1 Introduction to Electronics Objectives : After completing this Chapter, you will be able to : l l l l l l l l l l l l l State the meaning of electronics. Say how electronics has developed over the century. State the latest trends in the field of electronics. Name some of the electronic devices and draw their symbols. Recognize resistors, capacitors and inductors of different types. Read the values of resistors and capacitors from the colour code. State the voltage-divider and current-divider concepts. Draw the symbols of ideal voltage source, practical voltage source, ideal current source and practical current source. Give a graphical representation of voltage source and current source. Explain the difference between an ideal source and a practical source. Convert a given voltage source into an equivalent current source and vice-versa. State the difference between an analog signal and a digital signal. Write the SI units of various physical quantities used in electronics. 1.1 WHAT IS ELECTRONICS To put the tiny electrons to work is Electronics. The word ‘electronics’ is derived from electron mechanics. Electronics is the science and technology of the motion of electrons (or other such charges) in gas, vacuum, or in any semiconductor. It deals principally with the communication of information and/or data handling. Compared to the more established branches of engineering—civil, mechanical and electrical, the electronics is a newcomer—hardly 100 years old. Until around 1960, it was considered an integral part of electrical engineering. But due to the tremendous advancement over the last few decades, electronics has now gained its rightful place. The advancement has been so fast that many sub-branches of electronics—such as Computer Science Engineering, Communication Engineering, Control and Instrumentation Engineering, Information Technology— are now full-fledged courses in many universities. Everyone is familiar with electronic devices, be it the television, the computer, or the cellular phone. However, to most people, what goes on inside, is a mystery. An Electronics Engineer knows and understands the functioning of these devices. He acquires the capability to further improvise these devices as per the needs of the user. 2 Electronics Engineering 1.2 HISTORY OF ELECTRONICS Electronics took birth in 1897 when J.A. Fleming developed a vacuum diode. Useful electronics came in 1906 when vacuum triode was invented by Lee De Forest. This device could amplify electrical signals. Later, around 1925, tetrode and pentode tubes were developed. These tubes dominated the field of electronics till the end of World War II. The Transistor Era The era of semiconductor electronics began with the invention of the junction transistor in 1948. Bardeen, Brattain and Shockley were awarded the Nobel Prize in Physics in 1956 for this invention. This was the first Nobel award given for an engineering device in nearly 50 years. Soon, the transistors were replacing the bulky vacuum tubes in different electronic circuits. The tubes had major limitations : power was consumed even when they were not in use and filaments burnt out, requiring frequent tube replacements. By now, vacuum tubes have become history. Earlier, the transistors were made from germanium as it was easier to purify a sample of germanium. In 1954, silicon transistors were developed. These afforded operations upto 200 °C, whereas germanium device could work well only upto about 75 °C. Today, almost all semiconductor devices are fabricated using silicon. Invention of the Integrated Circuits (IC) In 1958, Kilby conceived the concept of building an entire electronic circuit on a single semiconductor chip. All active and passive components and their interconnections could be integrated on a single chip, during the manufacturing process. This drastically reduced the size and weight, as well as the cost per active component. The term ‘microelectronics’ refers to the design and fabrication of these high componentdensity ICs. The following approximate dates give some indication of the increasing component count per chip of area 3 × 5 mm2 and thickness 0.3 mm (about three times the thickness of human hair) : 1951 — Discrete transistors 1960 — Small-Scale Integration (SSI), fewer than 100 components 1966 — Medium-Scale Integration (MSI), 100 to 1000 components 1969 — Large-Scale Integration (LSI), 1000 to 10000 components 1975 — Very-Large-Scale Integration (VLSI), more than 10000 components The Field-Effect Transistor (FET) In 1951, Shockley proposed the junction field-effect transistor (JFET), using the effect of applied electric field on the conductivity of a semiconductor. A reliable JFET was produced in 1958. The techniques to make reliable JFETs led to an even more important device called metaloxide-semiconductor field-effect transistor (MOSFET). Subsequent improvements in processing and device design, and the growth of the computer industry have made MOS devices the most widely used transistors. Introduction to Electronics 3 Digital Integrated Circuits The growth of computer industry spurred new IC development. In turn, the new IC concepts resulted in new computer architecture. Speed, power consumption, and component density are important considerations in digital ICs. Transistor-transistor logic (TTL), emitter-coupled