Introduction to Electronics - newagepublishers.com integral part of electrical engineering. ... We are moving into the new era of Information Technology. 1.5 ELECTRONIC CIRCUIT

1Introduction to Electronics

Objectives : After completing this Chapter, you will be able to :

� 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 electronmechanics. Electronics is the science and technology of the motion of electrons (or other suchcharges) in gas, vacuum, or in any semiconductor. It deals principally with the communicationof information and/or data handling.

Compared to the more established branches of engineering—civil, mechanical and electri-cal, the electronics is a newcomer—hardly 100 years old. Until around 1960, it was consideredan integral part of electrical engineering. But due to the tremendous advancement over thelast few decades, electronics has now gained its rightful place. The advancement has been sofast that many sub-branches of electronics—such as Computer Science Engineering, Commu-nication 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 thecellular phone. However, to most people, what goes on inside, is a mystery. An ElectronicsEngineer knows and understands the functioning of these devices. He acquires the capabilityto 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 electronicscame in 1906 when vacuum triode was invented by Lee De Forest. This device could amplifyelectrical signals. Later, around 1925, tetrode and pentode tubes were developed. These tubesdominated 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 transistorin 1948. Bardeen, Brattain and Shockley were awarded the Nobel Prize in Physics in 1956 forthis invention. This was the first Nobel award given for an engineering device in nearly 50years.

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 andfilaments burnt out, requiring frequent tube replacements. By now, vacuum tubes have becomehistory.

Earlier, the transistors were made from germanium as it was easier to purify a sample ofgermanium. In 1954, silicon transistors were developed. These afforded operations upto200 °C, whereas germanium device could work well only upto about 75 °C. Today, almostall 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 singlesemiconductor chip. All active and passive components and their interconnections could beintegrated on a single chip, during the manufacturing process. This drastically reduced thesize and weight, as well as the cost per active component.

The term ‘microelectronics’ refers to the design and fabrication of these high component-density ICs. The following approximate dates give some indication of the increasing componentcount per chip of area 3 × 5 mm2 and thickness 0.3 mm (about three times the thickness ofhuman hair) :

1951 — Discrete transistors1960 — Small-Scale Integration (SSI), fewer than 100 components1966 — Medium-Scale Integration (MSI), 100 to 1000 components1969 — Large-Scale Integration (LSI), 1000 to 10000 components1975 — 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 ofapplied electric field on the conductivity of a semiconductor. A reliable JFET was produced in1958.

The techniques to make reliable JFETs led to an even more important device called metal-oxide-semiconductor field-effect transistor (MOSFET). Subsequent improvements in processingand device design, and the growth of the computer industry have made MOS devices the mostwidely used transistors.

Introduction to Electronics 3

Digital Integrated Circuits

The growth of computer industry spurred new IC development. In turn, the new IC conceptsresulted in new computer architecture.

Speed, power consumption, and component density are important considerations in digitalICs. 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 onhole flow. Improved fabrication methods led to the use of N-channel MOS (NMOS). Thesegave 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, bipolarjunction transistors (BJTs) were used in making random access memories (RAMs), which werecapable of both storing and retrieving data (i.e., writing and reading). These could store about100 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-bitchips available.

Read-only memories (ROMs), used for look-up tables in computers (e.g., to obtain the valueof 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) andnew 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 processingand communication.

Analog Integrated Circuits

The first operational amplifier (OP AMP) was developed in 1964. Since then the OP AMP hasbecome the “workhorse” in analog signal processing. Other circuits and systems developedsubsequently 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, toappreciate and to meet the challenges of these dynamic fields, it has become essential toequip 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 andtelevision receiver. The video section of a TV reproduces the picture signal received. Manyinteresting toys use ICs. The telephone systems now employ digital ICs for switching andmemory. Communication satellites became feasible because of microelectronics.


A new era of electronics, called digital signal processing (DSP), has evolved because ICs havemade 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 andinverters. 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 usedto detect the location of enemy jet fighters, war-ships and submarines, and then to control theaiming and firing of guns.

Guided missiles are completely controlled by electronic means. Electronic circuits providea means of secret communication between the head-quarter and different units. Such acommunication 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 ofthickness, quality, weight and moisture content of a material can be easily done by suchsystems.

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 controlledby tiny electronic devices and circuits.

