146 Electronics Engineering Technician UNIT 1 Semi Conductor Components Learning Objectives • To study the atomic models, inter automic bonds, conductivity of conductors, semiconductors and insulators. • Semiconductors, doping, formation of P N junction diode, Zener diode, Transistor, and the study V I characteristics and applications. • To observe data sheets specifications of semiconductor devices and applications. 1.0 Introduction 1.0 This chapter deals with physical phonomina involved in semi conductor devices operation. The material in nature, they are divided into 3 categories. they are 1. Conductors 2. Semi conductors 3. Insulaters. The above materials are classified by based on the conduction of current. Paper - II Electronic Devices and Circuits 147 Conductors The materials, which conducts electrical current through it, is known as conductors Examples - Cupper, Aluminium, Silver, brass, iron etc., The conductors possesses low resistivity, hence the resistance affected by it is low, conducts, allows to flow more current through it. Semiconductors The materials, which offer high resistance at a low termperature. These type of materials different behavior, with rise in each degree of temperature resistance decreases. These materials have negative temperature co-efficient of resistance, resulting semiconductors offers very high conductivity. At particularly high temperature high conductivity offers more current to flow through it. Example - Silicon, Germanium etc. The semiconductors resistivity is lies 0.038 cms to 0.04 cms. Insulators These materials consists of high resistance hence practically no current flows through it. No conductivity. Example Paper, Wood, Plastic etc., 1.1 Properties of Solid State Semiconductor The semi conductor devices had resistivity between 10-4 to 104 m. The semiconductor materials are Silicon and Germanium. These semiconductors family lies 4th group in periodic table. Those elements had 4 free electrons in outer orbit. 3rd Orbit 2nd Orbit 1st Orbit Fig. 1.1 Nucleus 148 Electronics Engineering Technician Fig. 1.2 Germanium automic structure Inter atomic bonds are three types. they are (i) Ionic Bond (ii) Covelent bond (iii) Co-ordinate covelent bond. (i) Ionic Bond Two atoms having mutually opposite charges, when they are in exited state combine chemically sharing electrons with opposite charges form a bond between opposite ions is known as ionic bond. Example : Sodium atomic number, Na11, electron configuration Na11 = 1S2, 2S2, 2P6, 3S1. The outer most orbit of sodium is 3rd orbit and it gives one outer electron it becomes as a donar i.e. Na+. The chlorine atomic number is Cl17. The electronic configuration of Chroline is Cl17 = 1S2, 2S2, 2P6, 3S2, 3P5 In the P-orbital only electron is needed to fill P-orbital. It takes one electron and it becomes as acceptor. When it is in exited state Cl- i.e. anion. When these combines together chemically forms a bond between donar and acceptor. forms a bond is known as ionic bond. Na Cl + e- Na+ + eCl- ---- doner ---- acceptor Paper - II Electronic Devices and Circuits Adding Na + Cl + e- 149 Na+ + e- + ClNaCl. (ii) Covelent bond It is the bond, which forms between same type of atoms, by sharing of electrons equally in outer most orbit. Example : Formation of Hydrogen, Chlorine molecules, which were diatomic in state. H-H H2 When hydrogen is in exited state one electron in outer most oribit, the same type of atoms comes together nearer and to get stable state forms as H2. Similarly Cl2 molecules also forms. (iii) Coordinate Covenlent Bond In this type of bond three hydrogen atoms shares three electrons with Px, Py, Pz of Nitrogen atom. When these are in exited state, to get stable condition NH3 molecule is formed. the formation co-ordinates covelent bond figure as follows. H N H NH3 H Fig. 1.3 Formation of Ammonia Molecule 1.1.2 Energy Band Diagram of Material The material are three types. They are (a) Conductors (b) Semiconductors (c) Insulators (a) Conductors In these materials the conductor band energy and valence band energy overlaps on each other. Hence there is no forbidden gap in between the bands, electric current flows, the materials allows low resistance hence more conductivity. Example : All metals are conductors, Copper, Silver, Aluminium etc., 150 Electronics Engineering Technician Conduction Band rgy Band Ene Forbidden Band Valence Band Fig. 1.4 Conductors Energy Band Diagram (b) Semiconductors Band Energy In this type of materials there is small energy gap. It posseses negative resistance characteristics and it changes with change of temperature. Particularly at higher temperature, the semiconductors conducts heavily. Hence these materials used as active devices. Conduction Band Forbidden Gap 0.7 eV Valance Band Fig. 1.5 Semiconductor Energy Band Diagram (c) Insulators In these type of materials forbidden energy gap is more in between conduction and valance bands. These materials there is no current flow. Hence the materials has very high resistance. Examples : Wood, plastic, glass etc., Band Energy Paper - II Electronic Devices and Circuits 151 Conduction Band Forbidden Gap 6.0 eV Valance Band Fig. 1.6 Insulators Energy Band Diagram 1.2 Formation of P Type and N Type Material The semiconductors alloys are in earth. To get pure of semiconductors purification chemical process is done. Classification of Semi Conductors The pure form of semiconductors are known as intrinsic semi conductors. The intrinsic semiconductors are two type (i) P-type intrinsic semiconductor. (ii) N-type intrinsic semiconductor. (i) P- Type Intrinsic Semiconductor It is pure form of P-type semiconductor. It consist of only positive charge carriers. ++++++++ ++++++++ ++++++++ Fig. 1.7 P-type Intrinsic Semiconductor 152 Electronics Engineering Technician (ii) N-type Intrinsic Semiconductor In this type almost all carriers are electrons. No positive charge carriers. -- -- -- -- -- -- -- --- -- -- -- -- -- -- -Fig. 1.8 N-type Intrinsic Semiconductor The pure form of semiconductors does not conduct electricity. To conduct electric current impurities are added. Doping :The process of adding impurities to the pure form of intrinsic semiconductor is known as doping. For 106 to108 atoms one impurity atom is added. For P type Intrinsic semiconductor, a trivalent impurity is added. For N type Intrinsic semiconductor, a tetravalent or pentavalent impurity is added. While adding such impurities the semiconductor becomes as extrinsic semiconductor. At room temperature its conductivity increases ten times. Every rise in 1oc of temperature the conductivity of extrinsic semiconductor rises ten times. 1.3 Extrinsic Semiconductor Extrinsic Semiconductor : The impure form of semiconductors is known as extrinsic semiconductors. These are two types. They are (i) P-type extrinsic semiconductor : In this type the semiconductor consists of majority carriers are positive charge carriers and minority charge carriers are electrons. - ++++++++ ++++++++ ++++++++ - - Fig. 1.9 P Type Extrinsic Semiconductor +ve, holes or positive -ve, electrons negative charge carriers (ii) N-type Extrinsic Semiconductors : In this the semiconductor consists Paper - II Electronic Devices and Circuits 153 majority carriers are electrons and minority charge carriers are holes i.e. positive charge carriers. +ve holes, -ve electrons -- +-- -- -- -- --+-- --+ - -+- - +- - -------Fig. 1.10 N-Type Extrinsic Semiconductors 1.4 PN Junction Diode Formation P Type semiconductor : The pure form of intrinsic P-type semiconductor is added trivalent impurity such as Gallium, Indium and it becomes extrinsic semiconductor which carries majority carries as holes (positive charge carriers) minority charge carriers are electrons. Fig. 1.11 PN layers Fig. 1.12 PN layers in placed liquid form Fig. 1.13 P N Junction Formation N-type Semiconductors When small amount of pentavelent or tetravelent impurity is added N-type intrisic semiconductor a N-type extrinsic semiconductor is formed. In this it carries majority carriers are electrons and minority charge carriers are holes. Pentravalent impurities are Arsanic and Anti many. 154 Electronics Engineering Technician Anode Cathode Fig. 1.14 PN Junction diode Symbol PN Junction : When a piece of P-type semiconductor a piece of N-type semiconductor being together and heat is applied in special container up to 5000 oC. At high temperature pores the material one upon other. separated by a junction known as PN junction. Anode Cathode No External field is applied Fig. 1.15 P N Junction diode without exteral field A N type semiconductor in the form crystal, a P-type indium piece is kept, which position shows in the fig. 1.15. Then the whole combination is kept in side the puddle. The puddile is heated upto 5000oC. The indium part of P-type layer is becomes in the liquid form on the N-type crystal shown in the fig1.12. Finally heat is withdrawn from the puddle a cooling process takes place. The whole system is cooled form PN junction diode. PN Junction Diode Working : At room temperature the majority carriers cross the both layers in the P, N-type layers on the semiconductors without any external supply shown in below. P, N type layers an electrodes are connected known as Anode, Cathode respectively. Potential Barrier : It is the charge carrier, which cross across the junction. Paper - II Electronic Devices and Circuits 155 The charge carriers potential on the junction is known as potential barrier or barrier potential. The minimum cut of voltage required to conduct Garmanium diodes are 0.2 V, . In the case of Silicon diodes the minimum cut off voltages are 0.7 V. Work function : The minimum amount of additional energy required to lift electrons from cathode surface is known as work function. Generally this work function refers to cathode materials such as Tungsten, Thoriated Tungstun and oxide coated materials. These materials are used as filaments in electrical bulbs or other heating elements to the electrodes. These are two types.They are directly heated / indirectly heated cathodes. 1.5 PN Junction Diode Forward Bias As soon as switch S is closed, the diode anode is connected to positive terminal of the power supply and cathode is connected to negative terminal of the power supply, the PN junction diode is said to be connected in forward bias mode. To measure cut off voltage a volt meter is connected across the PN junction diode and the ammeter is connected in series , to measure forward current. Biasing : It is the process in which supply giving to the semiconductor device is known as biasing. As soon as switch closed, increase the dc power supply form 0-1V, slowly. We can take / observe corresponding readings in Volt/ meter reading Ameter. At cut off voltages particularly in thecase of Ge is 0.2V and Si is 0.7V the anode current starts to increase and reaches the diode into saturation region. i.e. maximum current it reaches, while changing slightly from 0.6V to 0.8 V, reaches upto 12 to 25 mA of anode current. The forward V.I. characteristics are shown in the figure. V A K Ammeter + - S Switch DC Power Supply Fig. 1.16 PN Junction diode connected in Forward bias 156 Electronics Engineering Technician In this region the majority carriers attracts the interelectrodes of the dc power supply. Hence the junction offers low resistance. It behaves just like down fall of water from upto down. Fig. 1.17 PN Junciton diode Forward Bias VI Characteristics From the forward bias V.I. characteristics, the diode forward resistance is calculated. Draw the tangent at sharp curve say the point P (x,y), which is point of tangency. Diode Forward Resistance rf : It is the ratio of difference of change of anode to cathode voltage to change in difference in anode current is known as PN junction diode forward resistance. Diode forward resistance = Difference of Anode to Cathode voltage Difference of Inode current. = VAK / IA = 0.7V - OV / 2mA - omA = 0.7V / 2mA rf = 350 Diode Reverse Bias VI Characteristics Anode of the diode is connected to negative terminal of the dc power suppply and cathode is connected to positive terminal of the dc power supply, the connection is said to the diode is connected in reverse bias mode. As soon as switch is closed, Increase the VAK insteps of OV, 5V, 10V, 15V, 20V, 25V. A small amount of current flows at high voltage in mA. The diode reverse bias V-I characteristics is drawn in third co-quardrant, because VAK , IA are nagative. Paper - II Electronic Devices and Circuits 157 V A K Ammeter - + S Switch DC Power Supply Fig. 1.18 PN Junction diode reverse biased circuit Fig. 1.19 VI Characteristics In reverse bias the diode affects reverse resistance more than 100 k., the graphical representation as follows. Fig. 1.20 PN Junction diode VI Characteristics At particular point Q in reverse bias mode suddenly a small amount rises and stops with incresae in VAK . The break down point is known as Avelounch break down point. The correspoinding voltage current are known as Avelanch voltage, current respectively. Then it is represented as VAB, IAB. A tangent is drawn at sharp curve. The point of tangency gives us diode reverse resistance. The Diode Reverse Resistance : It is the ratio of difference in change in VAK to change IA is known as PN junction diode reverse resistance. Diode Reverse resistance = Change in VAK / Change in IA = VAK / IA 158 Electronics Engineering Technician From the graph = 18V / 25mA. = 0.72 x 106 = 720 k Applications : 1) Used as Rectifier 2) Switch 3) Used as clipper Types of Diode : a) Zoner Diode b) LED c) Photo Diode d) Varacter Diode e) Tunnel Diode (a) Zener Diode The doping concentration is 10% more compare to PN junction diode . The V.I characteristics differ change in doping concentration. Anode Cathode Fig. 1.21 Zener Diode Symbol Forward bias V.I characteristics is identical to PN junction diode characterisitics. In reverse bias at constant voltage current starts to increase from minimum to maxium corresponding point is known as Zener break down point. Corresponding voltage, currentt is known as Zener voltage current. Due to this V.I. characteristics zener is used as a voltage regulator. (b) Light Emiting Diode (LED) This is made with Gallium Arsanide (GaA) or Gallium Phosphate (GaP). LED when it is given forward bias the junction heats just like filament, it glows light energy comes. Hence it is known as Light Emiting Diode. . Anode Cathode Fig. 1.22 Light Emitting Diode Symbol Paper - II Electronic Devices and Circuits 159 (c) Photo Diode This diode is made with Cadmium Sulphide. When light falls on the junction of the photo diode it conducts. Hence it is known as photodiode. Applications : 1) Used in light flashes system. 