logic (ECL) and integrated-injection logic (I2L) technologies were developed. The use of MOSFETs is very attractive because very high component-densities are obtainable. Originally, reliable fabrication employed PMOS devices, in which operation depended on hole flow. Improved fabrication methods led to the use of N-channel MOS (NMOS). These gave higher speed performance. Later, the complementary metal oxide semiconductor (CMOS) technology employing both PMOS and NMOS in a circuit, was used. MOSFETs find major applications in semiconductor memories. Earlier in 1970s, bipolar junction transistors (BJTs) were used in making random access memories (RAMs), which were capable of both storing and retrieving data (i.e., writing and reading). These could store about 100 bits of information. Using MOS technology, 16000-bit RAMs were made available in 1973, 64000-bit RAMs in 1978, and 288000-bit in 1982. By now, you have more than a billion-bit chips available. Read-only memories (ROMs), used for look-up tables in computers (e.g., to obtain the value of sin x), were first introduced in 1967. Subsequent developments led to programmable ROMs (PROMs) and erasable PROMs (EPROMs) in which data stored could be removed (erased) and new data stored. The first microprocessor (mP) was developed by M.E. Hoft at Intel in 1969. It led to the “computer on a chip”. Another important development arising from MOS technology is the charge-coupled device (CCD). Recently, CCDs have found applications in camera manufacturing, image processing and communication. Analog Integrated Circuits The first operational amplifier (OP AMP) was developed in 1964. Since then the OP AMP has become the “workhorse” in analog signal processing. Other circuits and systems developed subsequently are analog multipliers, digital-to-analog (D/A) and analog-to-digital (A/D) converters, and active filters. Technological developments are taking place today at an awesome pace. To digest, to appreciate and to meet the challenges of these dynamic fields, it has become essential to equip oneself with the fundamental understanding of the subject. 1.3 APPLICATIONS OF ELECTRONICS Electronics is a big bag of tricks. It finds applications almost everywhere: 1. Communications and Entertainment An audio amplifier is the heart of a tape recorder, a public address (PA) system, radio and television receiver. The video section of a TV reproduces the picture signal received. Many interesting toys use ICs. The telephone systems now employ digital ICs for switching and memory. Communication satellites became feasible because of microelectronics. 4 Electronics Engineering A new era of electronics, called digital signal processing (DSP), has evolved because ICs have made the merging of communications and computation possible. 2. Controls and Instrumentation Silicon controlled rectifiers (SCRs) are used in the speed-control of motors, power rectifiers and inverters. Automation of industrial processes has been made possible by electronic circuits. With microelectronics, computers have become integral components of control systems. Very accurate and user-friendly instruments like digital voltmeter (DVM), cathode ray oscilloscope (CRO), frequency counter, signal generator, strain gauge, pH-meter, spectrum analysers, etc., are now available. 3. Defence Applications Defence services are using electronic equipments. Radar, sonar and infrared systems are used to detect the location of enemy jet fighters, war-ships and submarines, and then to control the aiming and firing of guns. Guided missiles are completely controlled by electronic means. Electronic circuits provide a means of secret communication between the head-quarter and different units. Such a communication has become absolutely essential to win a war. 4. Industrial Applications Use of automatic control systems in different industries is increasing day by day. Control of thickness, quality, weight and moisture content of a material can be easily done by such systems. Use of computers has made the reservations in railways and airways simple and convenient. Even the power stations, which generate thousands of megawatts of electricity, are controlled by tiny electronic devices and circuits. 5. Medical Sciences Doctors and scientists are finding new uses of electronic systems in the diagnosis and treatment of different ailments. Electrocardiographs (ECG), X-rays, short-wave diathermy units, ultrasound scanning machines, endoscopy, etc. are in common use. Even the thermometers, blood-pressure measuring instruments, blood-sugar measuring instruments, etc. are so userfriendly because of electronic circuits, that the patient can himself handle them. 6. Automobile Industry Manufacturing of different types of vehicles is done primarily under the supervision of electronic control systems. After the vehicle comes on the road, its efficient running depends on electronic controls. Electronic ignition system, multipoint fuel injection (MPFI) system, electronic battery charger, etc. are essential in modern high-speed vehicles. 