5. Medical Sciences

Doctors and scientists are finding new uses of electronic systems in the diagnosis and treatmentof 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 user-friendly 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 electroniccontrol systems. After the vehicle comes on the road, its efficient running depends on electroniccontrols. Electronic ignition system, multipoint fuel injection (MPFI) system, electronic batterycharger, 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, youcan instantly get any information from anywhere in the world. Soon, it will be possible toinstantly contact any person anywhere in the world.


The merging of extensive communication and inexpensive computers has already begunpenetrating nearly every aspect of the society. The ability and relative ease by which informationcan be stored, retrieved, manipulated and transmitted is affecting us in our homes, our placesof 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 typesof 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 theshape of electrical signals. Passive components, such as resistors, inductors and capacitors, donot change the shape of the signal. An electric circuit becomes an electronic circuit when it hasone or more active component(s). However, an active component all alone cannot performany useful function.

Active Components

There are many active components used in electronic circuits. Earlier, active devices were oftube-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 usednow-a-days.

Almost all electronic circuits today use semiconductor devices or ICs. Brief informationabout 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 greekcapital letter omega).

All resistors have power ratings. It is the maximum power that can be dissipated withoutraising the temperature too high. Thus, a 1-W resistor with a resistance of 100 W can pass a



maximum current of 100 mA. But a ¼ W resistor with the same resistance value (100 W) couldhandle a current of only 50 mA. If the current exceeds this limit for any length of time, theresistor 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 valuemust not exceed the rating.

The size of a resistor is bigger if its wattage rating is higher, so as to withstand higherdissipation losses.

A resistor is said to be linear, if the current through it is proportional to the pd (potentialdifference) across its terminals; otherwise it is nonlinear. Resistors made from semiconductormaterials are non-linear.

Resistors are made in many forms. But all belong to either of the two groups—fixed andvariable.

Fixed Resistors

Most resistors used in electronic circuits are fixed (or non-variable). And most of these aremoulded-carbon composition resistors. In such resistors, the resistive material is of carbon-claycomposition. The leads are made of tinned copper. Resistors of this type are readily availablein values ranging from a few ohms to about 22 MW, with tolerance range of 5 to 20 % , andwattage 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 eithertrimming 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 canbe obtained with an accuracy of ±1 %.

When ratings of more than 1 W are required, we use wire wound resistors. A thin nichromewire is wound on a ceramic or porcelain core. These resistors are available in the range of 1 Wto 100 kW, and power ratings upto about 200 W.

Resistance Colour Coding : Carbon-moulded and metal film resistors are small in size. Itbecomes almost impossible to print the ratings on their body. Therefore, a standard colourcoding 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 closestto it, as shown in Fig. 1.1.

Fig. 1.1 Colour coding of resistors


The first and second colour bands represent the first and second numbers (significantdigits) respectively, of the resistance value. The third band indicates the number of zeros thatfollow the second digit. In case the third band is gold or silver, it represents a multiplyingfactor of 0.1 or 0.01, respectively. The fourth band represents tolerance. It is a measure of theprecision with which the resistor was manufactured. In case the fourth band is not present, thetolerance is assumed ±20 %.

Table 1.1 Colour coding

Colour Digit Multiplier Tolerance

Black 0 100 = 1

Brown 1 101 = 10

Red 2 102

Orange 3 103

Yellow 4 104

Green 5 105

Blue 6 106

Violet 7 107

Gray 8 108

White 9 109

Gold – 0.1 = 10–1 ± 5 %

Silver – 0.01 = 10–2 ± 10 %

No colour – – ± 20 %

Mnemonics : As an aid to memory in remembering the sequence ofcolour codes given above, the student can remember any one of thefollowing 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. Findthe 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 2nd band 3rd band 4th band

Yellow Violet Orange Gold

4 7 103 ±5 % = 47 kW ± 5 %

Now, 5 % of 47 kW = ��

��

��

W W=

Therefore, the resistance should be within the range 47 kW ± 2.35 kW, or between 44.65 kWand 49.35 kW.

Example 1.2 A resistor has a colour band sequence : gray, blue, gold and silver. What is therange in which its value must lie so as to satisfy the manufacturer’s tolerance ?


Solution : The specification of the resistor can be found by using the colour coding table asfollows :

1st band 2nd band 3rd band 4th band

Gray Blue Gold Silver

8 6 10–1 ±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, or7.74 W and 9.46 W.