2) Cameras. Fig. 1.22 Photo Diode (d) Varacter Diode This diode is designed to work at high frequencies. It works depends on the voltage applied across the junction resulting junction capacitance changes in high frequencies. Application : Used in high frequency tune in T.V. tuners. Anode Cathode Fig. 1.23 Varacter Diode (e) Tunnel diode More doping is used in Tunnel diodes. This diodes offers positive / negative resistance characteristics. Anode Cathode Fig. 1.24 Tunnel Diode 160 Electronics Engineering Technician Fig. 1.25 1.6 Diode Manufacturers Specification Paper - II Electronic Devices and Circuits Fig. 1.26 161 Fig. 1.28 Electronics Engineering Technician Fig. 1.27 162 Fig. 1.30 163 Fig. 1.29 Paper - II Electronic Devices and Circuits Electronics Engineering Technician Fig. 1.31 164 Paper - II Electronic Devices and Circuits 165 Fig. 1.32 P N Junction Diode Epoxy lens/case Wire bond Reflective cavity Semiconductor die Anvil Post Lead frame Flat spot Anode Cathode Fig. Photo Diode 1.33 166 Electronics Engineering Technician Fig. 1.34 Zener Diode VI Characteristics Fig. 1.35 Photodiode VI Characteristics Paper - II Electronic Devices and Circuits Anode 167 Cathode Fig. 1.36 Varacter Diode Fig. 1.37 Tunnel Diode Symbol 168 Electronics Engineering Technician 1.7 Transistor A transistor is a three layer semiconductor, three electrodes device is known as Transistor. The transisters are two types. (1) P NP Transistor (2) NPN Transistor Collector Collector Base P N Base N P N P Emitter Emitter Fig. 1.40 PNP Transistor Fig. 1.41 NPN Transistor In a transistor base is thinly doped, collector is moderately doped and emitter is heavily doped. The PNP transistor is used where the current gain is less than 10. the NPN transistors are used where current gain is more than 40.-200. A transistor is three semiconductor layers, three electrodes, two junction device is a transistor.In the transister transformation of resitance takes place from input to output. In a transistor generally base to emitter junction is forward biased and collector to emitter junction is reverse biased. Based on transistor three electrodes commanly connected there are three types of configurations.They are .as follows (1) Common Base Transistor amplifier Configurations - CB (2) Common Emitter Transistor amplifier Configurations - CE (3) Common Collector Transistor amplifier configuration - CC In the transistor the arrow mark shows, current carrying direction. In PNP transistor the arrow mark is inside the junction. In NPN transistor the arrow marks is outside direction which shown in the figure. Paper - II Electronic Devices and Circuits 169 1.8 Working of PNP and NPN Transistor In a transistor emitter is heavily doped, collector is moderately droped and base is thinly doped. A PNP transistor emitter to base junction is forward biased and collector to base junction is reverse biased. Its emitter is connected to positive and collector is connected to negative terminal of the dc power supply. A small amount of negative cvoltage is appled to the base. Emitter Junction Collector Junction Fig. 1.42 Common Base Transistor In PNP transistor, due to majority carriers in P- type layer, the holes drift from emitter to collector moves, consequently the equal number of free electrons moving from emitter to collector thrugh dc power supply in the external circuit. The number of holes drifting from emitter region to collector region is controlled by the base bias and the transistor is used as an amplifier. Working of a NPN Transistor The NPN transistor emitter is connected negative terminal of the dc power supply and the positive terminal of the dc power supply is connected to base. Hence emitter base junctions is connected in forward bias mode. The collector is connected to positive terminal conneted collector and negative connected to base. Fig. 1.43 Common Base Transistor 170 Electronics Engineering Technician Fig. 1.44 Common Base Transistor The free electrons emitter attraction negative terminal. The emitter to base offers low resistance and collector to base offers very high resistance. Transistor Input / Output V-I Characteristics In common base amplifier with changing emitter to base voltage slowly corresponding IE mA changes. Draw the relation between VEB and IE mA in graphical method. Emitter to base Voltage Fig. 1.45 CB Input VI Characteristics It is shown in the curve with changing VEB keeping constant VEB = OV, 10V corresponding change is IE mA is noted and the curve between those two parameters. The emitter current mA is taken on Y-axis and VEB is taken on Xaxis approximate readings are noted. (i) The mitter current I increases rapdily with small increase in emitter base voltage VEB. It means the input resistance is small. (ii) The IE is almost independent of VCB. The IE is independent of VCB. Paper - II Electronic Devices and Circuits 171 1.9 Current Amplication Factor It is the ratio change collector current to change in emitter current as small signal current gain “ “ or CB amplifier current gain. = IC / IE at constant VCB. = IC / IE. The values lies between 0.9 to 0.99 in a transistor. Output V-I characteristics : The curve is drawn between VBE on X-axis and IcmA on Y-axis keeping constant IE in the base to emitter junction. The following points are noted. (i) The ICmA varies with VCB only at very low voltages less than 1V. (ii) When the VCB is from 1-2 V the Ic remains constant, which is horizontal line. Collector Current (iii) A large change in VCB produces small change in Ic. Hence output resistance is very high. Base to Emitter Voltage Fig. 1.46 Common Base Output VI Characteristics Large Signal Current amplification factor : It is ratio of collector current to base current. IC / IB 172 Electronics Engineering Technician Reverse Saturation current ICEO : It is knwon as collector to emitter reverse saturation current plays role when minority carriers comes into role. Common collector amplifiers current gain r = It is the raio of emitter current IE to IC collector current. r = IE / I C It is generally more than 1. Relation between and : At junction of the transister current equation as follows. I E = I C + IB (1) or IB = IE - IC CE amplifier current gain is given by substituting = Ic / IB = IC / IE - IC Dividing by IE both Numerator and denominator. right side of the above equation = Ic / IE = IE / IE - IC / IE = / 1- . lies in a transistor from 40 to 200 depend upon current applicaiton factor. 173 Fig. 1.47 Paper - II Electronic Devices and Circuits Electronics Engineering Technician Fig. 1.48 174 175 Fig. 1.49 Paper - II Electronic Devices and Circuits Electronics Engineering Technician Fig. 1.50 176 Paper - II Electronic Devices and Circuits Fig. 1.51 177 Electronics Engineering Technician Fig. 1.52 178 179 Fig. 1.53 Paper - II Electronic Devices and Circuits Electronics Engineering Technician Fig. 1.54 180 181 Fig. 1.55 Paper - II Electronic Devices and Circuits 1.11 FET AND MOSFET WORKING FET - Field Effect Transistor is a unipolar solid state semiconductor device. Electronics Engineering Technician Fig. 1.56 182 Paper - II Electronic Devices and Circuits Fig. 1.57 183 184 Electronics Engineering Technician Paper - II Electronic Devices and Circuits 185 Drain Gate Gate N Channel P Channel Source Fig. 1.59 N chanel FET P-Channel FET The FET can fabricated N-Channel, P-Channel both channels. A narrow N-type semiconductor is taken and diffused on opposite sides on mid points of either sides of channels. The P layer behaves as PN junction diodes. Gates and the area remained between two gates is known as Channels. These layers are inernally shorted. These leads are joined called Drain “D” and Source “S” are opposite sides of the channel. this devices posses very high input impedence. FET : V-I Characteristics In a N-channel FET, the source terminal is connected to the negative and drain is connected positive of the dc power supply. The gate is always reverse biased. The following points are to be noted. (i) At first, the drain current ID raises rapdily with VDS, but then becomes constant. The VDS above which ID becomes contant is known as Pinch-off Voltage. The path OA is pinch off voltage. (ii) Increase in ID is very small with VDS above pinch off voltage. Consequently ID remains constant. (iii) The V-I characteristics are identical to pentoid valve. 186 Electronics Engineering Technician Fig. 1.60 VI FET V-I characteristics 1.12 ADVANTAGES OF FET (1) High input impedence. (2) Small size, rugged and long life. (3) Low noise, good high frequency response. (4) Better thermal stability. (5) High power gain. (6) It is unipolar device. Applications: (1) Input stage is amplifiers, CRO’s and other electronic instrucments. (2) In logic circuits. (3) As unixer stage is Fm radio and T.V. receiver. (4) In computers for large scale integration LSI and memory circuit. 1.13 Understanding the Naming and Convention of Semi conductor Components The semiconductor devices namings of electrodes are taken from manufactureres data sheets that are taken as convention to identify the various electrodes in active devices. 187 Fig. 1.61 Paper - II Electronic Devices and Circuits Electronics Engineering Technician Fig. 1.62 188 189 Fig. 1.63 Paper - II Electronic Devices and Circuits Electronics Engineering Technician Fig. 1.64 190 Paper - II Electronic Devices and Circuits Fig. 1.65 191 Electronics Engineering Technician Fig. 1.66 192 Paper - II Electronic Devices and Circuits Fig. 1.67 193 Electronics Engineering Technician Fig. 1.68 194 195 Fig. 1.69 Paper - II Electronic Devices and Circuits Electronics Engineering Technician Fig. 1.70 196 Paper - II Electronic Devices and Circuits Fig. 1.71 197 Electronics Engineering Technician Fig. 1.72 198 Paper - II Electronic Devices and Circuits Fig. 1.73 199 Electronics Engineering Technician Fig. 1.74 200 Paper - II Electronic Devices and Circuits Fig. 1.75 201 Fig. 1.76 Electronics Engineering Technician Fig. 1.76 202 Paper - II Electronic Devices and Circuits Fig. 1.77 203 204 Electronics Engineering Technician Fig. 1.78 Paper - II Electronic Devices and Circuits Fig. 1.79 205 206 Electronics Engineering Technician Fig. 1.80 Paper - II Electronic Devices and Circuits Fig. 1.81 207 208 Electronics Engineering Technician Fig. .182 Paper - II Electronic Devices and Circuits Fig. .183 209 Electronics Engineering Technician Fig. 1.84 210 Paper - II Electronic Devices and Circuits Fig. 1.85 211 212 Electronics Engineering Technician Summary Materials are three types 1. Conductors : The material, which passes electrical current though it is known as Conductors. 2. Semiconductors : The materials, which conducts electricity partially at lower temperatures, with rise in every degree temperature conductivity increases in Semiconductors. 3. Insulaters : The materials, which does not conduct eletric current is known as insulaters. 4. Intrinsic Semiconductors are two types, P-type intrinsic semiconductor, N-type intrisic semiconductors. 5. Intrinsic Semiconductors : The pure form of semiconductor is known as intrinsic semiconductors. 6. P-type Intrinsic semiconductor : It carries only holes, no electrons is known as P-type intrinsic semiconductors. 7. N-type Intrinsic Semiconductor : It carries only electrons, no holes is known as N-type intrinsic semiconductors. 8. Doping : The process of adding impurities to pure form of semiconductors is known as dopoing. 9. P-Type Extrinsic Semiconductor : It carries majority carriers are holes, minority charge carriers are electrons is known as P-type intrinsic semiconductors. 10. N type extrinsic semiconductor : It carries majority carrieres are electrons and minority carriers are holes is known as N-type extrinsic semiconfuctors. 11. PN Junction Diode : A piece of P - type material , a piece of N-type material joined at higher temperature 5000oC and electrodes are connected to each layer is known as PN junction doide. 12. Transistor : Three layers, three electronodes device is a transistor. The terminals are base, collector and emitter. Two types (a) PNP Transistor b) NPN transistor. Application : - Used as switch and amplifiers. 13. = IC / IE, =IC / IB, b = /1- - Relation Paper - II Electronic Devices and Circuits 213 14. FET : Field Effect Transistor is a Unipolar, 3 electrodes device, had high input resistance and it is voltage sensitive device. 15. MOSFET : Metal Oxide Semiconductor Field Effect Transistor. Two types N-Channel, P-Channel MOSFET - High input impedence upto 1,00,000 M Ohms ,Voltage sensitive device - Used in memories. 16. Data Sheet of Semiconductors : It describes, physical and V-I characteristics, operating temperatures, power ratings, band width. Easily identification standard numbering - Alternative numbers, which were equivalents. Short Answer Type Questions 1. Write classification of material ? 2. Define ionic, covelent, coordinate covelent bond. 3. Write number electrons in an atom ? 4. Define intrinsic semiconductor. 5. Define doping . 6. Define extrinsic semiconductor. 7. What are applications of PN junction diode ? 8. What are the applications of Zonar, Photodiodes ? 9. What are the applications of Varacter, Tunnel diodes ? 10. What are the uses of semiconductor data sheet ? 11. Draw the symbol of Transister. 12. Define , , r ? 13. Write relation between and . 14. What is converter ? 15. Mention the types of rectifier circuits ? 16. Define average, RMS value of an a.c ? 17. Define efficiency of a rectifier ? 18. Define ripple factor. 19. Mention the ripple value of half / full wave rectifier. 214 Electronics Engineering Technician 20. Mention the type of filters used in rectifiers ? 21. Define voltage regulation. 22. Mention the types of regulator circuits ? 