1.4 THE FUTURE We already have big size TVs that can be hung on a wall like a calender. Through internet, you can instantly get any information from anywhere in the world. Soon, it will be possible to instantly contact any person anywhere in the world. Introduction to Electronics 5 The merging of extensive communication and inexpensive computers has already begun penetrating nearly every aspect of the society. The ability and relative ease by which information can be stored, retrieved, manipulated and transmitted is affecting us in our homes, our places of work. We are moving into the new era of Information Technology. 1.5 ELECTRONIC CIRCUIT COMPONENTS Howsoever complicated an electronic circuit may look, but luckily it contains only a few types of basic components—some active and some passive. Components such as PN-junction diodes, transistors, SCRs, FETs, etc. are said to be active because these are capable of changing the shape of electrical signals. Passive components, such as resistors, inductors and capacitors, do not change the shape of the signal. An electric circuit becomes an electronic circuit when it has one or more active component(s). However, an active component all alone cannot perform any useful function. Active Components There are many active components used in electronic circuits. Earlier, active devices were of tube-type. We had vacuum tubes (such as vacuum diode, vacuum triode, vacuum pentode, etc.) and gas tubes (such as gas diode, thyratron, etc.). These bulky tube devices are not used now-a-days. Almost all electronic circuits today use semiconductor devices or ICs. Brief information about commonly used semiconductor devices is given on the next page. Passive Components 1. Resistors A resistor is a device which provides a force opposing the charge-flow (or current) in a circuit. This opposing force is called resistance (R). It is measured in ohms (symbol is W, the greek capital letter omega). All resistors have power ratings. It is the maximum power that can be dissipated without raising the temperature too high. Thus, a 1-W resistor with a resistance of 100 W can pass a 6 Electronics Engineering Introduction to Electronics 7 maximum current of 100 mA. But a ¼ W resistor with the same resistance value (100 W) could handle a current of only 50 mA. If the current exceeds this limit for any length of time, the resistor would overheat and might even burn out. In electronic circuits, the common standard power ratings are ¼ W, ½ W, 1 W and 2 W. Thus, if we require a resistor to dissipate 0.6 W, we would select a 1 W resistor since the operation value must not exceed the rating. The size of a resistor is bigger if its wattage rating is higher, so as to withstand higher dissipation losses. A resistor is said to be linear, if the current through it is proportional to the pd (potential difference) across its terminals; otherwise it is nonlinear. Resistors made from semiconductor materials are non-linear. Resistors are made in many forms. But all belong to either of the two groups—fixed and variable. Fixed Resistors Most resistors used in electronic circuits are fixed (or non-variable). And most of these are moulded-carbon composition resistors. In such resistors, the resistive material is of carbon-clay composition. The leads are made of tinned copper. Resistors of this type are readily available in values ranging from a few ohms to about 22 MW, with tolerance range of 5 to 20%, and wattage ratings of ¼ W, ½ W and 1 W. Another variety of fixed resistors is made by depositing a homogeneous film of pure carbon (or some metal) over a glass or ceramic core. Desired values are then obtained by either trimming the layer-thickness or by cutting helical grooves of suitable pitch along its length. Contact caps are fitted on both ends. Tinned copper lead wires are then welded to these caps. This type of metal film resistor is sometimes called precision type, since the resistance value can be obtained with an accuracy of ±1%. When ratings of more than 1 W are required, we use wire wound resistors. A thin nichrome wire is wound on a ceramic or porcelain core. These resistors are available in the range of 1 W to 100 kW, and power ratings upto about 200 W. Resistance Colour Coding : Carbon-moulded and metal film resistors are small in size. It becomes almost impossible to print the ratings on their body. Therefore, a standard colour coding is used to indicate the ratings [Table 1.1]. The resistance is given in the form of four coloured signs (or bands) painted on the body. The coloured bands are always read from left to right from the end that has the bands closest to it, as shown in Fig. 1.1. Fig. 1.1 Colour coding of resistors 8 Electronics Engineering The first and second colour bands represent the first and second numbers (significant digits) respectively, of the resistance value. The third band indicates the number of zeros that follow the second digit. In case the third band is gold or silver, it represents a multiplying factor of 0.1 or 0.01, respectively. The fourth band represents tolerance. It is a measure of the precision with which the resistor was manufactured. In case the fourth band is not present, the tolerance is assumed ±20%. Table 1.1 Colour coding Colour Digit Black Brown Red Orange Yellow Green Blue Violet Gray White Gold Silver No colour 0 1 2 3 4 5 6 7 8 9 Multiplier Tolerance 0 10 = 1 101 = 10 102 103 104 105 106 107 108 109 0.1 = 101 0.01 = 102 ±5% ±10% ±20% Mnemonics : As an aid to memory in remembering the sequence of colour codes given above, the student can remember any one of the following sentences (all the capital letters stand for colours) : (a) Bill Brown Realized Only Yesterday Good Boys Value Good Work. (b) Bye Bye Rosie Off You Go Bristol Via Great Western. (c) B B ROY of Great Britain had Very Good Wife. Example 1.1 A resistor has a colour band sequence : yellow, violet, orange and gold. Find the range in which its value must lie so as to satisfy the manufacturer’s tolerance. Solution : With the help of the colour coding table (Table 1.1), we find : 1st band Yellow 4 2nd band Violet 7 3rd band Orange 103 4th band Gold ±5% = 47 kW ±5% 47 × 10 3 × 5 W = 2. 