistor and R3 is the resistance of 3rd resistor in the above resistor network. Voltage The total voltage that appears across a series resistors network is the addition of voltage drops at each individual resistances. In the above figure we have three different resistors which have three different values of voltage drops at each stage. Total voltage that appears across the circuit − $$V::=::V_{1}:+:V_{2}:+:V_{3}$$ Which means 1v + 5v + 9v = 15v is the total voltage. Where V1 is the voltage drop of 1st resistor, V2 is the voltage drop of 2nd resistor and V3 is the voltage drop of 3rd resistor in the above resistor network. Current The total amount of Current that flows through a set of resistors connected in series is the same at all the points throughout the resistor network. Hence the current is same 5A when measured at the input or at any point between the resistors or even at the output. Current through the network − $$I::=::I_{1}:=:I_{2}:=:I_{3}$$ Which means that current at all points is 5A. Where I1 is the current through the 1st resistor, I2 is the current through the 2nd resistor and I3 is the current through the 3rd resistor in the above resistor network. Resistors in Parallel Let us observe what happens, when few resistors are connected in Parallel. Let us consider three resistors with different values, as shown in the figure below. Resistance The total resistance of a circuit having Parallel resistors is calculated differently from the series resistor network method. Here, the reciprocal (1/R) value of individual resistances are added with the inverse of algebraic sum to get the total resistance value. Total resistance value of the resistor network is − $$frac{1}{R}::=::frac{1}{R_{1}}::+::frac{1}{R_{2}}::+frac{1}{R_{3}}$$ Where R1 is the resistance of 1st resistor, R2 is the resistance of 2nd resistor and R3 is the resistance of 3rd resistor in the above resistor network. For example, if the resistance values of previous example are considered, which means R1 = 1KΩ, R2 = 5KΩ and R3 = 9KΩ. The total resistance of parallel resistor network will be − $$frac{1}{R}::=::frac{1}{1}::+::frac{1}{5}::+frac{1}{9}$$ $$=::frac{45::+::9::+::5}{45}::=::frac{59}{45}$$ $$R::=::frac{45}{59}::=::0.762KOmega::=::76.2Omega$$ From the method we have for calculating parallel resistance, we can derive a simple equation for two-resistor parallel network. It is − $$R::=::frac{R_{1}::times::R_{2}}{R_{1}::+::R_{2}}:$$ Voltage The total voltage that appears across a Parallel resistors network is same as the voltage drops at each individual resistance. The Voltage that appears across the circuit − $$V::=::V_{1}:=:V_{2}:=:V_{3}$$ Where V1 is the voltage drop of 1st resistor, V2 is the voltage drop of 2nd resistor and V3 is the voltage drop of 3rd resistor in the above resistor network. Hence the voltage is same at all the points of a parallel resistor network. Current The total amount of current entering a Parallel resistive network is the sum of all individual currents flowing in all the Parallel branches. The resistance value of each branch determines the value of current that flows through it. The total current through the network is $$I::=::I_{1}:+:I_{2}:+:I_{3}$$ Where I1 is the current through the 1st resistor, I2 is the current through the 2nd resistor and I3 is the current through the 3rd resistor in the above resistor network. Hence the sum of individual currents in different branches obtain the total current in a parallel resistive network. A Resistor is particularly used as a load in the output of many circuits. If at all the resistive load is not used, a resistor is placed before a load. Resistor is usually a basic component in any circuit. Print Page Previous Next Advertisements ”;

Basic Electronics – Fixed Capacitors ”; Previous Next The Capacitors whose value is fixed while manufacturing and cannot be altered later are called as Fixed Capacitors. The main classification of fixed capacitors is done as polarized and non-polarized. Let us have a look at Non-polarized capacitors. Non-Polarized Capacitors These are the capacitors that have no specific polarities, which means that they can be connected in a circuit, either way without bothering about the placement of right lead and left lead. These capacitors are also called as Non-Electrolytic Capacitors. The main classification of Non-Polarized capacitors is done as shown in the following figure. Among the types of capacitors, let us first go through the Ceramic Capacitors. Ceramic Capacitors The common capacitors used among fixed type are Ceramic Capacitors. The Ceramic capacitors are fixed capacitors that have ceramic material as a dielectric. These ceramic capacitors are further classified as class1 and class2 depending upon their applications. For instance, Class1 has high stability and works best for resonant circuit applications, while class2 has high efficiency and gives its best for coupling applications. A hollow tubular or plate like ceramic material such as titanium dioxide and barium titanate is coated with a deposition of silver compound on both walls, so that both sides act as two capacitor plates and ceramic acts as a dielectric. Leads are drawn from these two surfaces and this whole assembly is encapsulated in a moisture-proof coating. The most often used modern ceramic capacitors are Multi-Layer