23. What is transister biasing ? 24. Mention the types of transister biasing methods. 25. Define stabilization of an amplifier. 26. Mention the different regions of output transister V-I characteristics. 27. Mention the transistor configurations. 28. What are the types of power amplifiers ? 29. Define efficiency of power amplifier. 30. What are advantages of class B push pull amplifier. 31. Write applications power amplifiers. 32. Mention the IC nos used in power amplifiers. Long Answer Type Questions 1. Draw and explain energy band diagrams. 2. Draw and explain formation of PN junction diode V-I characteristics with neat graph. 3. Explain transister CB input / output - V-I characteristics with neat diagram. 4. Draw and explain CE amplifier input / output V-I characteristics. 5. Derive relation between , and . 6. Draw and explain FET - VI. Characteristics. OJT /Practical Questions 1. Study of PN Junction diode, VI characteristics and applicaitons. 2. Study of applications of Zener diode, LED, Photo diode, Verector diode and tunnel diode. Identification of terminals and applicaitons. 3. Study of Transistor, VI Characteristics, Datasheets and applications. 4. Study of FET, MOSFET, VI characteristics, data sheets and applications. UNIT 2 Power Supplies and Filters Learning Objectives • Definitions of AC , DC power supply. • Block diagram of DC power supply. • Study of half wave rectifier with filter and applicaiton. • Definitions of RMS , average, ripple factor and efficiency of half wave rectifier. • Study of full wave rectifier with filter and applicaitons. • Study of bridge rectifier with filter and its applications. • Study of comparisions of rectifiers circuits. • Study of filters, Capacitor Inductor Filter, Capacitor Inductor Capacitor filter. • Definition of voltage regulations, Series and Shunt regulator by using zener diode and its applicaitons. 216 Electronics Engineering Technician 2.0 Introduction Every electronic circuit is used DC power supply. The DC power supply is obtained from converting AC supply into DC supply. The process of conversion of AC supply into DC power supply is known as Convertors. The AC supply is used for electroplating, electro typing, electro metal refining, arc lamp, battery charging, electromagnetic, valve or transistorized electronic equipments. The main blocks of DC power supply are a step down transformer, a rectifier circuit, a filter circuit and regulator. 2.1 DC Power Supply AC power is converted into dc power is known as converters. Every electronic circuit consist of 30% circuit is dc power supply circuit with different current /voltage / power ratings as per the requirement and specifications of the customer needs and specific applicaitons. The dc power supply consist of the following blocks. They are as follows. AC Mains 230V / 50HZ Stepdown Transformer Rectifiers Filters Regulators L O A D Fig. 2.01 Blocks are (1) AC main supply 230 V / 50 Hz. (2) Step down transformer. (3) Rectifiers (4) Filters (5) Regulators (6) Loads (i) AC Main supply 230V / 50 Hz World wide accepted standard line voltage specifications are 115V / 60Hz and 230 V / 50 Hz. In India adopted ac power transmission is 230 V / 50 Hz. as per the our power requirement of application, is drawn from the ac mains. Paper - II Electronic Devices and Circuits 217 (ii) Step - Down Transformers The transformer main function is to draw the ac power from primary to secondary at constant. Transformers are desinged depend up on current transfered from the windings frequency. Fig. 2.2 Transformer Symbol. Transformer primary consists of Inductance = Lp - H No. of turns = Np Voltage = Vp - V Current = Ip - A Transformer secondary consists of Inductance = Ls - H No. of turns = Ns Voltage = Vs -V Current = Is - A Fig. 2.3 Bawin used for Copper Winding Working : As soon as ac supply is given to the primary winding of the transformer, flux induces in the first turn of the primary windings links, to second turn, process continuous and flux links entire turns of the winding. Faradays First Law : Whenever flux links the winding an emf is induced. Faradays Second Law : The rate of change of flux linkages is equal to the induced emf. 218 Electronics Engineering Technician The emf is induced in the primary winding of the transformer. By mutual induction flux flows from winding to the secondary winding through magnetic core. Fig. 2.4 Transformer Core and Windings The flux induces in the secondary winding of the transformer from first turn, continuous process links to second turn and entire turns of secondary winding. As above Faraday’s laws of electromagneic induction emf and current induces which is an a.c. power. The voltage ratings are 24V, 20V, 18V, 12V, 9V, 6V, 4.5V, 3V, 1.5V with different current ratings 1A, 500mA, 300mA, 250mA, 200 mA, 150 mA, 100mA etc. (iii) Rectifiers It is the circuit in which the a.c. sine wave rectifies as pulsating positive half cycles are rectified and it depends upon rectifier circuits. The rectifiers are three types, they are 1. Half Wave rectifier 2. Full wave rectifier. 3. Bridge rectifier above rectifiers are discussed in separate is rectifier circuits. (iv) Filters A filter circuit is one which converts / filters the pulsating positive half cycles Paper - II Electronic Devices and Circuits 219 into dc voltage. The dc voltage obtined is output is with ripple. Now the types of filter circuits are 1. Capacitor Input filter 2. Capacitor, Inductor filter or LC -type filter. 3. Capacitor, Inductor, Capacitor filter or T-type filters II- type filters. (v) Regulator The regulater is one in which it gives output constant dc voltage. Defination : It is ratio of difference of no load voltage tothe load voltage to no load voltage is known as voltage regulator. The types of voltage regulaters are (a) Zener regulaters (series and shunt type) (b) IC regulaters. (vii) Load It is an electronic system in which it takes a dc power in generally known as load. The loads are three types. (a) Resistive load (b) Capacitive load (c) Inductive load. b,c are used in communication networks. 2.2 Halfwave Rectifier It is the rectifier circuit in which half of the ac sine wave conducts and produces, remaining negative half cycle could not conduct is known as half wave is known as half wave rectifier. The circuit is as follows. Fig. 2.5 Half Wave Rectifier 220 Electronics Engineering Technician The half wave rectifier conducts only positive half cycles of ac sine wave because forward biased. Whenever a negative half cycle receives the diodes goes to reverse bias mode hence there is no conduction. Fig. 2.6 Half Wave Rectifier wave forms The electrolyte capacitor 1000 mF/ 25V filters the half of the wave gaves dc voltage with 120% ripple. This type rectifiers are used in atomic power units. Input voltage to the rectifier Vi = Vm Sin = Vm Sin2 ft Im = Vm / Rs + Rl. Irms = Im / 2 rms value of half wave. Average value or dc value half wave rectifier Idc = Im / Ripple factor = = Irms Idc = Irms2 - I2dc Idc Im/2 Im/ Idc 2 -1 Irms = 2 = 2 2 -1 - 1.21 -1 Paper - II Electronic Devices and Circuits 221 Efficiency of half wave rectifier. = Pdc / Pac = (Vm / (Rs+RL))2 RL / Vm2 4(Rs+RL) = Pdc / Pac = 4 / 2 (1+Rs/RL) x 100 = 40.6 / 1 +Rs/RL 2.3 Center Tapped Fullwave Rectifier Center tapping of the transformer, it inverts the polarity at tappings. Hence it doubles the voltage and frequency. Working : At point A, the positive half cycles receives the diode D, and it is in forward bias mode conducts positive half cycle between time period 0 to the negative half cycle receives the D1 goes to reverse bias mode. Hence D1 wont conducts at the same time, at the time period 0 - , D2 is in forward bias mode, conducts - 2 period. Hence in first cycle D1 conducts 0- times and D2 conducts - 2 time. Hence full wave conduction takes place. This process repeats remaining cycles. The circuit operation is one cycle full two half conducts called fullwave rectification. With two positive half cycles of operations. even 1/2 AC Input cycles Current flows when D1 conducts odd 1/2 cycles Current flows when D2 conducts Resultant Output Waveform Fig. 2.7 Full Wave rectifier with wave forms 222 Electronics Engineering Technician 2.4 Bridge Rectifier In bridge rectifier circuit, the center tapping is eliminated. Conduction of this circuit is identical to fullwave center tapped transformer rectifier circuit. In working the time period 0-, two diodes D1 and D2 conducts ‘t’ receives the +ve half cycle of the wave. The D1, D2 moves into forward bias mode and it conducts. During time period -2 the D1 and D2 goes to reverse bias mode, no conduction. at the same time D3, D4 conducts because it moves into forward bias mode. Conducts - 2, period in 1st cycle. This operation repeats entire cycles during the operation. Hence the circuit is known as full wave bridge rectifier. In calculations such as Irms, Idc, Pdc, Pac, ripple, efficiency calculations are identical to full name rectifier. AC Input 230V 50HZ Ripples C Charges C Discharges V Resultant Output Waveform Waveform with capacitor Waveform without capacitor Fig. 2.8 Bridge Rectifier 2.5 RMS Value of Fullwave Rectifier Irms = Im / 2 Average value Idc = Im / for half wave rectifier Irms / Idc = / 2 = 1.57 and Im / Idc = = 3.14 Ripple factor F.W rectifier Idc = 2 Im / ; Irms = Im / 2 Irms / Idc = / 2 2 = 1.11 Paper - II Electronic Devices and Circuits Ripple Factor = 223 (Irms / Idc)2 -1 x 100 = 1.112 - 1 x 100 = 48% Efficiency of FW rectifier = Pdc / Pac x 100 = (4 Vm2 / 2( Rs + RL)2) / Vm2 / 2 (Rs + RL) = 81.2 / 1 + Rs/ RL = 81.2 2.6 Comparision of Rectifier Circuits Circuit Vdc Vrms/ Vdc Vm/ Vdc Average dc rms volts Peak Volt Volt Output Output Output Heat Wave PIV Ripple factor Efficiency Maximum 1 1.57 3.14 1.57 121% 40.6 Full wave Rectifier C.T 1 1.11 1.57 3.14 48% 81.2 FW Bridge 1 1.11 1.57 1.57 48% 81.2 Rectifier Rectifier 2.7 Filters Defination : It is the circuit in which it converts the pulsating the positive half cycles into dc voltage is known as filter circuit. The filter circuits are follows. (i) Inductor fulter - L type filter (ii) Condenser Inductor input filter - CL input filter (iii) Inductor Condenser filter - LCL or CLC , filter T-type and P - type 224 Electronics Engineering Technician 2.8 Operation of Rectifier Circuit Using Capacitor Input.Series Inductor and clc Filters (i) L-type Filter This type of filter is used, when it required large currents with low voltages power supplies this filter circuit is used. For better regulation a shunted capaciter is used. Hence it is called LC type filter. Fig. 2.9 L-Type Filter (ii) Capacitor input filter This type filter is used for high voltage, low dc current requirement applications. Fig. 2.10 Capacitor Input Filter (iii) LC Filter For heavy current loads an LC filter is used. It also gives better regulation at high load current. Followed C shunted capacitor provides high output voltage and reduced ripple. Paper - II Electronic Devices and Circuits 225 Fig. 2.10 LC Filter Fig. 2.11 LC Filter 2.9 Voltage Regulation It is ratio of difference of load to Vdc full load to Vdc full load multiplied by 100 is known as voltage regulation. Voltage Regulation = Vdc no load - Vdc full load / Vdc full load x 100 2.10 Zener Regulation A zener-diode is used to get constant output DC voltage to the load from source. The zener diode is connected shunt path across the load, because a zener diode breaks at constant voltage is reverse bias mode. The Rs is a series, resistant connected in the circuit to absorb load fluxations. Hence the output voltage does not change kept constant as Vz. Fig. 2.12 Zener Regulater The zener diode, when it is connected in reverse bias mode, at constant voltage Vz, the zener current only changes from minimum to maximum, without change in Vz the load resistance may change. However, shunted zener gives output constant voltage irrespective of intenal current changes. Hence the zener is used as voltage regulator. 226 Electronics Engineering Technician Let V1 = Input voltage to Regulator Vo = Output voltage Current through R is = Iz + IL R can calculated as from ohms law. R = E1 - Eo / Iz + IL. Switched Mode Power Supply By using switched mode diodes such IN 4001 and IN4007 connecting in the form of bridge dc power may be generated. The diodes basically two types. 1. Rectifying diodes - Conducts heavy current in forward and a low current in reverse bias mode. By using these diodes series regulated power supplies (SRPS) are assembled as per the customer ratings requirements. Later the positive half cycles are filtered and regulated. 2. Switched mode Diodes - These diodes work in forward bias mode. In reverser bias mode does not work and act as switch off mode. By using switched mode diodes bridge ac line voltage is rectified, to obtain pulsating positive half cycles. (b) The pulsating the positive half cycles are filtered by using suitable filtering capaciors. (c) The filtered dc voltage is equal to 230 2 = 324 V. Then it is given switching transister. This device convers dc voltage in to square wave. (d) The square wave is given to pulse transformer primary winding. (e) One part of the primary winding is given error amplifier to get constant voltage secondary of the pulse transformer. Irrespective input variations a constant output is obtained from the secondary of the pulse transformer. (f) Taking number tappings from the secondary of the pulse transformer output voltages are rectified and taken as dc supply voltages. Fig. 2.13 SMPS Paper - II Electronic Devices and Circuits 227 Summary The vocational EET student note that any electronic circuit consists of mainly depends on dc power. how it is generated. The main types conversion from ac to dc power circuit is known as converters. The converter circuits are mainly two types. they are (1) Series regulated power supplies (SRPS) (2) Switched mode power supplies - (SMPS) To generated higher power SRPS is costly and more weight, but SMPS is light weight low cast. Hence in consumer eletronics smps is used. Powre Supply : It is the electornic circuit which converts a.c. supply into d.c supply. Rectifier : Rectifiers are three types (a) Half wave rectifier (b) Full wave rectifier (3) bridge rectifier. RMS Value : It is the value is taken some of the square root of squares of the voltages or currents divided number of parts average value in RMS value D.C. Value : It is average value of a.d.c quantity for half wave rectifier average value = maximum value / . Ripple Factor : It is the ratio of a.c. content presenting d.c. output to d.c. output multiplied by 100 is known as Ripple factor. Efficiency : It is the ratio of output power to input power. For half wave, full wave rectifiers the efficiences are 40.6%, 81.2% respectively. Filter : By using an electrolytic capacitor the pulsating half waves are convered into d.c. voltage. Voltage Regulator : It is the ratio of difference of no load voltage to load voltage to no voltage nultiplied by 100. Generally for small power applicaitons zener diode using as voltage regulator. Short Answer Type Questions 1. Define (dc) power supply or converter ? 