35 k W 100 Therefore, the resistance should be within the range 47 kW ± 2.35 kW, or between 44.65 kW and 49.35 kW. Now, 5 % of 47 kW = Example 1.2 A resistor has a colour band sequence : gray, blue, gold and silver. What is the range in which its value must lie so as to satisfy the manufacturer’s tolerance ? Introduction to Electronics 9 Solution : The specification of the resistor can be found by using the colour coding table as follows : 1st band Gray 8 2nd band Blue 6 3rd band Gold 10–1 4th band Silver ±10% = 86 × 0.1 W ±10% = 8.6 W ±10% 10% of 8.6 W = 0.86 W The resistance should lie somewhere between the values (8.6 – 0.86) W and (8.6 + 0.86) W, or 7.74 W and 9.46 W. Standard Resistor Values : In most electronic circuits, it is not necessary to use resistors of exact values. The circuit works satisfactory even if the resistances differ from the designed values by as much as ±20%. Therefore, we don’t have to manufacture resistors of all values. In market, resistors of standard values as given in Table 1.2, are readily available. Table 1.2 Standard values of commercially available resistors (having 10% tolerance) Ohms (W) 1.0 1.2 1.5 1.8 2.2 2.7 3.3 3.9 4.7 5.6 6.8 8.2 10 12 15 18 22 27 33 39 47 56 68 82 100 120 150 180 220 270 330 390 470 560 680 820 Kilohms (kW) 1.0 1.2 1.5 1.8 2.2 2.7 3.3 3.9 4.7 5.6 6.8 8.2 10 12 15 18 22 27 33 39 47 56 68 82 Megohms (MW) 100 120 150 180 220 270 330 390 470 560 680 820 1.0 1.2 1.5 1.8 2.2 2.7 3.3 3.9 4.7 5.6 6.8 8.2 10 12 15 18 22 Variable Resistors Big size variable resistors are usually called rheostats. In electronic circuits, we use small size variable resistors, and they are called potentiometers (usually abbreviated to ‘pots’). Figure 1.2a shows the basic construction of a pot. A resistance wire is wound over a dough-shaped core of bakelite or ceramic. There is a rotating shaft at the centre of the core. The shaft moves an arm and a contact point from end to end of the resistance element. The outer two terminals are the end points of the resistance element and the middle leads to the rotating contact. Its symbol is given in Fig. 1.2b. The arrow indicates the movable contact. The variation of resistance in a potentiometer can be either linear or nonlinear. As shown in Figure 1.3, the linear type has the core (or former) of uniform height. The core of the non-linear type is made from a tapered strip. The pots used as volume control in sound equipments are generally of nonlinear type (with logarithmic variation). 10 Electronics Engineering Fig. 1.2 Variable resistor (potentiometer) Fig. 1.3 Wire-wound potentiometer Some Special Resistors There are some resistors which have special characteristics. These are used in specific applications. Thermistor : The conventional wire-wound metallic resistors have positive temperature coefficient of resistance. It means that their resistance increases when the temperature rises. A thermistor is a device whose resistance decreases with increasing temperature. When the temperature of a semiconductor [such as germanium (Ge) or silicon (Si)] rises, more covalent bonds break. This produces larger number of free electrons and holes. It now conducts better. In other words, the resistance decreases. Although Ge or Si could be used to make a thermistor, but in practice we don’t use Ge or Si. The conducting property of these materials (Ge or Si) is too sensitive to impurities. Commercial thermistors are made of sintered mixtures of Mn2O3, NiO2 and Co2O3. Typical thermistor structures are shown in Fig. 1.4. Fig. 1.4 Thermistor structures There are some electronic circuits in which the variation of resistance-value with temperature cannot be tolerated. In such circuits, thermistors are used to compensate for the change in resistance of ordinary components. Introduction to Electronics 11 Light Dependent Resistor (LDR) The resistance of this resistor depends on the intensity of light falling on it. It is also called photoresistive cell, or photoresistor. Figure 1.5 shows the structure and the symbol of an LDR. It is made of zig-zag strips of light-sensitive semiconductor material. The ends of the strips are attached to the external pins. The whole assembly is protected by a transparent plastic or glass cover. The lightsensitive material in an LDR is either cadmium sulfide (CdS) or cadmium selenide (CdSe). Fig. 1.5 A light-dependent resistor (LDR) Fig. 1.6 Variation of resistance with light intensity for a typical LDR Figure 1.6 shows a plot of resistance versus light intensity for a typical LDR. The resistance can change over a very wide range (from 100 kW to a few hundred ohms). The high resistance of the LDR when not illuminated is called its dark resistance. The LDRs are used in light