Standard Resistor Values : In most electronic circuits, it is not necessary to use resistors ofexact values. The circuit works satisfactory even if the resistances differ from the designedvalues 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) Kilohms (kW) Megohms (MW)

1.0 10 100 1.0 10 100 1.0 10

1.2 12 120 1.2 12 120 1.2 12

1.5 15 150 1.5 15 150 1.5 15

1.8 18 180 1.8 18 180 1.8 18

2.2 22 220 2.2 22 220 2.2 22

2.7 27 270 2.7 27 270 2.7

3.3 33 330 3.3 33 330 3.3

3.9 39 390 3.9 39 390 3.9

4.7 47 470 4.7 47 470 4.7

5.6 56 560 5.6 56 560 5.6

6.8 68 680 6.8 68 680 6.8

8.2 82 820 8.2 82 820 8.2

Variable Resistors

Big size variable resistors are usually called rheostats. In electronic circuits, we use small sizevariable resistors, and they are called potentiometers (usually abbreviated to ‘pots’). Figure 1.2ashows the basic construction of a pot. A resistance wire is wound over a dough-shaped coreof bakelite or ceramic. There is a rotating shaft at the centre of the core. The shaft moves anarm and a contact point from end to end of the resistance element. The outer two terminals arethe end points of the resistance element and the middle leads to the rotating contact. Itssymbol 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 inFigure 1.3, the linear type has the core (or former) of uniform height. The core of the non-lineartype is made from a tapered strip. The pots used as volume control in sound equipments aregenerally of nonlinear type (with logarithmic variation).


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 specificapplications.

Thermistor : The conventional wire-wound metallic resistors have positive temperaturecoefficient of resistance. It means that their resistance increases when the temperature rises. Athermistor 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 nowconducts better. In other words, the resistance decreases. Although Ge or Si could be used tomake a thermistor, but in practice we don’t use Ge or Si. The conducting property of thesematerials (Ge or Si) is too sensitive to impurities. Commercial thermistors are made of sinteredmixtures 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 temperaturecannot be tolerated. In such circuits, thermistors are used to compensate for the change inresistance of ordinary components.


Light Dependent Resistor (LDR)

The resistance of this resistor depends on the intensity of light falling on it. It is also calledphotoresistive cell, or photoresistor.

Figure 1.5 shows the structure and the symbol of an LDR. It is made of zig-zag strips oflight-sensitive semiconductor material. The ends of the strips are attached to the externalpins. The whole assembly is protected by a transparent plastic or glass cover. The light-sensitive material in an LDR is either cadmium sulfide (CdS) or cadmium selenide (CdSe).

Figure 1.6 shows a plot of resistance versus light intensity for a typical LDR. The resistancecan change over a very wide range (from 100 kW to a few hundred ohms). The high resistanceof 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 field-

effect 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 dependsupon the gate voltage.

2. Inductors

Inductors are made by winding a conducting wire (with very low-resistance) in the form of acoil. It may have either air-core or iron-core. The current flowing through the core produces

Fig. 1.6 Variation of resistance with light intensity

for a typical LDR

Fig. 1.5 A light-dependent resistor (LDR)

Fig. 1.7 Drain characteristics

of a JEFT


a magnetic field. This field reacts so as to oppose any change in current, by developing an inducedemf. 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 differentapplications.

Filter chokes are the inductors used in smoothing the pulsating currents produced by arectifier (a circuit converting ac into dc). A typical filter choke has many turns of wire woundon an iron core. Many power supply units use filter chokes of 5 to 20 H, capable of carryingcurrent upto 0.3 A.

Audio-frequency chokes (AFCs) are used to providehigh impedance to audio frequencies (say, between 60 Hzand 5 kHz). These are comparatively smaller in size andhave lower inductance. The radio-frequency chokes (RFCs)have still smaller inductance.

Variable inductors are used in tuning circuits for radiofrequencies. The permeability-tuned coil has a ferromagneticshaft 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 capacitoropposes 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, practicalcapacitors are specified in microfarads(mF), or picofarads(pF).