2. Mention the types of rectifier circuits. 3. How many diodes are used in HW, FW, BR ? 4. Write average values of HW, FW rectifiers. 228 Electronics Engineering Technician 5. Define ripple factor. 6. Define efficiency of rectifier (HW / FW) ? 7. Mention the types of filter circuits. 8. Define voltage regulation. Long Answer Type Questions 1. Draw the block diagram of power supply. Explain working of each block. 2. Draw and explain working of half wave rectifier. 3. Draw and explain working full wave rectifier. 4. Draw and explain working of bridge rectifier. 5. Write comparisions of rectifier circuits. OJT / Practical Questions 1. Study of power supplies(AC / DC). 2. Study of halfwave rectifier with filter and applications. 3. Study of full wave rectifier with filter and regulators applications. 4. Study of Bridge rectifier with filter and regulator applications. 5. Study of Filter circuits. 6. Study of regulator ( by using Zener diode). UNIT 3 Small Signal Amplifiers Learning Objectives • Definition of transistor biasing • Study of types of transistor biasing, - base emmitter biasing, Collector feedback resistor biasing, Voltage divider biasing. • Definition of stabilization • Comparision of amplifier circuits. • CC , CB and CE • Study of transistor output to VI characteristics drawing load line, operating point, fixing - cut of region, saturation region, and active region identification. 3.0 Introduction A properly biased transistor raises the strength of a weak signal to strengthen signal and thus acts as an amplifier. Almost all electronic equipments must include means for amplification of electrical signals. For instance radio receivers amplify very weak signals sometimes a few millionth of a volt at antenna until they are strong enough to fill a room with sound. The transducers are used in the medical and scientific investigations generate signals in the microvolt ( V) and milli volt (mV) range. These signals must be amplified thousands and million times before 230 Electronics Engineering Technician they will be strong enough to operate indicating instruments. Therefore electronic amplifiers are a constant and important ingredient of electronic systems 3.1 Proper Biasing in Amplifier Circuit and List of Biasing The transistor basic formation is top strength weak signal into strengthen signal. In the transistor current transfer from low resistance input to high resistance output. The transformation resistance takes place when current flows from input to output, hence device is known as transistor. The small signal amplifier are working input low voltages up to 2V and output voltages maximum up to 10V.in amplifier input signal is very low. These low voltage signal are also amplifier. The transistor amplifier circuit taking on of the terminal is common to both input and output is three types. They are (1) Common Base Amplifier – CB amplifier (2) Common emitter amplifier- CE amplifier (3) Common collector amplifier-CC amplifier. Transistor Biasing : The process of giving supply to both input and output is known as transistor biasing. The transistor biasing it as to fulfill the following condition. (1) Minimum VBE = 0.7 V Silicon transister = 0.3 V Germanium Transister (2) Minimum VCE = 0.2 V Both Si, Ge (3) Zero signal Collector. When above three biasing conditions are fulfilled by the transister works as an amplifier. There are three types of transistor biasing. They are as follows. (1) Base Emitter Biasing (2) Collector feedback resister biasing (3) Voltage divider biasing Base Emitter Biasing: In Base Emitter Biasing transistor amplifier circuit is used two de power supplies are Vec,Vbb, Keeping Vcc =OV, 5V, 10V Vbb is valid from OV to 2V for OV =Vec, there is no change in IB from 0 A Paper - II Electronic Devices and Circuits 231 For change Vec=5V constant vary the VBB from OV to 2V corresponding Ic and VCE is noted. the VBE =0.7V Si, 0.3 Ge transister ,the device comes from cut off condition to condition region. Vce also exceeds0.3V and reaches zero signal collector current, when these three conditions are fulfilled, the transistor works as an amplifier. Fig. 3.1 Transistor Biasing The conditions are i) Minimum VBE = 0.7V for Si transistor =0.3V for Ge transistor ii) Minimum VCE= 0.3V for both side Ge transistor iii) Fulfill zero signal collector current IC. 2. Collector feedback Resister biasing: This type of amplifier needs only single dc power supply. The circuit is shown in Fig. As soon as supply Vcc is given to the circuit, base current IB comes through Rcb to the base of the transistor. Transistor VBE= 0-7V reaches comes into conductor region. At the junction current Ic flows consider as zero signal collector current. The Ic flows across the collector to emitter voltage Vce exceeds 0.3V comes the junction into conductor region .The transistor fulfilled biasing conditions, works as an amplifier. 232 Electronics Engineering Technician Fig. 3.2 Collector Feedback Resister Biasing While designing Rcb, the base flow is taken into account. 3.Voltage Divider Biasing : In shown is said to be voltage divide biasing. The R1 R2 resistors are designed 1:10 ratio. Most of the current passes through R2 small amount of current flows through R, The difference of the current i.e. IR2 – IR1 flows to the base of the transistor. Fig. 3.3 Voltage Divider Biasing IB reaches base of the transistor the base junction voltage VBE exceeds 07V comes the input junction into conduction region. Ic flows through the collector to emitter junction comes into conduction region and exceeds VCE =0.3V. The Paper - II Electronic Devices and Circuits 233 transistor fulfill biasing conditions i.e. VBE=0.7,VCE=0.3V and zero signal collector current. In this biasing mode the transistor works as an amplifier. 3.2 Stabilization In small signal amplifier the transistor fulfilled biasing condition i.e. VBE=0.7V VCE=0.3V and zero signal collector. supply ratings should be remaining constant. Keeping constant supply ratings, junction temperature plays important role. The variation IB,IC,VBE,VCE. influences change of temperatures. The large signal current gain < variation takes place. In this context transistor parameters are to be keeping in our mind. And V-I output characteristics. In small signal amplifier operating point Q, Q signal swing varies from Q1 to Q2.At Q1 operating allows low voltage more current At Q2 low current, more voltage. In this context value changes 10% same as value. Silicon transistor operates -65oc to 120oc. Generating transistor operates -65oc to 65oc. at particular higher temperatures reverse saturation junction current flows in between two layers of the transistor. Fig. 3.4 Voltage Divider Biasing Stabilization It is the point in which a small variations at input and output ratings, small change in temperature, gains also varies but operating swing should not exceed. Transistor acts as amplifier. 234 Electronics Engineering Technician The transistor output V-I characteristics are divided into three regions. They are (1) Cut-off region (2) Active region (3) Saturation region In cut-off region the transistor acts as off-switch. Base to emitter, collector to emitter Junctions acts as reverse bias mode. In active region Base to emitter Junction is in forward biasing and collector to emitter junction acts as reverse bias mode. In this mode the transistor works as an amplifier. In saturation region both junction are forward biased, a high saturation current reaches a small amount VCE voltages. 3.3 Classification of Amplifier According to frequency, mode of operation, type and methods of coupling, R,C. coupled ,Transformer coupled and directly coupled. Classification of Amplifier Various type of amplifier circuit can be classified on the following four bases. 1.On The Base of Frequency: (i) A.F Amplifier (ii) R.F Amplifier (iii) I.F Amplifier (iv) VideoAmplifier 2. On the base of ability (mode of operation) (i) Class ‘A’Amplifier (ii) Class ‘B’Amplifier (iii) Class ‘AB’Amplifier (iv) Class ‘C’Amplifier 3. On the base of coupling (i) RC Coupled Amplifier Paper - II Electronic Devices and Circuits 235 (ii) LC Coupled Amplifier (iii) Transformer Coupled Amplifier (iv) Direct Coupled Amplifier 4. On the base of Power. (i) Voltage Amplifier (ii) Power Amplifier 3.4 Know the Frequency Response,Gain of The Above Amplifier When a single amplifier stage cannot produce sufficient amplification, two or more stages are coupled together for this purpose. The method of applying the output signal of the first amplifier stage of the input circuit of second stage is called coupling. Fig. 3.5 1.R.C.Coupled Amplifier In the R-C coupling the signal is performed by employing two resistors and a capacitor that is why it is called R-C coupling. This method is most economical and amplifies a wide frequency. The resistor Rc is the load resistor which acts as collector load resistor for the first transistor. The ac component of first transistor reaches to the base of second transistor through coupling capacitor Cc. The capacitive reactance of the coupling capacitor should be lesser than the load resistance, otherwise, the ac component of the signal will also pass through the load resistor. The coupling capacitor also prevents the d.c. voltage to reach the base of the second transistor and does not allow the later to become over loaded. DIAGRAM: 236 Electronics Engineering Technician Hence, it is also known as blocking capacitor. An R-C coupled Amplifier is shown in the above fig. Frequency response FIG: Fig. 3.6 Frequency Response The fig shows the frequency response of a typical R-C coupled Amplifier. It is clear that the voltage drops at low (<50Hz) and high (>20KHz) frequencies whereas it is uniform over mid - frequency range (50Hz to 20KHz). At low frequencies (<50Hz), the reactance of coupling capacitor Cc is quite high and hence very small part of a signal will pass from one stage to the next stage. At high frequencies (>20KHz), the reactance of Cc is very small and it behaves as a short circuit. This increase the loading effect of next stage and serves to reduce the voltage gain. At mid frequencies (50Hz to 20KHz), the voltage gain of the Amplifier is constant. The effect of coupling capacitor in this frequency range is such so as to maintain a uniform voltage gain. Applications:They are widely used as voltage amplifier i.e. in the initial stages of public address system. It is cheap and provides excellent audio fidelity over a wide range of frequency. Paper - II Electronic Devices and Circuits 237 Transformer Coupled Amplifier Fig. 3.7 Transformer - Coupled Amplifier ga in In the transformer coupling method an interstate or a driver transformer is employed for coupling. The primary winding of the transformer acts as an inductive load for the first transistor and the secondary winding acts as the signal source for the second transistor. Frequency Fig. 3.8 Frequency response The frequency response of a transformer couples amplifier is shown in the above fig. It is clear that frequency response is rather poor i.e. gain is constant only over a small range of frequency. The output voltage is equal to the collector current multiplied by reactance of primary. At low frequencies, the reactance of primary begins to fall, resulting in decreased gain. At high frequencies, the capacitance between turns of windings acts as a bypass condenser to reduce the output voltage and hence gain. Therefore there will be disproportionate amplification of frequencies in a complete signal such as music, speech etc. Hence, transformer – coupled amplifier introduces frequency distortion. 