meters, lighting controls, automatic door openers, thief-detectors, etc. Voltage Dependent Resistor (VDR) : It is possible to have a device in which the resistance between its two terminals is dependent on the voltage at its third terminal. A junction fieldeffect transistor (JFET) has three terminals—drain (D), source (S) and gate (G). The source terminal is normally grounded. For a given value of gate voltage (VGS), the drain current (ID) varies linearly with the drain voltage (VDS), till pinch-off occurs. This region is called ohmic region or triode region. As shown in Figure 1.7, if we change VGS, the current ID still remains linearly dependent on VDS. But the slope of the straight line changes. It shows that in this region, the resistance between drain and source depends upon the gate voltage. 2. Inductors Fig. 1.7 Drain characteristics of a JEFT Inductors are made by winding a conducting wire (with very low-resistance) in the form of a coil. It may have either air-core or iron-core. The current flowing through the core produces 12 Electronics Engineering a magnetic field. This field reacts so as to oppose any change in current, by developing an induced emf. The inductance (L) of an inductor is measured in henrys (H). Sometimes, we have two or more windings on the same core. It is then called a transformer, or a set of coupled (magnetically) coils. An inductor can be fixed or variable. Different types of inductors are available for different applications. Filter chokes are the inductors used in smoothing the pulsating currents produced by a rectifier (a circuit converting ac into dc). A typical filter choke has many turns of wire wound on an iron core. Many power supply units use filter chokes of 5 to 20 H, capable of carrying current up to 0.3 A. Audio-frequency chokes (AFCs) are used to provide high impedance to audio frequencies (say, between 60 Hz and 5 kHz). These are comparatively smaller in size and have lower inductance. The radio-frequency chokes (RFCs) have still smaller inductance. Variable inductors are used in tuning circuits for radio Fig. 1.8 Permeability-tuned frequencies. The permeability-tuned coil has a ferromagnetic variable coil shaft which can be screwed in or out of the core (Fig. 1.8). 3. Capacitors A capacitor can store energy in its electric field, and release it whenever desired. A capacitor opposes any change in the pd (potential difference) applied across its terminals. The capacitance (C) of a capacitor is measured in farads (F). However, as this unit is too large, practical capacitors are specified in microfarads(mF), or picofarads(pF). A capacitor offers low impedance to ac, but very high impedance to dc. So, capacitors are used when we want to couple alternating voltage from one circuit to another, while at the same time blocking the dc voltage from reaching the next circuit. In such usage, it is called coupling capacitor or blocking capacitor. It is also used as a bypass capacitor, where it bypasses the ac so that the ac does not go through the circuit across which it is connected. A capacitor also forms a tuned circuit in series or parallel with an inductor. A capacitor consists of two conducting plates, separated by an insulating material known as a dielectric. Capacitors, like resistors, can either be fixed or variable. Some of the most commonly used fixed capacitors are mica, ceramic, paper, and electrolytic. Variable capacitors are mostly air-gang capacitors. Mica Capacitors : Mica capacitors are made from plates of aluminium foil separated by sheets of mica, as shown in Fig. 1.9. The plates are connected to two electrodes. The mica capacitors have excellent characteristics under stress of temperature variations and high voltage applications (~500 V). Available capacitances range from 5 to 10 000 pF. Its leakage current is very small (Rleakage is about 1000 MW). Ceramic Capacitors : Ceramic capacitors are made in many shapes and sizes. A ceramic disc is coated on two sides with a metal, such as copper or silver. These coatings act as two plates (Fig. 1.10). After attaching tinned-wire leads, the entire unit is coated with plastic and marked with its capacitance value—either using numerals or colour code. The colour coding is similar to that used for resistances. Ceramic capacitors are very versatile. Their working Introduction to Electronics 13 Fig. 1.9 The construction of mica capacitor Fig. 1.10 Basic construction of a ceramic capacitor voltage ranges from 3 V (for use in transistors) up to 6000 V. The capacitance value ranges from 3 pF to about 3 mF. Ceramic capacitors have a very low leakage current (Rleakage is about 1000 MW) and can be used in both dc and ac circuits. Paper Capacitors : This capacitor consists of two metal foils separated by strips of paper. This paper is impregnated with a dielectric material such as wax, plastic or oil. Since paper can be rolled between two metal foils, it is possible to concentrate a large plate area in a small volume. (Fig. 1.11). Paper capacitors have capacitances ranging from 0.0005 mF to several mF, and are rated from about 100 V to several thousand volts. They can be used for both dc and ac circuits. Its leakage resistance is of the order of 100 MW. Fig. 1.11 Basic construction of a paper capacitor Electrolytic Capacitors : An electrolytic capacitor consists