A capacitor offers low impedance to ac, but very high impedance to dc. So, capacitors areused when we want to couple alternating voltage from one circuit to another, while at thesame time blocking the dc voltage from reaching the next circuit. In such usage, it is calledcoupling capacitor or blocking capacitor. It is also used as a bypass capacitor, where it bypasses theac so that the ac does not go through the circuit across which it is connected. A capacitor alsoforms a tuned circuit in series or parallel with an inductor.

A capacitor consists of two conducting plates, separated by an insulating material knownas a dielectric. Capacitors, like resistors, can either be fixed or variable. Some of the mostcommonly used fixed capacitors are mica, ceramic, paper, and electrolytic. Variable capacitorsare mostly air-gang capacitors.

Mica Capacitors : Mica capacitors are made from plates of aluminium foil separated bysheets of mica, as shown in Fig. 1.9. The plates are connected to two electrodes. The micacapacitors have excellent characteristics under stress of temperature variations and highvoltage applications (~500 V). Available capacitances range from 5 to 10 000 pF. Its leakagecurrent is very small (Rleakage is about 1000 MW).

Ceramic Capacitors : Ceramic capacitors are made in many shapes and sizes. A ceramicdisc is coated on two sides with a metal, such as copper or silver. These coatings act as twoplates (Fig. 1.10). After attaching tinned-wire leads, the entire unit is coated with plastic andmarked with its capacitance value—either using numerals or colour code. The colour codingis similar to that used for resistances. Ceramic capacitors are very versatile. Their working

Fig. 1.8 Permeability-tuned

variable coil


voltage ranges from 3 V (for use in transistors) up to 6000 V. The capacitance value rangesfrom 3 pF to about 3 mF. Ceramic capacitors have a very low leakage current (Rleakage is about1000 MW) and can be used in both dc and ac circuits.

Paper Capacitors : This capacitor consists of twometal foils separated by strips of paper. This paperis impregnated with a dielectric material such aswax, plastic or oil. Since paper can be rolled betweentwo metal foils, it is possible to concentrate a largeplate area in a small volume. (Fig. 1.11).

Paper capacitors have capacitances ranging from0.0005 mF to several mF, and are rated from about 100V to several thousand volts. They can be used forboth dc and ac circuits. Its leakage resistance is ofthe order of 100 MW.

Electrolytic Capacitors : An electrolytic capacitor consists of an aluminium-foil electrodewhich has an aluminium-oxide film covering on one side. The aluminium plate serves as thepositive plate and the oxide as the dielectric. The oxide is in contact with a paper or gauzesaturated 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 electricalcontact between one of the terminals and the electrolyte. In most cases, the negative plate isdirectly connected to the metallic container of the capacitor. The container then serves as thenegative terminal for external connections.

The aluminium oxide layer is very thin. Therefore, the capacitor has a large capacitance ina small volume. It has high capacitance-to-size ratio. It is primarily designed for use in circuitswhere 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, thereare capacitors available that can be used in ac circuits (for starting motors) and in cases wherethe polarity of the dc voltage reverses for short periods of time.

The capacitance value may range from 1 mF to several thousand microfarads. The voltageratings may range from 1 V to 500 V, or more.

Fig. 1.9 The construction of mica capacitor Fig. 1.10 Basic construction of a ceramic

capacitor

Fig. 1.11 Basic construction of a paper

capacitor


Variable Capacitors : The most common variable capacitor is the air-gang capacitor, shownin Fig. 1.12. The dielectric for this capacitor is air. By rotating the shaft at one end, we canchange the common area between the movable and fixed set of plates. The greater thecommon area, the larger the capacitance.

In some applications, the need for variation in the capacitance is not frequent. One settingis sufficient for all normal operations. In such situations, we use a variable capacitor called atrimmer (sometimes called padder). Both mica and ceramic are used as the dielectric fortrimmer capacitors (Fig. 1.13).

Fig. 1.12 Air-gang capacitor (variable)

Fig. 1.15 Current divider

Fig. 1.14 Voltage divider

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 =��

� ��

� �+

or V1 = ��

� �

�

� �+

...(1.1)

and V2 = ��

� �

�

� �+

...(1.2)

Current Divider

In the simple circuit of Fig. 1.15, the currents in the parallel resistors are given as

I1 = ��

� �

�

� �+

...(1.3)

and I2 = ��

� �

�

� �+

...(1.4)

Note that in Eq. 1.1 for V1, we have R1 in thenumerator; but in Eq. 1.3 for I1 we have R2.