238 Electronics Engineering Technician Applications Transformer coupling is mostly employed for impedance matching. In general, the last stage of a multistage amplifier is the power stage. Here, a concentrated effort is made to transfer maximum power to the output device e.g. a loud speaker. For maximum power transfer, the impedance of power source should be equal to that of load. Transformer coupled amplifier is used for power amplification. Direct Coupled Amplifier The latest method of coupling is direct coupling. Its circuit is very simple as shown in the fig. In this method, the collector of the first transistor is directly connected to the base of the second transistor. Hence, D.C. will be present on the base too. The advantage of direct coupling are – no distortion and uniform response over a wide frequency range. These circuits are used for very low frequency amplification purposes. Fig. 3.9 Direct Coupled Amplifier Applications The transformer coupled amplifiers are used for amplifying extremely low frequencies (as low as a fraction of a Hertz). Frequency response of the amplifier: Fig shows the frequency response of a typical R- C coupled amplifier. It is clear that voltage gain drops at off low (<50Hz) and high (>20KHz) frequencies where as it is uniform over mid frequency range (50Hz to 20KHz). This behavior of the amplifier is briefly explained below: Paper - II Electronic Devices and Circuits 239 (i) At low frequencies ((<50Hz), the reactance of coupling capacitor Cc is quite high and hence very small part of signal will pass from one stage to the next stage. Moreover, CE cannot shunt the emitter resistance RE effectively because of its large reactance at low frequencies. These two factors cause a falling off voltage gain at low frequencies. (ii) At high frequencies (>20KHz), the reactance of CC is very small and it behaves as a short circuit. This increases the loading effect of next stage and serve to reduce the voltage gain. Moreover, at high frequency, capacitive reactance of base-emitter junction is low which increase the base current. This reduces the current amplification factor (beta symbol). Due to these two reasons, the voltage gain drops off at high frequency. (iii) At mid-frequencies (50Hz to 20KHz), the voltage gain of the amplifier is constant. The effect of coupling capacitor in this frequency range is such so as to maintain a uniform voltage gain. Thus, as the frequency increases in this range, reactance of CC decreases which tends to increase the gain. However, at the same time, lower reactance means higher loading of first stage and hence lower gain. These two factors almost cancel each other, resulting in a uniform gain at mid frequency. Advantages (i) It has excellent frequency response. The gain in constant over audio frequency range which is the region of most importance for speech, music etc. (ii) It has lower cost since it has employs resistor and capacitors which are cheap. (iii) The circuit is very compact as the modern resistors and capacitors are small and extremely light. Disadvantages (i) The R-C coupled amplifier have low voltage and power gain. It is because the low resistance presented by the input of each stage to the preceding stage decreases the effective load resistance (RAC) and hence the gain. (ii) They have the tendency to become noisy with age, particularly in moisture climates. (iii) Impedance matching is poor. It is because the output impedance of R-C coupled amplifier is several hundred ohms whereas that of a speaker is only a few ohms. Hence, little power will be transferred to the speaker. 240 Electronics Engineering Technician Applications The R-C coupled amplifier excellent audio fidelity over a wide range of frequency. Therefore, they are widely used as voltage amplifier e.g. in the initial stages of public address system. If other type of coupling (e.g. transformer coupling) is employed in the initial stages, this results in frequency distortion which may be amplifier in next stages. However, because of poor impedance matching, R-C coupling is rarely used in the final stages. 3.5 Comparision of CB,CE, and CC Amplifiers S. No Characteristics CB CE CC 1. Voltage Gain 100-200 300-600 <1 2. Current Gain <1 20-100 20-100 3. Power Gain Medium High 100 4. Input Impedance 5. Output Impedance Very High 6. Phase Inversion 7 Applications Very Low 00 HF Low Very High Medium Very Low 1800 00 AF For Impedance matching Methods of Transistor Biasing The following are the most commonly used methods of obtaining transistor biasing from one source of supply i.e. Vcc (i) Base resistor method (ii) Biasing with feedback resistor (iii ) Voltage divider bias. Stabilization The collector current in a transistor changes rapidly when (i) The temperature changes (ii )The transistor is replaced by another of the same type. This is due to the inherent variations of transistor parameters. When the temperature changes or the transistor is replaced, the operating point Ic and VCE also changes. However, the faithful amplification , it is essential that operating point remains fixed. Paper - II Electronic Devices and Circuits 241 The process of making operating point independent of temperature changes or variations in transistor parameters is known as stabilization. A good biasing circuit always ensures the stabilization of operating point. Summary Amplifiers: There are transistor configurations. They are common base amplifier, common collector amplifier, and common emitter amplifier. Biasing: The process of giving supply to the transistor is known as biasing. They are three types biasing (1) Base resistor biasing (2) Collector feedback resistor biasing (3) Voltage divider biasing. In a transistor biasing a transistor should fulfill following requirement to acts as an amplifier. (1) Zero signal collector current (2) Minimum collector to emitter voltage VCE = 0.3 V (3) Minimum VBE in the case of Silicon transistor 0.7 volts, Germanium transistor 0.3 volts. Stabilization: The proper operating point, reverse saturation currents and large signal current gain important role in stabilization. The Coupling Networks: In the transistor the coupling components used as RC, LC, RL, Transformer and Direct Coupling. Short Answer Type Questions (1) What are the applications of CB amplifier.? (2) What are the applications of CE amplifier.? (3) What are the applications of CC amplifier.? (4) What is stabilization.? (5) Define stabilization.? (6) Name the different types of coupling networks.? Long Answer Type Questions (1) Compare the characteristics of CB, CE, CC amplifiers.? (2) Explaining biasing and stabilization of transistors.? (3) Draw the two stage RC coupled amplifier explain working.? 242 Electronics Engineering Technician (4) Draw the transformer couple amplifier explain working.? (5) Draw the direct couple amplifier explaining working.? (6)Draw the frequency response curve of an RC coupled amplifier and explain AF and RF frequency response.? OJT / Practical Questions. 1. Study the transistor biasing methods and its applicaitons. 2. Study the stabilization 3. Study the transistor output VI characteristics, mark cut off region , active region and saturation region, fixing of operating point on load line. 4. Study of comparision of CB, CC, CE configurations. 5. Study two stage RC coupled amplifier, working with gain versus logitherm of frequency characters. 6. Study the two stage transformer couple amplifier with its applications. 7. Study the direct couple amplifier and its application. UNIT 4 Power Amplifiers Learning Objectives • Study defination of Voltage,Power amplifiers. • Study the differences between Voltage and Power amplifiers. • Study the types of Power amplifiers-Class A,Class B,Class C, Class B push pull . • Study the Power amplifiers applications. • Study the different ICs used in Power amplifiers. 4.0 Introduction The circuit in which raises the strength of a weak signal is kown as amplifier. Almost all electronic equipment must include means for amplifying electrical signals. For instance, radio receivers amplify very weak signals. A practical amplifier always consists of a number of stages that amplify a weak signal until sufficient power is available, to operate to loudspeaker or other output devices. The first few stages in the multistage amplifier have the function only voltage amplification. However the last stage is designed to provide maximum power. Therefore the final stage is power amplifier. In some applications, feedback technique is used to alter some of the 244 Electronics Engineering Technician properties like gain, bandwidth, input and output impedances of the amplifer. The amplifier which employs the feedback technique is known as feedback amplifiers. An opreational amplifier is basically a direct coupled high gain amplifer with feedback available in form of integrated circuit. The object of this chapter is to study the different types of power amplifiers, feedback amplifier and different applications of operational amplifier. Definition of Power Amplifier: A transistor amplifier in which raises the power level of the signals that have audio frequency range is known as transistor power amplifier. In general the last stage of multistage amplifier is the power stage. A power amplifier differs from the voltage amplifier. A transistor that is suitable for power amplification is generally called as power transistor. Difference between Voltage and Power Amplifier A voltage amplifier is designed to have maximum voltage amplification. However, there is no importance of power amplification. On the other hand power amplifier is designed to achieve maximum power output. 4.1 Voltage Amplifier An electronic circuit whose function is to accept an input voltage and produce a magnified, voltage as an output voltage. The voltage gain of the amplifier is the amplitude ratio of the output voltage to the input voltage. Voltage amplifiers are distinguished from other categories of amplifiers whose ability to amplify voltages, or lack thereof, is of secondary importance. Amplifiers in other categories usually are designed to deliver to power gain or to isolate one part of a circuit from another. Power amplifiers may or may not have voltage gain, while buffers and emitter followers generally produce power gain without a corresponding voltage gain. To obtain high gain, cascades ofsingle amplifier circuits are used, usually with a coupling network, actually a simple filter, inserted between the stages of amplification. One such filter is a high-pass network formed by a coupling capacitor, the output resistances of the driving stage, and the input resistance of the driven stage. Since dc voltages are blocked by the capacitor, this ac coupling permits independently setting dc bias voltages for each amplifier stage in the cascade. The coupling network also rejects signal with ac frequency components below a cutoff. The capacitor must be sufficiently large not to attenuate any of the frequencies that are to be amplified. If dc is to be amplified, a direct-coupled amplifier is required., and the design is some what more complicated since dc Paper - II Electronic Devices and Circuits 245 bias voltages on each transistor now cannot be set independently. The amplifiers discussed above are called single-ended amplifiers, since their input and output voltages are referred to a common reference point which by convention is called ground. These single-ended circuits, while satisfactory for most non critical applications, have several weaknesses which degrade their performance in high-gain, weak-signal applications. Their unbalanced construction and their use of a common ground point for return currents makes them susceptible to noise pickup. To minimize noise on sensitive signal lines, special balanced differential amplifier circuits are often used in critical amplifier applications. Differential amplifiers are designed to have equal impedances to ground for each side of the signal line and to have a output voltage proportional to the differece of the voltages from each signal line to ground. This symmetry cancels common-mode noise voltages, voltages which tend to appear on each of the signal lines as equal voltages to ground. Proper circuit design, which attention to the symmetry of the input circuit construction, can ensure that the majority of undesred noise pickup will be common-mode noise and, hence, will be attenuated by the differential amplifier. In cases where a voltage amplifier is required for some special purpose, operational amplifiers are used to fill the need. The operational amplifier is an integrated circuit containing a cascade of differential amplifer stages, usually followed by a push-pull amplifier acting as a buffer. The different voltage gain of the operational amplifier is very high, about 100,000 at low frequencies, while its input impedance is in the megohm range and its output impedance is usually under 100 ohms. The amplifier is designed to be used in a negative-feedback configuration, where the desired gain is controlled by a resistive voltage divider feeding a fraction of the output voltage to the inverting input of the operational amplifier. Power Amplifier Power amplifier is designed to obtain maximum output power(ie product gains of voltage and current). 246 Electronics Engineering Technician 4.2 Comparisions Between Voltage and Power Amplifier Particular Voltage Amplifier Power Amplifier High > 100 Low (20 to 50) Rc High (4-10) k Low (5-20) Coupling Usually R-C coupling Transformer C coupling Input Voltage Low of few mV High (2-4 V) Collector current Low( 1mA) High (>100 mA) Power output Low High Output impedance High ( 12 k Low (200 Power dissipation rating of active device Need not be large. Should have large rating Necessity of cooling Not necessary arrangements. Cooling arrangements and heat sinks are needed 4.3 Classification of Power Amplifiers The power amplifiers can be classified in the following ways. 1.According to the Usage of Frequency Signals: (i) Audio frequency power amplifiers. (ii) Radio frequency power amplifiers. (iii) Video frequency power amplifiers. Paper - II Electronic Devices and Circuits 247 2. According to the Period of Conduction: (i) Class A Power Amplifiers: The period of cnduction is for total 3600 (full cycle). (ii) Class AB Power Amplifiers: The period of conduction is for 1800 only (half cycle). (iii) Class AB Power Amplifiers: The period of conduction is greater than 1800 but less than 3600 (in between class A and class B). (iv) Class C Power Amplifiers: The period of conduction is for less than 1800. 3. According to the Configuration Used: (i) Single ended amplifier. (ii) Push pull amplifier. (iii) Complementary symmetry push pull amplifier. 4. Applications of Power Analysis: 1. Used in public addressing systems. 2. In audio systems like radio, tape recorders, record players. 3. In T.V. receivers. 4. In broadcast and T.V transmitters. 5. In repeater circuits. 6. In all communications systems. 7. In nuclear research centres. 4.4 Efficiency of Power Amplifier Amplifier coverts of d.c power obtained from d.c. supply to a.c. power delivered to the load. The conversion efficiency of an amplifier is defined as “The ratio of the a.c. output power to the d.c. power supplied to the intput circuit. The conversion efficiency also called as collector circuit efficiency in case of transistor amplifier”. Thus % of collector circuit efficiency, = signal power delivered to the load 248 Electronics Engineering Technician d.c. power supplied to the output circuit. x 100 = ((1/2 VmIm) / (VCC1C) x 100%) = (50VmIm) / (VCCIC) % 4.5 Class-A, PowerAmplifier Working Fig 4.1 shows a simple series single ended class A amplifier with resistive load RC. The transistor used here is under fixed biasing condition. As the input signal is applied, the transistor operates in active-region and hence the amplified power output appears across the load resistor. The static output characteristics along with input, output waveforms are shown in Fig 4.1. Fig. 4.1 Series Fed Class A Amplifier For the purpose of analysis, we assume the static output characteristics to the equidistant for equal increments of the input excitation. Fig. 4.2 Series Fed Class A Amplifier - VI Characteristics When the applied input signal is a siusoidal sigal the base current varies Paper - II Electronic Devices and Circuits 249 sinusoidally and causes the transistor to amplify these sinusoidal variations. Thus the amplified output signals are also inthe form of sine wave forms as shown in Fig. 4.2. The power output of this circuit is P = VCIC = I2C RC ....................