Chip Capacitors (MLCC). These capacitors are made in surface mounted technology and are mostly used due to their small size. These are available in the order of 1ηF to 100µF. Film Capacitors The Film Capacitors are the ones which have a film substance as a dielectric material. Depending upon the type of film used, these are classified as Paper and Metal film capacitors. These film capacitors are both paper dielectric capacitors whereas a paper capacitor uses a waxed paper while a metallic film capacitor uses a metallized paper. The arrangement is almost same as shown below. Paper Capacitors Paper capacitors use Paper as a dielectric material. Two thin tin foil sheets are taken and placed between thin waxed or oiled paper sheets. This paper acts as a dielectric. Now-a-days paper is being replaced by plastic. These sheets are sandwiched and are rolled into a cylindrical shape and encapsulated in a plastic enclosure. Leads are drawn out. The following figure shows an example of Paper Capacitors. Paper capacitors are available in the order of 0.001µF to 2µF and the voltage rating can be as high as 2000volts. These capacitors are useful in high voltage and current applications. Metal Film Capacitors Metal Film capacitors are another type of film capacitors. These are also called as Metal Foil Capacitors or Metallized Paper Capacitors as the dielectric used here is a paper coated with metallic film. Unlike in paper capacitors, a film of Aluminum or Zinc is coated on a paper to form a dielectric in this metallic film capacitors. Instead of Aluminum sheets placing between papers, the paper itself is directly coated here. This reduces the size of the capacitor. The Aluminum coating is preferred over zinc coating so as to avoid destruction of capacitor due to chemical reduction. The Aluminum coated sheets are rolled in the form of a cylinder and leads are taken. This whole thing is encapsulated with wax or plastic resin to protect the capacitor. These capacitors are useful in high voltage and current applications. Other Capacitors These are the miscellaneous capacitors that are named after the dielectric materials used. This group includes Mica Capacitors, Air Capacitors, Vacuum Capacitors and Glass Capacitors etc. Mica Capacitors The Mica Capacitors are made by using thin Mica sheets as dielectric materials. Just like paper capacitors, thin metal sheets are sandwiched with mica sheets in between. Finally the layers of metal sheets are connected at both ends and two leads are formed. Then the whole assembly is enclosed in plastic Bakelite capsule. The following image shows how a Mica capacitor looks like. Mica Capacitors are available in the range of 50pF to 500pF. The Mica capacitors have high working voltage up to 500volts. These are most commonly used capacitors for electronic circuits such as ripple filters, Resonant circuits, Coupling circuits and high power, high current RF broadcast transmitters. Air Capacitors The Air Capacitors are the ones with air as dielectric. The simplest air capacitors are the ones with conducting plates having air in between. This construction is exactly the same as the variable tuning capacitor discussed above. These capacitors can be fixed and variable also but fixed are very rarely used as there are others with superior characteristics. Vacuum Capacitors The Vacuum Capacitors uses high vacuum as dielectric instead of air or some other material. These are also available in fixed and variable modes. The construction of these capacitors is similar to vacuum tubes. They are mostly seen in the form of a glass cylinder which contain inter-meshed concentric cylinders. The following image shows a variable vacuum capacitor. The following image shows how a fixed vacuum capacitor looks like − Variable vacuum capacitors are available at a range of 12pF to 5000pF and they are used for high voltage applications such as 5kV to 60kV. They are used in main equipment such as high power broadcast transmitters, RF amplifiers and large antenna tuners. Glass Capacitors Glass capacitors are very exclusive ones with many advantages and applications. As all of the above types, here glass is the dielectric substance. Along with glass dielectric, Aluminum electrodes are also present in these capacitors. Plastic encapsulation is done after taking out the leads. The leads can be axial leads or tubular leads. There are many advantages of a glass capacitor such as − The temperature coefficient is low. These are Noise-free capacitors. They produce high quality output with low loss. They have the capability of handling high operating temperatures. These capacitors can handle large RF currents. There