1.7 SOURCES OF ELECTRICAL POWER

A source supplies power to the load. A dc source supplies dc (direct current) and an ac sourcesupplies ac (alternating current). Some example of dc sources are battery, dc generator andrectification-type dc supply. Examples of ac sources are alternators, oscillators and signalgenerators.

A source has an emf (electromotive force). It represents the driving influence that causes acurrent to flow. Actually, emf is not a force, but it represents the energy expended during thepassing 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 acircuit. The pd may be either passive or active. A pd is passive when it has no tendency tocreate current in a circuit.

Note from Fig. 1.16 that the current flow leaves the source at positive terminal and thereforeis in the same direction as the emf (indicated by arrow). The current enters the load at positiveterminal. 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 voltagesource and ideal current source are very helpful.

Fig. 1.17 Symbols for ideal voltage sourcesFig. 1.16 Transfer of energy from

source to load

Ideal Voltage Source

The ideal voltage source maintains its prescribed voltage independent of its output current. It meansthat 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 anideal 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 isdrawn from the source, its terminal voltage is found to drop by some amount. The larger thecurrent 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)’. Thisresistance is called the internal resistance of the source. In Fig. 1.18 RSV represents the internalresistance of the practical voltage source.

The linear relationship between the load voltage VL and load current IL is

VL = VS – RSVIL ...(1.5)


The open-circuit voltage (VLOC) and short-circuit current (ILSC) are

VLOC = VS ...(1.6)

ILSC =�

�

�

��

...(1.7)

Ideal Current Source

An ideal current source produces its prescribed current,independent of its output voltage. Thus, the output currentof such a source remains unchanged from zero load(open-circuit condition) to an infinite load (short-circuitcondition). This could be possible only if the source wascapable of supplying unlimited amount of power.

Practical Current Source

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 practicalcurrent source is modelled as an ideal current source in parallel with an internal resistance RSI, asshown in Fig. 1.20.]

Fig. 1.20 A practical current source

The load current is given as

IL = ��

��

�

��

- ...(1.8)

Fig. 1.18 A practical voltage source

Fig. 1.19 Symbols for ideal current

sources


The open-circuit voltage (VLOS) and the short-circuit current (ILSC) are

VLOC = IS RSI ...(1.9)

ILSC = IS ...(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 sourcemay be treated either as a voltage source or as a current source, depending upon which typesuits better for the analysis of the network. Thus, the transformation of one type of source intoanother type is frequently needed while analysing a network.

The two sources would be equivalent if they produce identical values of IL and VL, whenthey are connected to the same load RL, whatever be its value. The two equivalent sourcesshould also provide the same open-circuit voltage and short-circuit current. The conditions ofequivalence 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

�

�

�

��

= IS ...(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 whathappens at the load terminals. However, they are not equivalent internally.

Fig. 1.21 A source connected to a load


Example 1.3 Find the equivalent current source representationof the dc voltage source given in Fig. 1.22. Take a load resistanceof 1 W and show that the two equivalent source representationsgive same load current and load voltage.

Solution : From Eq. 1.13, the current IS of the equivalent currentsource is

IS =�

�

�

�

=��

�W = 2 A

The internal resistance remains the same but it is now connected in parallel with the currentsource 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 = ��

� ��

�

� �+

= ��

� ��

+

= 1 A

\ VL = IL RL = 1 × 1 = 1 V

From Fig. 1.23b, using the voltage-divider concept, we get

VL = ��

� ��

�

� �+

= ��

� ��

+

= 1 V

\ IL =�

�

�

�

=�

� = 1 A

Thus, we find that the two representations give the same values of IL and VL.

Example 1.4 Consider a practical ac voltage source having an open-circuit voltage of 10 Vand an internal resistance of 100 W. Find the percentage variation in load current and loadvoltage, if the load connected across its terminals varies (a) from 1 W to 10 W, (b) from 1 kW to10 kW.

Fig. 1.22 A voltage source


Solution :

(a) RL varies from 1 W to 10 W (Fig. 1.24a)

The currents for the two extreme values of RL are

��

=�

� �

�

� ��

��

� ��+

=

+

= 0.0990 A

��

=�

� �

�

� ��

��

�� +

=

+

= 0.0909 A

\ Percentage variation in current =� ��

� ��

� �

�

-

× 100 = 8.1 %

Now, the load voltages for the two extreme values of RL are

��

= IL1 RL

1 = 0.0990 × 1 = 0.099 V

��

= IL2 RL

2 = 0.0909 × 10 = 0.909 V

\ Percentage variation in voltage = � ��

� ��

� �

��

-

= 89.1 %

We find that the percentage variation in load current is much less than that in loadvoltage. Thus, the behaviour of the source is nearer to the ideal current source, under thiscondition.