(1) Where VC and IC are the rms values of output voltage and current respectively. The magnitudes of VC and IC may be found graphically from Fig. 4.2. In the Fig. 4.2 Im and Vm represent the peak sinusoidal output current and voltage swings respectively. Then 1C = Im/ 2 = ((Imax - I min) / 22) ..............................(2) Vc =Vm / 2 = ((Vmax - Vmin)/ 22) ..............................(3) P =VcIc= (Vmax - Vmin) (Imax - Imin) /8 Collector Dissipation and Conversion Efficiency: Collector Dissipation: In a power amplifier, it is of significance to know, that what fraction of the total d.c power is effectively converted into a.c output power. In this analysis, we assume the load impedance to be pure resistor. The average power input from the d.c. supply is VCC IC. The power absorbed by the output circuit is I2C Rl + IC VC where IC and VC are the rms values of output current and voltage respectively. R1 is the static load resistance. The average power dissipated in the transistor is PD. Then, by using law of conservation of energy. Vcc Ic = I2c R1 + IcVc + PD But Vcc = Vc + Ic Rc .......................................(4) .....................................(5) Where VC is d.c. collector voltage. By submitting the value of VCC from Eqn (5 ) into Eqn (4 ). PD = VcIc - VcIc .......................................(6) The PD is the power is dissipated in the active device. If the input signal is zero, then a.c. power output VcIc also zero. Therefore, in accordance with Eqn. (6), the collector dissipation PD is maximum and has value equal to VcIc. Thus the device is cooler when delivering power to a load tha with zero signal condition. 250 Electronics Engineering Technician 4.6 Class - A, Power Amplifier Efficiency It is the ratio of output power to input power is known as efficiency. = (P output max / PD max)x100 = 0.5 Vec - ICQ / Vcc Icq x 100 4.7 Single Ended Class-A, Power Amplifier With Transformer Load The single ended class -A ,power amplifier with transformer collector load is shown in Fig 4.3. The resistors R1 , R2 and R3 form the biasing and stabilization network. The emitter bypass capacitor CE offers low reactance path to the signal. Here, we use transformer in the collector load. In the resistence coupled stage, the quiescent current passes through the load resistance. Thus these appears a cosiderable waste of power due to drop across load resistance due to passage of quiescent collector current which does not contribute to the a.c. output power. In addition to this usually many electronic systems have the loud-speaker as the load. The output impedance of the amplifier is high. The loud-speaker voice coil impedance is of 8 Ohms(symbol). For maximum transfer of power from amplifier to speaker, the impedance has to be matched. This can be done by using a stepdown transformers. Fig. 4.3 Single Ended Transformer Coupled Power Amplifer Working: The operating point is so selected that the transistor works only in the linear portion of its characteristics. The input signal varies with the base current. This produces a variation in the collector current. As the collector current in the Paper - II Electronic Devices and Circuits 251 primary of the output transformer varies, with the induced voltage in the secondary of the output transformer varies 4.8 Class-B, Push - pull Amplifier Efficiency It is the ratio of power output to power input power of the class B pushpull amplifier is known as efficiecy. Efficiency = Po / Pdc = / 4 x Vem / Vec x IC / IC x 100 But for practical purpose = 3.14*100 / 4= 75.50 ~ 75% 4.9 Advantages and Disadvantages of Push-Pull Amplifier Advantages: 1. Even harmoics are absent in the output. 2. The problem of core saturation and non-linear distortions will not appear because of cancellation of d.c. components of collector current. 3. The output is double as that offered by a single ended stage. 4. The effect of ripple voltage of the power supply due to inadequate filtering are balanced out because of flow of ripple current in opposite direction in the primary of the output transformer. Disadvantages: 1. Two transistors have to be used. 2. It requires two equal and oppsite voltages at the input. Therefore push pull circuit requires, the use of driver stage, to furnish these signals. 3. If the parameter of the two transistors are not the same, there will be unequal amplification of two halves of the signal. 4. The circuit gives more distortion. 5. Transformer used are bulky and expensive. 4.10 Class-B, Push-Pull Amplifier By complementary symmetry is meant a princple of assembling push-pull class amplifier without using centre-tapped transformers. Fig 4.4 shows the transistor push-pull amplifier using complementary symmetry. It employs one npn and pnp transistors and requires no centre tapped transformers. 252 Electronics Engineering Technician Fig. 4.4 Complementary Symmetry Push-Pull Amplifier Working: During the positive half cycle of the input signal, transistor T1 (the npn transistor) conducts current while T2 (the pnp transistor) is cutoff. During the negative half cycle of the signal, transistor T2 conducts while T1 is cut off. In this way, npn transistor amplifies the positive half cycle of the signal while the pnp transistor amplifies the negative half cycle of the signal. Here output transformer is used for impedace matching. Advantages and Disadvantages of Complementary-Symmetry Push-Pull Amplifier Advantages: 1. This circuit does not require transformers. This saves weight and cost. 2. Equal and opposite input signal voltages are not required. Disadvantages: 1. It is difficult to get a pair of transistor, that have similar characteritics. 2. Two separate collector power supplies are needed. 3. Power supply float with respect to the ground i.e., neither side of the power supply is grounded. Paper - II Electronic Devices and Circuits 253 4.11 Class-B, Push-Pull Amplifier Efficiency It is the ratio of power output to power input power of the class B pushpull amplifier is known as efficiecy. Efficiency = Po / Pdc = / 4 x Vem / Vec x IC / IC x 100 But for practical purpose = 3.14*100 / 4= 75.50 ~ 75% 4.12 Power Amplifier Applications of Power Amplifier Power Amplifier are used in (i) Public address system amplifiers. (ii) Radio receivers. (iii) Radio and T.V Transmitter. The term power amplifier is a relative term with respect to the power delivered to the load ,from the source, by the supply circuit. In general a power amplifier is desigated as the last amplifier,a transmission chain (the output stage) and is the amplifier stage that typically requires most attention to power efficiency. Efficiency cosiderations lead to various classes of power amplififer based o the biasing of the output transistors or tubes. Power amplifiers by Application: • Audio amplifier,Power which is Audio power amplifiers. • RF power amplifier, such as for transmitter final stages. • Servo motor controllers, where linearity is not important. Power amplifier circuits Power can be divivded into: • Vacuum tube/Valve, Hybrid or Transistor power amplifiers. • Push-pull output or Single-ended output stages. 4.13 Power Amplifiers IC Numbers 1. CA 3007,Class -AB, power amplifier Power output is low up to 30mw. 2. A 1ow power amplifier system by using IC mc1554. 254 Electronics Engineering Technician 3. A 20W, Class-B, power amplifier by using IC mc1533. Summary AF amplifier: these are used in audio frequency (20Hz to 20KHz) range. RF amplifier: these are used in RF frequency(20KHz to 30MHz) range. Voltage amplifier: it is an amplifier which gives only voltage amplification. Power amplifier: It is an amplifier, which gives power amplification. Power Amplifier: 1) Class-A power amplifier. 2) Class-B power amplifier. 3) Class-B push-pull power amplifier. 4) Class-C power amplifier. 5) Power amplifier and voltage amplifiers are used for amplification purpose in commuication networks. Short Answer Type Questions 1. Define voltage amplifier? 2. Define power amplifier? 3. Mention the types of coupling components used in between the amplifier stages. 4. Draw the Class-A, Class-B, Class-B push- pull amplifier wave forms. 5. Mention max efficiency of Class-A, Class-B push-pull amplifier. 6. Write application of Class-A amplifier. 7. Write applications of Class-B push-pull amplifier? 8. Mention the IC numbers used in power amplifiers. Long Answer Type Questions 1. Draw and explain working of Voltage amplifier? 2. Draw and explain working of Power amplifier? 3. Explain briefly amplifiers based on their mode of operation. Paper - II Electronic Devices and Circuits 255 4. Draw and explain class-A transformer coupled amplifier. 5. Draw and explain working of Class-B push pull amplifier. OJT/Practical Questions • Study the Voltage/Power amplifiers types. • Study the Class-A,Class-B,Class-C,Class-B push-pull amplifiers input/ output wave forms,efficienies and applications. • Study the ICs numbers used for Voltage/Power amplifiers. 256 Electronics Engineering Technician UNIT 5 Feedback Amplifier & Oscillators 5.1 Learning Objectives • Study definations of positive /negative feedback. • Study the camparions of positive and negative feedback. • Study the block diagram and working of negative feedback types of negative feedback. • Study the conditions to get oscillations,block diagram of positive feedback,derivation over all gain of an oscillator. • Study of types of oscillators working,expressions of frequency of RC Phase shift, Collector tuned,Heartly,Collpits oscillators. • Study of comparisions of RC and LC oscillators. • Study of crystal oscillators working advantages. • Study of applications of oscillators. Paper - II Electronic Devices and Circuits 257 5.0 Introduction of Feedback Amplifiers The phenomenon of feeding a portion of the output energy back to the input circuit is known as feedback. The effect results in a dependence between the output and the input and an effective control can be obtained in the working of the circuit. Feedback is of two types. 1. Positive Feedback 2. Negative Feedback Positive or regenerate feedback: When the feedback voltage or current, is in phase with the input signal, it is called positive or regenerative feedback. The positive feedback increases the amount of amplification. Negative or Degenerate feedback: When the feedback voltage or current,is out of phase to the input signal,it is called negative or degenerative feedback. Negative feedback decreases the magnitude of amplification. Its main advantage is the reduction in the distortion of the amplifier. Feedback: The process of injecting a fraction of output energy of some device back to the input is known as feedback. Depending upon whether the feedback energy aids or opposes the input signal, there are two basic types of feedbacks in amplifiers. These are. 1. Positive Feedback 2. Negative Feedback 1. Positive Feedback: In positive feedback, the feedback energy (voltage or currents), is in phase with the input signal and thus aids it. Positive feedback increases gain of the amplifier also increases distortion, noise and instability. Because of these disadvantages, positive feedback is seldom employed in amplifiers. But the positive feedback is used in oscillators. 2. Negative Feedback: In negative feedback, the feedback energy (voltage or current), is out of phase with the iput signal and thus opposes it. Negative feedback reduces gain of the amplifier. It also reduce distortion, noise and instability. This feedback increases bandwidth and improves input and output impedances. Due to these advantages, the negative feedback is frequetly used in amplifiers. 258 Electronics Engineering Technician 5.1 Comparision Between Positive and Negative Feed Back The difference between positive and negative feedback is, Negative Feedback Positive Feedback 1. Feedback energy is out phase with their input signal Feedback energy is in phase with the input signal. 2. Gain of the amplifier decreases Gain of the amplifier increases Gain stability increases Gain stability decreases S.No. 3. 5. Noise and distortion decreases. Noise and distribution increases. Increase the band width Decreases bandwidth 6. Used in amplifiers 4. Used in Oscillators 5.2 Expression for the Gain of Feedback Amplifier The configuration of the feedback amplifer in its shortest form in shown in Fig 5.1 The feedback factor of the feedback network is given by = Xf / Xo where Xf and Xo are feedback and output signals respectively. The input to the amplifier is Xs. The gain of the basic amplifiers is A. Therefore, Output sigal Xo = AXi where Xi is the input signal to the basic amplifier which is equal to difference signal Xd. Therefore Xo = AXd But for negative feedback Xd = Xs - Xf = Xi Therefore Xo = A(Xf - Xf) We know that = Xf / Xo or Xf = Xo Substituting this value is Eqn Xo = A(Xs - Xo) Xo + AXo = AXs Xo (1+A) = AXs Paper - II Electronic Devices and Circuits 259 Xo = AXs / 1 + A The gain of feedback amplifier is Af = Xo / Xs = A / 1+A Here, Af is less than A giving in reduction in gain. If positive feedback employs, in deominator is - (minus) ad therefore gain increase. Fig. 5.1 Block Diagram of Simplified Single loop Negative Feedback amplifier Effects of Negative Feedback: The following are the advantages of negative feedback in amplifies. 