of an aluminium-foil electrode which has an aluminium-oxide film covering on one side. The aluminium plate serves as the positive plate and the oxide as the dielectric. The oxide is in contact with a paper or gauze saturated with an electrolyte. The electrolyte forms the second plate (negative) of the capacitor. Another layer of aluminium without the oxide coating is also provided for making electrical contact between one of the terminals and the electrolyte. In most cases, the negative plate is directly connected to the metallic container of the capacitor. The container then serves as the negative terminal for external connections. The aluminium oxide layer is very thin. Therefore, the capacitor has a large capacitance in a small volume. It has high capacitance-to-size ratio. It is primarily designed for use in circuits where only dc voltages are applied across the capacitor. The terminals are marked +ve and – ve. Ordinary electrolytic capacitors cannot be used with alternating currents. However, there are capacitors available that can be used in ac circuits (for starting motors) and in cases where the polarity of the dc voltage reverses for short periods of time. The capacitance value may range from 1 mF to several thousand microfarads. The voltage ratings may range from 1 V to 500 V, or more. 14 Electronics Engineering Variable Capacitors : The most common variable capacitor is the air-gang capacitor, shown in Fig. 1.12. The dielectric for this capacitor is air. By rotating the shaft at one end, we can change the common area between the movable and fixed set of plates. The greater the common area, the larger the capacitance. In some applications, the need for variation in the capacitance is not frequent. One setting is sufficient for all normal operations. In such situations, we use a variable capacitor called a trimmer (sometimes called padder). Both mica and ceramic are used as the dielectric for trimmer capacitors (Fig. 1.13). Fig. 1.12 Air-gang capacitor (variable) Fig. 1.13 Basic construction of a mica trimmer 1.6 VOLTAGE AND CURRENT DIVIDERS The concepts of voltage divider and current divider are very useful in analysing networks. Voltage Divider In the simple circuit of Fig. 1.14, the voltages across the series resistors are given as V1 = R1I = R 1 V R1 + R 2 or V1 = V R1 R1 + R 2 ...(1.1) and V2 = V R2 R1 + R 2 ...(1.2) Fig. 1.14 Voltage divider Current Divider In the simple circuit of Fig. 1.15, the currents in the parallel resistors are given as and I1 = I R2 R1 + R 2 ...(1.3) I2 = I R1 R1 + R 2 ...(1.4) Note that in Eq. 1.1 for V1, we have R1 in the numerator; but in Eq. 1.3 for I1 we have R2. Fig. 1.15 Current divider Introduction to Electronics 15 1.7 SOURCES OF ELECTRICAL POWER A source supplies power to the load. A dc source supplies dc (direct current) and an ac source supplies ac (alternating current). Some example of dc sources are battery, dc generator and rectification-type dc supply. Examples of ac sources are alternators, oscillators and signal generators. A source has an emf (electromotive force). It represents the driving influence that causes a current to flow. Actually, emf is not a force, but it represents the energy expended during the passing of a unit charge through the source. The emf and pd (potential difference) are similar quantities. Both are measured in volts (V). However, an emf is always active in the sense that it tends to produce an electric current in a circuit. The pd may be either passive or active. A pd is passive when it has no tendency to create current in a circuit. Note from Fig. 1.16 that the current flow leaves the source at positive terminal and therefore is in the same direction as the emf (indicated by arrow). The current enters the load at positive terminal. In a load (passive element) the current and pd are in opposite direction. To describe the characteristics of a source of electrical energy, the concepts of ideal voltage source and ideal current source are very helpful. Fig. 1.16 Transfer of energy from source to load Fig. 1.17 Symbols for ideal voltage sources Ideal Voltage Source The ideal voltage source maintains its prescribed voltage independent of its output current. It means that even if the current drawn from such a source varies from zero value (open-circuit condition) to infinity (short-circuit condition), its terminal voltage remains unchanged. It implies that an ideal voltage source is capable of supplying unlimited amount of power. Practical Voltage Source In practice, no source can be represented as an ideal voltage source. When some current is drawn from the source, its terminal voltage is found to drop by some amount. The larger the current drawn, the larger is the voltage drop. To account for this lowering of the terminal voltage, a practical voltage source is represented as ‘an ideal voltage source in series with a resistance (or impedance, in case of an ac source)’. This resistance is called the internal resistance of the source. In Fig. 1.18 RSV represents the internal resistance of the practical voltage source. The linear relationship between the load voltage VL and load current IL is VL = VS – RSVIL ...(1.5) 16 Electronics Engineering Fig. 1.18 A practical voltage source The open-circuit voltage (VLOC) and short-circuit current (ILSC) are VLOC = V S ILSC = ...