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 =

�

� �

�

� ��

��

�� +

=

+

= 0.00909 A = 9.09 mA

IL2 =

�

� �

�

� ��

��

�� +

=

+

= 0.000999 A = 0.999 mA

\ Percentage variation in current = � ��

� ��

� �

��

-

= 89 %


Now, the load voltage for the two extreme values of RL are

= � ��

= 9.09 × 10–3 × 1 × 103 = 9.09 V

��

= � ��

= 0.999 × 10–3 × 10 × 103 = 9.99 V

\ Percentage variation in voltage = � ��

� ��

� �

��

-

= 9 %

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 theother 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 informa-tion that is keyed into a calculator goes to the microprocessor inside, and then finally to thedisplay. Or, a system might collect information about the pressure of gas in a pipe andtransmit it to a control room. Or, a system might transmit a television picture from one sideof the world to the other.

When we transmit information, we require it to be translated into some equivalent electricalvalue (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 samepurpose, we would call the varying current, the signal.

Signals fall under two categories—analog and digital. Analog signals first came intoprominence with telephone. Digital signals first appeared when telegraph system usingMorse code was introduced.

An analog signal has continuous variation (Fig. 1.25a). It can have any value from an infinitenumber of values. Such variation is associated with production of sound (as in the telephoneor 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


1.9 SI UNITS

This system of units is coherent, rational and comprehensive. It is now followed everywherein the world—at least in engineering. It has seven base units and many derived units.

Some Derived Units

Physical quantity Name of SI unit Symbol

Frequency hertz Hz = cycles/s = 1/s

Force newton N= kg m/s2

Work, energy, quantity of heat joule J = N m

Power watt W = J/s

Electric charge coulomb C = A/s

Electric potential volt V = W/A

Electric capacitance farad F = A/s/V

Electric resistance ohm W = V/A

Electric conductance siemens* S = A/V

Magnetic flux weber Wb = V/s

Magnetic flux density tesla T = Wb/m2

Inductance henry H = Vs/A

Customary temperature degree celsius °C

Pressure pascal Pa = N/m2

*The unit siemens is same as mho (É) which was used earlier.

SI Prefixes

Factor Prefix Symbol Factor Prefix Symbol

101 deca da 10–1 deci d

102 hecto h 10–2 centi c

103 kilo k 10–3 milli m

106 mega M 10–6 micro m109 giga* G 10–9 nano n

1012 tera T 10–12 pico p

1015 peta P 10–15 femto f

1018 exa E 10–18 atto 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 strongly-

felt need.

1. Multiples of the fundamental unit should be chosen in powers of ±3n where n is aninteger. Centimetre, owing to its established usage and its convenient size, cannot begiven up lightly.


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 areequivalent.

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 energygained 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. What is electronics ? How has it affected our daily life ?

2. What are the modern trends in electronics ?

3. Name any three electronic components.

4. What are the different types of resistors ?

5. What are the various types of capacitors used in electronics industry ?

6. What is meant by active and passive components ?

7. What is used as a dielectric in an electrolytic capacitor ? Why is an electrolytic capacitorpolarized ?

8. Name three primary uses of capacitors in electronics.

9. While tuning your radio receiver to a desired station, which component inside the set areyou varying ?

10. When you adjust the volume control knob of your radio receiver, which component isvaried inside the set ?

11. What is an inductor? What is the unit of inductance ?

12. Explain the condition under which a practical voltage source is considered to be a goodvoltage source.

13. 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 ?

14. What is the difference between an analog signal and a digital signal ?

15. Is the output of a microphone analog or digital ?

16. What is the SI unit of electric conductance ?

17. Define electron volt, the unit of energy.

18. How is the use of eV as the unit of energy more convenient in the field of electronics ?

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Introduction to Electronics - newagepublishers.com integral part of electrical engineering. ... We are moving into the new era of Information Technology. 1.5 ELECTRONIC CIRCUIT