1. Gain Stability: An important advantage of negative feedback is that the resultant gain of the amplifier can be made independent of transistor parameters or the supply voltage variations. Af = (A) /(1+ A) The product of A is much greater than unity. Therefore in above relation 1 can be neglected as compared to A. Then, the expression becomes. Af = (A / A = (1 / It may seen that the gain now depends only upon feedback fraction . The feedback circuit is usually resistive network. Therefore, it is uneffected by changes in temperature variations in transistor parameters ad frequency. Hence, the gain of the amplifier is extremly stable. 2. Reduces Non-Linear Distortion: The negative feedback reduces,with the non linear distortion in large signal amplifiers. It can be proved mathematically ,giventhat Df = (D) / (1 + A) 260 Electronics Engineering Technician It is clear from the above equation that, a negative feedback reduces the distortion by factor 1 + A. 5.3 Types of Nagative Feedback Amplifiers The feedback amplifiers can be classified according to mixing and sampling employed to it as follows: 1. Voltage series feedback amplifier 2. Current series feedback amplifier 3. Current shunt feedback amplifier 4. Voltage shunt feedback amplifier 1. Voltage Series Feedback Amplifier: This uses output voltage sampling and series mixing. 2. Current Series Feedback Amplifier: This uses output current sampling and series mixing. 3. Current Shunt Feedback Amplifier: This uses output current sampling and shunt mixing. 4. Voltage Shunt Feedbac Amplifier: This uses output voltage sampling and shunt mixing. 5.4 Conditions of an oscillators - Barkhausen Criteron Oscillations produced by adequate positive feedback in an amplifier is called a feedback oscillator. Fig. 5.02 gives the block diagram of feedback oscillator. An amplifier is an essential part of an oscillator. Oscillations may be produced by adequate positive feedback in an amplifier. Fig. 5.2 Block diagram of an Oscillator Paper - II Electronic Devices and Circuits 261 Consider an external signal Xs applied directly to the input terminals of the amplifier shown in Fig.5.03. This results in an output signal Xo. The output of the feedback network is. Xf = Xo = AXs This output of the mixing network is X1f = -Xf = -AXs Let it be so arranged that X1f is identical with Xs. If now the external source is removed and terminal 2 is connected to terminal 1, the amplifier continues to provide the same output voltage Xo as before without any exteral input signal. The system then functions as an oscillator. The condition necessary for oscillations is that X1f = Xs. Thus the instantaneous values X1f and Xs are identical at all times. Since X1f = -AXs implies that -A=1 i.e., the loop gain must be equal to unity and phase angle of -A is zero. This condition for sustained oscillations is called the Barkhausen criterion. Barkhausen Criterion: 1. Sustained oscillations are produced in a sinusoidal oscillators at a frequency for which the total phase shift introduced,as the signal travels from the input terminal through the basic amplifier, feedback network and mixing network back to the input terminals its precisely zero or a integral multiple of 2 radians. 2. Sustained oscillations are not produced if at the oscillation frequency the magnitude of the loop gain i.e., the product of the transfer gain A, of amplifer and magnitude of the feedback factor of the feedback network is less than unity. Requisites of an Oscillator 1. Tank Circuit: It consists of inductor connected in parallel with capacitor C. The frequency of oscillations in the circuit depends upon the values of inductance (L) ad capacitace (C). In RC oscillators inductor replaced by resistor(R). 2. Transistor Amplifier: The transistor amplifier receives d.c power from the battery and changes it into a.c. power for supplying to the tank circuit. The oscillations occurring in the tank circuit are applied to the input of the transistor amplifier. The amplified output of oscillations is due to the d.c. power supplied by the battery. The output of the transistor can be supplied the tank circuit to meet the losses. 3. Feedback Circuit: The feedback circuit supplies a part of collector energy 262 Electronics Engineering Technician to the tank circuit in correct phase to aid the oscillations i.e., it provides positive feed back. In oscillator is to satisfy Barkhausen criteria has to get sustained oscillations. 5.5 Classification of Oscillators The oscillators can be classified in the following ways. 1. According to the generated waveform. (a) Sine wave oscillators. (b) Relaxation or non-sinusoidal oscillators. 2. According to the fundemental mechanism involved (a) Feedback oscillators. (b) Negative resistance oscillators. 3. According to the associated circuit components (a) RC oscillators (b) LC oscillators (c) Cyrstal oscillators 4. According to the frequency range: (a) Audio frequency (AF) oscillators (b) Radio frequency (RF) oscillators (c) VHF or microwave oscillators. 5.5.1 | A| > 1 When the total phase shift around a loop is 00 or 3600 and |A >1, then the output oscillates but the oscillations are of growing type. The amplitude of oscillations goes on increasing as shown in Fig. 5.3 Fig. 5.3 Growing type of Oscillations Paper - II Electronic Devices and Circuits 5.5.2 263 | A = 1| As stated by Barkhausen criterion, when total phase shift around a loop is 0 or 3600 ensuring positive feedback and | A| = 1 then the oscillations are with constant frequency and amplitude called sustained oscillations. Such oscillations are shown in Fig 5.4 0 Fig. 5.4 Sustained Oscillations 5.5.3 |A < 1| When total phase shift around a loop is 00 or 3600 but | A< 1| then the oscillations are of decaying type i.e. such oscillation amplitude decreases exponentially and the oscillations finally cease. Thus circuit works as an amplifier without oscillations. The decaying oscillations are shown in Fig 5.5. Fig. 5.5 Exponentially decaying Oscillations 264 Electronics Engineering Technician Classification of Oscillators As type of tank circuit employ to the amplifier circuit in positive feedback the following oscillators. 1. RC Phase Shift Oscillator 2. Collector Tuned Oscillator 3. Hartley Oscillator 4. Collpitt’s Oscillator 5.6 RC Phase Shift Oscillator Fig.5.06 shows the circuit of a phase shift oscillator. It consists of a conventional single transistor amplifier and a RC Phase shift network. The phase shoft network consists of three sections R1 C1, R2 C2 and R3 C3. At some particular frequency f0, the phase shift of each section is 600, so that the total phase-shift produced by the RC network is (3 x 60) = 1800. The frequency of oscillations is given by fo = ( 1 ) / ( 2 RC 6) where R1 = R2 = R3 = R C1 = C2 = C3 = C Figure 5.6 RC Phase Shift Oscillator Paper - II Electronic Devices and Circuits 265 With the circuit is switched ON, it produces oscillations. The output E0 of the amplifier is feedback to RC feedback network. This network produces a phase shift of 1800 and a voltage E1 appears at its output which is applied to the transistor amplifier. The feedback factor = E1 / E0. It can be shown that the feedback factor of the RC network is = 1/29. This expression has an important significance. For self starting the oscillations we must have A >1. It means that gain A of the amplifier must be greater than 29. Only then the oscillations can start. The feedback phase is correct. A phase shift of 1800 is produced by the transistor amplifier. A further phase shift of 1800 is produced by the RC network. As a result, the phase around the entire loop is 3600. Advamtages : 1. It does not require transformers or inductors. 2. It can be used to produce very low frequencies. 3. The circuit provides good frequency stability. Disadvantages : 1. It is difficult for the circuit to start oscillations as the feedback is generally small. 2. The circuit gives small output. 5.7 Tuned Collector Oscillator The tuned collector oscillator contains tuned circuit L1 -C1 in the collector load .The feedback coil L2 in the base circuit is magnetically coupled to the tank circuit coil L1.and hence the name. The frequency of oscillations depends upon the values of L1 and C1 and is given by f = ( 1 ) / (2 L1C1) The figure coil L2 in the base circuitis magnetically coupled to the tank circuit L1. In practice L1 and L2 form the primary and secondary of the transformer. The biasing is provided by potential divider arrangement. The capacitor C connected in the base circuit provides low reactance path to the oscillations. 266 Electronics Engineering Technician Fig. 5.7 Circuit Operation: When switch S is closed. Collector current starts increasing and charges the capacitor C1. When this capacitor is fully charged, it discharges through Coil L1, setting up oscillations of frequency. f = ( 1 ) / (2 L1C1) These oscillations induce some voltage in coil L2 by mutual induction. The frequency of voltage of coil L2 is the same as that of tank circuit but its magnitude depends upon the number of turns of L2 and coupling between L1 and L2. The voltage acaross L2 is applied between base and emitter and appears in the amplified form in the collector circuit, thus overcoming the losses occurring in the tank circuit. The number of turns of L2 and coupling between L1 and L2 are so adjusted that oscillations across L2 are amplified to a level just sufficient to supply losses to the tank circuit. It may be noted that the phase of feedback is correct i.e., energy supplied to the tank circuit is in phase with the generated oscillations. A phase shift of 1800 is created between the voltages of L1 and L2 due to transformer action. A further phase shift of 1800 takes place between base-emitter and collector circuit due to transistor properties. As a result the energy feedback to the tank circuit is in phase with the generated oscillations. 5.8 Hartly Oscillator Hartly oscillator is very popular and is commonly used as a local oscillator in radio receivers. Fig.5.8 shows the circuit of Hartley oscillator. The tank circuit is made up Paper - II Electronic Devices and Circuits 267 of C L1 and L2. The coil L1 is inductively coupled to coil L2, the combination functions as auto-transformer. The self bias is provided here for biasing. the capacitor Cb blocks the d.c. component. When the power is ON, collector current starts rising and charges the capacitor C. When the capacitor is fully charged, it discharges through coils L1 and L2 setting up oscillations of frequency. f = ( 1 ) / ( 2 (L1+L2) C) Fig. 5.8 Hartley Oscillator The oscillations across L1 are applied to the base-emitter junction and appears in the amplified form in the collector circuit. The coil L2 couples the collector circuit energy back into the tank circuit by means of mutual inductance between L1 and L2. In this way, energy is being continuously supplid to the tank circuit to overcome the losses occurring in it. It may be seen that the phase of feedback is correct. The capacitor C and L1 - L2 are 1800 out of phase. A further phase shift of 1800 is produced by transistor circuit. In this way, energy feedback to the tank circuit is in phase with oscillations. Advantages : 1. Easy to tune. 2. Adaptability to a wide range of frequencies. 5.9 Colpitt’s Oscillator Fig 5.9 shows the circuits of colpitt’s oscillator. The tank circuit is make up 268 Electronics Engineering Technician of C1 C2 and L. The biasing is provided by self biasing. When power is ON, collector current starts rising and charges the capacitors C1 and C2. These capacitors discharges through coil L setting up oscillations. The frequency of oscillations is given by f = (1) / (2 LCT) where CT = (C1C2) / (C1 + C2) The oscillations across C1 are applied to the base-emitter junction and appear in the amplified form in the collector circuit and supply losses to the tank circuit. The amount of feedback depends upon the relative capacitance values of C1 and C2. Fig. 5.9 Collpitt’s Oscillator It may be noted that the phase of feedback is correct. The capacitors C1 and C2 act as a simple alternating voltage divider. Therefore the tank circuit of L C1 C2 produce 1800 phase shift. A further 1800 phase shift is produced by the transistor. In this way feedback is properly phased to produce continuous undamped oscillations. 5.10 Oscillators Frequency Equations as Follows a) Collector tuned oscillator frequency f = (( 1 ) / (2 CTL1) b) RC Phase Shift Oscillator frequency fo = ((1) / (2 RC 6) Paper - II Electronic Devices and Circuits 269 where R1 = R2 = R3 = R C1 = C2 = C3 = C c) Hartly Oscillator frequency (formula) where fo = ((1) / (2CL1) where L1 = L1 + L2 + - 2M d) Colpitt’s Oscillator frequency fo = ((1) / 2 CTL) where CT = C1C2 / C1+C2 5.11 Comparison of LC and RC Oscillators: S.No. Particulars LC Oscillators RC Oscillators 1. Requirements of Inductor / transformer Yes No 2. Cost More Less 3. Output Frequency High Low 4. Frequency stability Poor Good 5. Output voltage More Less 5.12 Piezo Electric Crystals Certain crystalline materials, exhibit the piezo-electric effect i.e., when we apply an a.c. voltage across them, they vibrate at the frequency of the applied voltage. Conversely, if the crystals are forced mechanically to vibrate, they generate an emf at the fundamental frequency of the crystal. This nature is found in materials namely: Rochelle salt, quartz and tourmaline. Of the various piezoelectric crystals quartz is most commonly used. The advantages of quartz crystal is. 1. Optimum value of mechnical strength 2. Inexpensive 3. Readily available in nature 270 Electronics Engineering Technician The nature shape of the quartz crystal is a hexagonal prism. The useful crystal is obtained by cutting the nature crystal. The crystal is usually mounted in an oscillator circuit to vibrate best at one of its resonant frequencies, usually the fundamental frequency. The formula of the fundamental frequency of crystal is given by. f=k/t where t = Thickness of crystal k = constant that depends o its cut and other physical factors. In order to use crystal in an electronic circuit, it is placed between two metal plates. A crystal can be conveniently replaced by an electrical equivalent circuit. When the crystal is not vibrating, it is equivalent to capacitance Cm because it has two metal plates separated by a dielectric (crystal). However when crystal is vibrating, it is equivalent to series tuned circuit RLC. Therefore, the electrical equivalent circuit of the crystal is shown in Fig.5.10. In this figure. Cm = Mounting capacitance Cs = Series capacitance introduced by air gap R-L-C : Electrical equivalent of vibrational characteristics of crystal. Fig. 5.10 The series resonant frequency of crystal is the resonant frequency of LCR branch is given by fs = ((1) / (2 LCs)) The parallel resonant the frequency of the crystal is the frequency at which the loop current il reaches the maximum value. Since C is in series with Cm the loop capacitance CT equal to (CmC) / (C + Cm). So the parallel resonant frequency is given by fp = ((1) / 2 LCT)) Paper - II Electronic Devices and Circuits 271 5.13 Transistor Crystal Oscillator The Fig.5.11 shows the crystal oscillator. This circuit is same as Colpitt’s oscillator. In this circuit the crystal is mounted to act as an inductor which forms the tuned circuit with C1 and C2. The positive feedback is provided by the capacitive voltage divider network. The crystal now acts as an inductor that resonants with C1 and C2 and the oscillating frequency of the circuit now lies in between series and parallel resonant frequencies of the crystal. The resistors R1, R2 for biasing and RE for stabilization. The CE configured transistor provides 1800 phase shift where as the remaining 1800 phase shift is provided by the feedback network. Fig. 5.11 Crystal Oscillator Advantages 1. It can produce highest oscillating frequencies. 