Basic Electronics – Inductance ”; Previous Next The property of an inductor to get the voltage induced by the change of current flow, is defined as Inductance. Inductance is the ratio of voltage to the rate of change of current. The rate of change of current produces change in the magnetic field, which induces an EMF in opposite direction to the voltage source. This property of induction of EMF is called as the Inductance. The formula for inductance is $$Inductance::=::frac{volatge}{rate:of:change:of:current}$$ Units − The unit of Inductance is Henry. It is indicated by L. The inductors are mostly available in mH (milli Henry) and μH (micro Henry). A coil is said to have an inductance of one Henry when an EMF of one volt is self-induced in the coil where the current flowing changed at a rate of one ampere per second. Self-Inductance If a coil is considered in which some current flows, it has some magnetic field, perpendicular to the current flow. When this current keeps on varying, the magnetic field also changes and this changing magnetic field, induces an EMF, opposite to the source voltage. This opposing EMF produced is the self-induced voltage and this method is called as self-inductance. The current is in the figure indicate the source current while iind indicates the induced current. The flux represents the magnetic flux created around the coil. With the application of voltage, the current is flows and flux gets created. When the current is varies, the flux gets varied producing iind. This induced EMF across the coil is proportional to the rate of change in current. The higher the rate of change in current the higher the value of EMF induced. We can write the above equation as $$E::alpha::frac{dI}{dt}$$ $$E::=::L::frac{dI}{dt}$$ Where, E is the EMF produced dI/dt indicates the rate of change of current L indicates the co-efficient of inductance. Self-inductance or Co-efficient of Self-inductance can be termed as $$L::=::frac{E}{frac{dI}{dt}}$$ The actual equation is written as $$E::=::-L::frac{dI}{dt}$$ The minus in the above equation indicates that the EMF is induced in opposite direction to the voltage source according to Lenz’s law. Mutual Inductance As the current carrying coil produces some magnetic field around it, if another coil is brought near this coil, such that it is in the magnetic flux region of the primary, then the varying magnetic flux induces an EMF in the second coil. If this first coil is called as Primary coil, the second one can be called as a Secondary coil. When the EMF is induced in the secondary coil due to the varying magnetic field of the primary coil, then such phenomenon is called as the Mutual Inductance. The current is in the figure indicate the source current while iind indicates the induced current. The flux represents the magnetic flux created around the coil. This spreads to the secondary coil also. With the application of voltage, the current is flows and flux gets created. When the current is varies, the flux gets varied producing iind in the secondary coil, due to the Mutual inductance property. The change took place like this. $$V_{p}::I_{p}rightarrow::B::rightarrow::V_{s}::I_{s}$$ Where, Vp ip Indicate the Voltage and current in Primary coil respectively B Indicates Magnetic flux Vs is Indicate the Voltage and current in Secondary coil respectively Mutual inductance M of the two circuits describes the amount of the voltage in the secondary induced by the changes in the current of the primary. $$V(Secondary)::=::-Mfrac{Delta I}{Delta t}$$ Where $frac{Delta I}{Delta t}$ the rate of change of current with time and M is the co-efficient of Mutual inductance. The minus sign indicates the direction of current being opposite to the source. Units − The units of Mutual inductance is $$volt::=::Mfrac{amps}{sec}$$ (From the above equation) $$M::=::frac{volt.:sec}{amp}$$ $$=::Henry(H)$$ Depending upon the number of turns of the primary and the secondary coils, the magnetic flux linkage and the amount of induced EMF varies. The number of turns in primary is denoted by N1 and secondary by N2. The co-efficient of coupling is the term that specifies the mutual inductance of the two coils. Factors affecting Inductance There are a few factors that affect the performance of an inductor. The major ones are discussed below. Length of the coil The length of the inductor coil is inversely proportional to the inductance of the coil. If the length of the coil is more, the inductance offered by that inductor gets less and vice versa. Cross sectional area of the coil The cross sectional area of the coil is directly proportional to the inductance of the coil. The higher the area of the coil, the higher the inductance will be. Number of turns With the number of turns, the coil affects the inductance directly. The value of inductance gets square to the number of turns the coil has. Hence the higher the number of turns, square of it will be the value of inductance of the coil. Permeability of the core The permeability (μ) of the core material of inductor indicates the support the core provides for the formation of a magnetic field within itself. The higher the permeability of the core material, the higher will be the inductance. Coefficient of Coupling This is an important factor to be known for calculating Mutual inductance of two coils. Let us consider two nearby coils of N1 and N2 turns respectively. The current through first coil i1 produces some flux Ψ1. The amount of magnetic flux linkages is understood by weber-turns. Let the amount of magnetic flux linkage to the second coil, due to unit current of i1 be $$frac{N_{2}varphi_{1}}{i_{1}}$$ This can be understood as the Co-efficient of Mutual inductance, which means $$M::=::frac{N_{2}varphi_{1}}{i_{1}}$$ Hence the Co-efficient of Mutual inductance between two coils or circuits is understood as the weber-turns in one coil due to 1A of current in the other coil. If the self-inductance of first coil is L1, then $$L_{1}i_{1}::=::{N_{1}varphi_{1}}::=>::frac{L_{1}}{N_{1}}::frac{varphi_{1}}{i_{1}}$$ $$M::=::frac{N_{2}L_{1}}{N_{1}}$$ Similarly, coefficient of mutual inductance due to current i2 in the second coil is $$M::=::frac{N_{1}varphi_{2}}{i_{2}}:dotsm:dotsm:dotsm:dotsm::1$$ If self-inductance of