(1.6) VS RSV ...(1.7) Ideal Current Source An ideal current source produces its prescribed current, independent of its output voltage. Thus, the output current of such a source remains unchanged from zero load (open-circuit condition) to an infinite load (short-circuit condition). This could be possible only if the source was capable of supplying unlimited amount of power. Practical Current Source Fig. 1.19 Symbols for ideal current sources Like an ideal voltage source, an ideal current source is also non-existent in the real world. Certain transistor circuits can deliver a constant current to a limited range of load. A practical current source is modelled as an ideal current source in parallel with an internal resistance RSI, as shown in Fig. 1.20.] Fig. 1.20 A practical current source The load current is given as IL = I S - VL R SI ...(1.8) Introduction to Electronics 17 The open-circuit voltage (VLOS) and the short-circuit current (ILSC) are VLOC = IS RSI ...(1.9) ILSC = I S ...(1.10) Equivalence between Voltage Source and Current Source An ideal voltage source and an ideal current source are non-existent in practice. A practical source may be treated either as a voltage source or as a current source, depending upon which type suits better for the analysis of the network. Thus, the transformation of one type of source into another type is frequently needed while analysing a network. The two sources would be equivalent if they produce identical values of IL and VL, when they are connected to the same load RL, whatever be its value. The two equivalent sources should also provide the same open-circuit voltage and short-circuit current. The conditions of equivalence can now be established. Equating the open-circuit voltage (VLOC) in the two cases (from Eqs. 1.6 and 1.9), we get VS = ISRSI ...(1.11) Equating the short-circuit currents in the two cases (from Eqs. (1.7) and (1.10)), we get VS = IS RSV ...(1.12) From the above two equations, it follows that RSV = RSI = RS (say) and VS = RS IS ...(1.13) where RS would represent the internal resistance of either of the sources. As shown in Fig. 1.21, such two practical sources will be equivalent with respect to what happens at the load terminals. However, they are not equivalent internally. Fig. 1.21 A source connected to a load 18 Electronics Engineering Example 1.3 Find the equivalent current source representation of the dc voltage source given in Fig. 1.22. Take a load resistance of 1 W and show that the two equivalent source representations give same load current and load voltage. Solution : From Eq. 1.13, the current IS of the equivalent current source is IS = VS 2 V = =2A RS 1 W Fig. 1.22 A voltage source The internal resistance remains the same but it is now connected in parallel with the current source IS, as shown in Fig. 1.23a. Fig. 1.23 Equivalent representations Now, we connect a load resistance RL = 1 W across the terminals of the two representations, and find IL and VL. From Fig. 1.23a, using the current-divider concept, we get IL = I S \ RS = 2× 1 = 1 A 1+1 RS + RL VL = IL RL = 1 × 1 = 1 V From Fig. 1.23b, using the voltage-divider concept, we get VL = VS RL = 2× 1 = 1 V RS + RL 1+1 VL 1 = =1A RL 1 Thus, we find that the two representations give the same values of IL and VL. \ IL = Example 1.4 Consider a practical ac voltage source having an open-circuit voltage of 10 V and an internal resistance of 100 W. Find the percentage variation in load current and load voltage, if the load connected across its terminals varies (a) from 1 W to 10 W, (b) from 1 kW to 10 kW. Introduction to Electronics 19 Solution : (a) RL varies from 1 W to 10 W (Fig. 1.24a) The currents for the two extreme values of RL are I L1 = VS 10 = 0.0990 A = RL1 + R S 1 + 100 I L2 = VS 10 = 0.0909 A = RL2 + RS 10 + 100 0.0990 - 0.0909 × 100 = 8.1% 0. 0990 Now, the load voltages for the two extreme values of RL are \ Percentage variation in current = VL1 = IL RL = 0.0990 × 1 = 0.099 V 1 1 VL2 = IL RL = 0.0909 × 10 = 0.909 V 2 2 0.909 - 0.099 × 100 = 89.1% 0. 909 We find that the percentage variation in load current is much less than that in load voltage. Thus, the behaviour of the source is nearer to the ideal current source, under this condition. \ Percentage variation in voltage = Fig. 1.24 (b) RL varies from 1 kW to 10 kW (Fig. 1.24b) The currents for the two extreme values of RL are IL1 = VS 10 = 0.00909 A = 9.09 mA = RL1 + RS 1000 + 100 IL = VS 10 = 0.000999 A = 0.999 mA = RL2 + RS 10000 + 100 2 \ Percentage variation in current = 9. 09 - 0. 999 × 100 = 89% 9. 09 20 Electronics Engineering Now, the load voltage for the two extreme values of RL are VL1 = I L1 RL1 = 9.09 × 10–3 × 1 × 103 = 9.09 V VL2 = I L2 RL2 = 0.999 × 10–3 × 10 × 103 = 9.99 V \ Percentage variation in voltage = 9. 99 - 9. 09 × 100 = 9% 9. 99 We find that the percentage variation in load voltage is much less than that in load current. Thus, the behaviour of the source is nearer to the ideal voltage source, under this condition. We, therefore, conclude that a source should better be treated as a constant current source, if its internal resistance RS is much larger than the load resistance RL (i.e., if RS >> RL). On the other hand, if RS << RL, it is better to treat the source as a constant voltage source. 