Basic Electronics – Energy Bands ”; Previous Next In gaseous substances, the arrangement of molecules is not close. In liquids, the molecular arrangement is moderate. But, in solids, the molecules are so closely arranged, that the electrons in the atoms of molecules tend to move into the orbitals of neighboring atoms. Hence the electron orbitals overlap when the atoms come together. Due to the intermixing of atoms in solids, instead of single energy levels, there will be bands of energy levels formed. These set of energy levels, which are closely packed are called as Energy bands. Valance Band The electrons move in the atoms in certain energy levels but the energy of the electrons in the innermost shell is higher than the outermost shell electrons. The electrons that are present in the outermost shell are called as Valance Electrons. These valance electrons, containing a series of energy levels, form an energy band which is called as Valence Band. The valence band is the band having the highest occupied energy. Conduction Band The valence electrons are so loosely attached to the nucleus that even at room temperature, few of the valence electrons leave the band to be free. These are called as free electrons as they tend to move towards the neighboring atoms. These free electrons are the ones which conduct the current in a conductor and hence called as Conduction Electrons. The band which contains conduction electrons is called as Conduction Band. The conduction band is the band having the lowest occupied energy. Forbidden gap The gap between valence band and conduction band is called as forbidden energy gap. As the name implies, this band is the forbidden one without energy. Hence no electron stays in this band. The valence electrons, while going to the conduction band, pass through this. The forbidden energy gap if greater, means that the valence band electrons are tightly bound to the nucleus. Now, in order to push the electrons out of the valence band, some external energy is required, which would be equal to the forbidden energy gap. The following figure shows the valance band, conduction band, and the forbidden gap. Depending upon the size of the forbidden gap, the Insulators, the Semiconductors and the Conductors are formed. Insulators Insulators are such materials in which the conduction cannot take place, due to the large forbidden gap. Examples: Wood, Rubber. The structure of energy bands in Insulators is as shown in the following figure. Characteristics The following are the characteristics of Insulators. The Forbidden energy gap is very large. Valance band electrons are bound tightly to atoms. The value of forbidden energy gap for an insulator will be of 10eV. For some insulators, as the temperature increases, they might show some conduction. The resistivity of an insulator will be in the order of 107 ohm-meter. Semiconductors Semiconductors are such materials in which the forbidden energy gap is small and the conduction takes place if some external energy is applied. Examples: Silicon, Germanium. The following figure shows the structure of energy bands in semiconductors. Characteristics The following are the characteristics of Semiconductors. The Forbidden energy gap is very small. The forbidden gap for Ge is 0.7eV whereas for Si is 1.1eV. A Semiconductor actually is neither an insulator, nor a good conductor. As the temperature increases, the conductivity of a semiconductor increases. The conductivity of a semiconductor will be in the order of 102 mho-meter. Conductors Conductors are such materials in which the forbidden energy gap disappears as the valence band and conduction band become very close that they overlap. Examples: Copper, Aluminum. The following figure shows the structure of energy bands in conductors. Characteristics The following are the characteristics of Conductors. There exists no forbidden gap in a conductor. The valance band and the conduction band gets overlapped. The free electrons available for conduction are plenty. A slight increase in voltage, increases the conduction. There is no concept of hole formation, as a continuous flow of electrons contribute the current. Important Terms There is a necessity to discuss a few important terms here before we move on to subsequent chapters. Current It is simply the flow of electrons. A continuous flow of electrons or charged particles, can be termed as Current. It is indicated by I or i. It is measured in Amperes. This can be alternating current AC or direct current DC. Voltage It is the potential difference. When there occurs a difference in potentialities, between two points, there is said to be a voltage difference, measured between those two points. It is indicated by V. It is measured in Volts. Resistance It is the property of opposing the flow of electrons. The possession of this property can be termed as resistivity. This will be discussed later in detail. Ohm’s Law With the terms discussed above, we have a standard law, which is very crucial for the behavior of all the electronic components, called as Ohm’s Law. This states the relation between current and voltage in an ideal conductor. According to Ohm’s law, the potential difference across an ideal conductor is proportional to the current through it. $$V:alpha::I$$ An ideal conductor has no resistance. But in practice, every conductor has some resistance in it. As the resistance increases, the potential drop also increases and hence the voltage increases. Hence the voltage is directly proportional to the resistance it offers. $$V:alpha::R$$ $$V = IR $$ But the current is inversely proportional to the resistance. $$V:alpha::I:alpha::frac{1}{R}$$ $$I = V/R $$ Hence, in practice, an Ohm’s law can be stated as − According to Ohm’s law, the current flowing through a conductor is proportional to the potential difference across it, and is inversely proportional to the resistance it offers. This law is helpful in determining the values of unknown parameters among the three which help to analyze a circuit. Print Page Previous Next Advertisements ”;

Basic Electronics – Materials ”; Previous Next Matter is made up of molecules which consists of atoms. According to Bohr’s theory, “the atom consists of positively charged nucleus and a number of negatively charged electrons which revolve round the nucleus in various orbits”. When an electron is raised from a lower state to a higher state, it is said to be excited. While exciting, if the electron is completely removed from the nucleus, the atom is said to be ionized. So, the process of raising the atom from normal state to this ionized state is called as ionization. The following figure shows the structure of an atom. According to Bohr’s model, an electron is said to be moved in a particular Orbit, whereas according to quantum mechanics, an electron is said to be somewhere in free space of the atom, called as Orbital. This theory of quantum mechanics was proven to be right. Hence, a three dimensional boundary where an electron is probable to found is called as Atomic Orbital. Quantum Numbers Each orbital, where an electron moves, differs in its energy and shape. The energy levels of orbitals can be represented using discrete set of integrals and half-integrals known as quantum numbers. There are four quantum numbers used to define a wave function. Principal Quantum number The first quantum number that describes an electron is the Principal quantum number. Its symbol is n. It specifies the size or order (energy level) of the number. As the value of n increases, the average distance from electron to nucleus also increases, as well, the energy of the electron also increases. The main energy level can be understood as a shell. Angular Momentum Quantum number This quantum number has l as its symbol. This l indicates the shape of the orbital. It ranges from 0 to n-1. l = 0, 1, 2 …n-1 For the first shell, n = 1. i.e., for n-1, l = 0 is the only possible value of l as n = 1. So, when l = 0, it is called as S orbital. The shape of S is spherical. The following figure represents the shape of S. If n = 2, then l = 0, 1 as these are the two possible values for n = 2. We know that it is S orbital for l = 0, but if l = 1, it is P orbital. The P orbital where the electrons are more likely to find is in dumbbell shape. It is shown in the following figure. Magnetic Quantum number This quantum number is denoted by ml which represents the orientation of an orbital around the nucleus. The values of ml depend on l. $$m_{l}= int (-l::to:+l)$$ For l = 0, ml = 0 this represents S orbital. For l = 1, ml = -1, 0, +1 these are the three possible values and this represents P orbital. Hence we have three P orbitals as shown in the following figure. Spin Quantum number This is represented by ms and the electron here, spins on the axis. The movement of the spinning of electron could be either clockwise or anti-clockwise as shown here under. The possible values for this spin quantum number will be like, $$m_{s}= +frac{1}{2}::up$$ For a movement called spin up, the result is positive half. $$m_{s}= -frac{1}{2}::down$$ For a movement called spin down, the result is negative half. These are the four quantum numbers. Pauli Exclusion Principle According to Pauli Exclusion Principle, no two electrons in an atom can have the same set of four identical quantum numbers. It means, if any two electrons have same values of n, s, ml (as we just discussed above) then the l value would definitely be different in them. Hence, no two electrons will have same energy. Electronic shells If n = 1 is a shell, then l = 0 is a sub-shell. Likewise, n = 2 is a shell, and l = 0, 1 is a sub-shell. Shells of electrons corresponding to n = 1, 2, 3….. are represented by K, L, M, N respectively. The sub-shells or the orbitals corresponding to l = 0, 1, 2, 3 etc. are denoted by s, p, d, f etc. respectively. Let us have a look at the electronic configurations of carbon, silicon and germanium (Group IV – A). It is observed that the outermost p sub-shell in each case contains only two electrons. But the possible number of electrons is six. Hence, there are four valence electrons in each outer most shell. So, each electron in an atom has specific energy. The atomic arrangement inside the molecules in any type of substance is almost like this. But the spacing between the atoms differ from material to material. Print Page Previous Next Advertisements ”;