1.8 SIGNALS Electronic systems are used to process and to transmit information. For example, the information that is keyed into a calculator goes to the microprocessor inside, and then finally to the display. Or, a system might collect information about the pressure of gas in a pipe and transmit it to a control room. Or, a system might transmit a television picture from one side of the world to the other. When we transmit information, we require it to be translated into some equivalent electrical value (say, a voltage or a current). For example, when we vary a voltage to represent, say, sound, then the varying voltage is called signal. Equally, if we were to vary a current for the same purpose, we would call the varying current, the signal. Signals fall under two categories—analog and digital. Analog signals first came into prominence with telephone. Digital signals first appeared when telegraph system using Morse code was introduced. An analog signal has continuous variation (Fig. 1.25a). It can have any value from an infinite number of values. Such variation is associated with production of sound (as in the telephone or radio). Digital signals have one of a limited number of discrete values (Fig. 1.25b). In most applications, there are only two discrete values. These values are described in crude terms—on and off, closed and open, 1 and 0, or high and low. Fig. 1.25 Electrical signals Introduction to Electronics 21 1.9 SI UNITS This system of units is coherent, rational and comprehensive. It is now followed everywhere in the world—at least in engineering. It has seven base units and many derived units. Some Derived Units Physical quantity Name of SI unit Frequency Force Work, energy, quantity of heat Power Electric charge Electric potential Electric capacitance Electric resistance Electric conductance Magnetic flux Magnetic flux density Inductance Customary temperature Pressure hertz newton joule watt coulomb volt farad ohm siemens* weber tesla henry degree celsius pascal Symbol Hz = cycles/s = 1/s N= kg m/s2 J=Nm W = J/s C = A/s V = W/A F = A/s/V W = V/A S = A/V Wb = V/s T = Wb/m2 H = Vs/A °C Pa = N/m2 *The unit siemens is same as mho (Á) which was used earlier. SI Prefixes Factor Prefix Symbol Factor 101 102 103 106 109 1012 1015 1018 deca hecto kilo mega giga* tera peta exa da h k M G T P E 101 102 103 106 109 1012 1015 1018 Prefix deci centi milli micro nano pico femto atto Symbol d c m m n p f a *Pronounced as jeega. Note : (i) The prefixes for factors greater than unity have Greek origin; those for factors less than unity have Latin origin (except femto and atto, recently added, which have Danish origin). (ii) Almost all abbreviations of prefixes for magnitudes <1, are English lowercase letters. An exception is micro (Greek letter m). (iii) Abbreviations of prefixes for magnitudes >1 are English upper-case letters. Exceptions are kilo, hecto, and deca. (iv) The prefixes hecto, deca, deci and centi should not be used unless there is a stronglyfelt need. 1. Multiples of the fundamental unit should be chosen in powers of ±3n where n is an integer. Centimetre, owing to its established usage and its convenient size, cannot be given up lightly. 22 Electronics Engineering 2. Double or compound prefixes should be avoided, e.g., instead of micromicrofarad (mmF) or millinanofarad (mnF), use picofarad (pF). 3. To simplify calculations, attach the prefix to the numerator and not to the denominator. Example : use MN/m2 instead of N/mm2, even though mathematically both forms are equivalent. 4. The rules for binding-in indices are not those of ordinary algebra, e.g., cm2 means (cm)2 = (0.01)2 m2 = 0.0001 m2, and not c × (m)2 = 0.01 m2. Another Unit of Energy : The Electron Volt Although joule (J) is the standard unit of energy in SI system, another unit—the electron volt (eV)—proves more useful in electronics. One electron volt is defined as the amount of energy gained by an electron when it moves through a potential rise of one volt. 1 eV = charge on an electron × 1 volt = 1.602 × 10–19 J REVIEW QUESTIONS 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. What is electronics ? How has it affected our daily life ? What are the modern trends in electronics ? Name any three electronic components. What are the different types of resistors ? What are the various types of capacitors used in electronics industry ? What is meant by active and passive components ? What is used as a dielectric in an electrolytic capacitor ? Why is an electrolytic capacitor polarized ? Name three primary uses of capacitors in electronics. While tuning your radio receiver to a desired station, which component inside the set are you varying ? When you adjust the volume control knob of your radio receiver, which component is varied inside the set ? What is an inductor? What is the unit of inductance ? Explain the condition under which a practical voltage source is considered to be a good voltage source. A practical source can be represented either as a voltage source or as a current source. How can you convert one representation into the other ? What is the difference between an analog signal and a digital signal ? Is the output of a microphone analog or digital ? What is the SI unit of electric conductance ? Define electron volt, the unit of energy. How is the use of eV as the unit of energy more convenient in the field of electronics ?