2. The quality factor (Q) of the crystal is very high. The Q factor of the crystal may be as high as 10,000 compared to about 100 of LC tank circuit. 3. They have a high order of frequency stability. 4. Low cost. 5. Simple in construction. 272 Electronics Engineering Technician Disadvantages : 1. They are fragile and consequently can only be used in low power circuits. 2. The frequency of oscillation cannot be changed appreciably. Summary Oscillator Circuit or Tank Circuit: A circuit which produce electrical oscillations of any desired frequency is known as an oscillatory circuit. Frequency of oscillations is given by f = (1) / (2LC) Feedback Oscillator : Oscillations produced by adequate positive feedback in an amplifier is called a feedback oscillator. Barkhensans Condition for Sustained Oscillations: 1. A = 1 2. Phase angle of - A is zero. Colpitt’s Oscillator : The tank circuit of this oscillator is made up of C1 C2 and L. The frequency of oscillations is given by f = (1) / (2LCT) where CT = (C1C2) / (C1 + C2) Hartley Oscillator : The tank circuit of this oscillator is made up of CL1 and L2. The frequency of oscillations is given by f = 1 / (2(L1+L2) C) RC Phase Shift Oscillator : The phase shift network of this oscillator consists of three identical RC sections. The phase shift of each session is 600. Frequency of oscillations is given by f = (1) / RC6) Crystal Oscillator : It is used to get high frequency stability. This is possible by employing crystal in a transistor oscillator. Relaxation Oscillator : An oscillator which produces non-sinusoidal wavesl like square, sawtooth, rectangular, triangular etc., is called a relaxation oscillator. Paper - II Electronic Devices and Circuits 273 Short Answer Type Questions 1. Define an oscillator. 2. Explain how oscillations produce in tank circuit. 3. Explain the condition for oscillation. 4. Explain the classifications of oscillators. 5. State the requisites of an oscillator. 6. Draw the circuit of a Collpitt’s oscillator and explain its working? 7. With a near diagram explain the action of Hartley oscillator. 8. Draw the circuit diagram of an RC phase shift oscillator and explain. 9. Mention the advantages and disadvantages of phase shift oscillator. 10. List the applications of oscillators. 11. Draw the circuit diagram of crystal oscillator and explain its working. Also list its advantages and disadvantages. 12. Draw the circuit diagram of UJT relaxation oscillator and explain its working. 13. What are the requisites of an oscillators? Long Answer Type Questions 1. Write camparisions of negative and positive feedback. 2. Draw and explain positive feedback. 3. What are the requirements a transister amplifier works as an oscillator. Explain ?. 4. Explain working of RC phase shift oscillator. 5. Explain working of tuned collector oscillator with neet diagram. 6. Explain working of Hartely oscillator. 7. Explain working of Colpitts oscillator. 8. Explain working of Crystal oscillator. 274 Electronics Engineering Technician Practical/OJT Questions • Study the oscillators-RC phase shift,Hartely,Colpitts,Tuned collector and Crystal oscillators. UNIT 6 Analogic’s Learning Objectives • Study of different IC s used as Voltage regulators. • Study of working of siries/shunt voltage regulators. • Study oft the advantages of IC s regulators. • Study of the positive/negative voltage regulator by using IC 7800 and 7900series. • Study the operation of LM317 adjustable voltage regulator. • Study the operation of differential amplifier. • Study operational amplifier working and Input impedence,Open loop gain,Slew rate, CMRR,Input offset voltage,Input offset Current and specifications. • Study the block diagram of IC 741 working. • Study the operational amplifier working as summer, integrator, diffentiator, inverter, multiplier,voltage follower,voltage to current converter,current to voltage converter,camparitor and square wave generator. • Study the block diagram of IC 555 and working. • Study the working of PLL. 276 Electronics Engineering Technician • Study the block diagram of PLL-LM565 and working. • Study the working of operation VCO, LM566. 6.0 Introduction The function of a voltage regulator is to provide a stable dc voltage for powering other electronic circuits. A voltage regulator should be capable of providing substantial output current. Voltage regulators are classified as: Series regulator Shunt regulator Series regulator use as power transistor connected in series between the unregulated dc input and the load. The output voltage is controlled by the continuous voltage drop taking place across the series pass transistor. Since the transistor conducts in the active or linear region, these regulators are also called linear regulators. Linear regulators may have fixed or variable signal output voltage ad could be positive or negative. The schematic, important characteristics, data sheet, short circuit protection, current fold-back, current boosting techniques for linear voltage regulators such as 78 XX, 79 XX series, 723 IC are discussed. Switching regulators, on the other hand, operate the power transistor as a high frequency on/off switch, so that the power transistor does not conduct current continuously. This gives improved efficiency over series regulator. In the principle of switching power supply and its advantages over linear type of voltage regulator are discussed. 6.1 Series Voltage Regulator In your robot, the energy is derived from batteries. Specifically, there are two sets of batteries wired up to act as voltage sources,a 9V, battery, and two 1.5V batteries connected in series that act as a 3V source. Since different circuits in your robot require different voltage sources, it is not always possible to hook up the battery directly to power the circuits. The ICs in your robot circuit are designed to work with a constant 5V source. Therefore, it is important to convert the 9V source into a 5V source. Since a DC voltage (one that is fixed over time such as a battery) is being converted to another DC voltage, the circuit that does this is called a DC-to-DC converter or a voltage regulator. If we were to convert 110V AC (alternatig current - like the power in a wall outlet) into a 5V DC source, the circuit would be an AC-DC converter. In this lab, you will build the voltage regulator circuit which converts the 9V Paper - II Electronic Devices and Circuits 277 batteries output into a constant 5V voltage source. The circuit has already been designed for you.You task will be to build and test its operation. You will also do some experiments that will allow you to develop an understanding of how the circuit works. The voltage regulator circuit consists of 5 different components; a 9V battery, a resistor, a diode, a transistor, and a capacitor. You may wish to review the description of the operation of these components in the Lab Guide.The circuit you will be building is shown in Fig. 6.1. The pin out for the transistor is shown in Fig. 6.2 - please be very careful, interchanging the base and collector will result in immediate destruction of the transistor. Fig. 6.1 Power Supply Regulator If you are using the replacement transistor rather than the one that comes in the robot kit, its leads come out in a different order (shown on the left in Fig. 6). For more details on identifying the E, B, and C terminals of your transistors see the Lab Guide. Another potential problem is that the capacitor is electrolytic which means that it can only stand to have voltages applied in one direction. If voltages are applied contrary to the sign of the label on the capacitor it will be destroyed. Note, this circuit is also shown in the Graymark Robot Assembly Manual in Figure T2 - but it may be WRONG. The Zener diode may be incorrectly connected with the arrow pointing toward ground. Follow the schematic shown above in Fig.6.1 or in the schematic. A brief description of the circuit operation is as follows. Before the 9V battery is attached, all points on the circuit are at 0V (ground). Let us first consider the operation of the circuit without the capacitor (C11). When the switch is closed, a voltage is applied to the Zener diode through R14. The value of R14 is chosen so that the Zener diode is in the reverse break-down region. Consequently, the voltage across the diode is held constant at 5.6V. This 5.6V also appears across the base-emitter junction of the transistor and the load resistor in series. Since this voltage is much greater than 0.7V, the base-emitter 278 Electronics Engineering Technician diode is forward biased and current flows intothe base of the transistor. The voltage across the resistor is therefore fixed at 5.6-0.7=4.9 V as long as the Zener diode is in the reverse breakdown region and the base-emitter diode is forward biased. To see how this circuit is always able to hold the voltage across the load at approximately 5 V, let us consider the current flows in the circuit. The current flowing through R14 is split between the Zener diode and the base-emitter diode of the transistor. If we denote the current flowing into the base as IB, a current equal to (symbol)IB flows from the battery into the Collector. Since the transistor is connected to operate in the “Forward Active” region, a current equal to (symbol+1)IB flows out of the emitter and through the Zener diode changes so that the base current and therefore the emitter current has the proper value to give the required 5V across the load resistor. Fig. 6.2 Transistor Lead Configurations The capacitor is a circuit element that stores electrical change. It is used in this circuit to help keep the voltage regulator’s output voltage constant over time. The rate of change of the voltage across a capacitor is proportional to the current flowing out of it divided by the capacitance. Therefore, the larger the capacitor, the smaller the changes in voltage at the output of the regulator over time for a fixed current drain. 1. First, build the circuit ,on your proto board. The transistor is TR3, to determine how the voltage regulator circuit should be wired together. Connect the voltage regulator output to a 4.7K resistor (which will give you a load current of about 1mA) which acts as the load resistance, R load. For testing purposes, we are replacing the battery with the +20V power supply. Connect the Common terminal to the bottom of the diode and C11. Make sure you set Paper - II Electronic Devices and Circuits 279 the +20V output of the power supply output voltage to 0V before you connect the +20V terminal of the power supply to the transistor collector and R14. Measure the output capacitor voltage, VOUT,for varying values of power supply voltage, V1N, starting at 0 volts and increasing the supply by 1 volt steps until it reaches 9 volts. As you increase the power supply volage keep an eye on the ammeter on your power supply. If it moves noticeably, immediately turn the voltage back down and check your circuit. 2. One important characterization of a voltage regulator is how well it holds the output constant in the face of changing input voltage - this is called line regulation. Line regulation is characterized by the change in output voltage divided by the change in input voltage. That is, Aline = VOUT / VIN The line regulations error for the ideal voltage regulator is 0%. With a single 4.7K resistor as a load for the voltage regulator output, and a power supply voltage of 9V, measure the output voltage. Change in the power supply voltage to 8V and then measure the output voltage. What is Aline for your voltage regulator? 3. Next, we will explore what is called the “load regulation” of your voltage regulator. Good load regulation means that the output voltage does not change much with changing load resistance. To characterize the load regulation of your regulator circuit, set the power supply voltage at 9V, and see how the output voltage varies as you draw current from (load down) the voltage regulator output. Measure the regulator output voltage with a 4.7K resistance (which means a load current of about 1mA) and with a load resistance of wo 4.7K resistors in parallel (which means a load current of approximately 2mA. We formulate the voltage regulator’s load regulation in terms of its incremental output resistance change in load voltage divided by the change in load current. Rsupply = VOUT / IOUT = VOUT / 1mA Note, an ideal voltage source would have Rsupply = 0 What is Rsupply for your regulator? 4. After you have verified that the circuit functions properly, solder the voltage regulator circuit onto the robot PC board. Note, do not out off the leads of the components flush with the board. You should leave enough wire protruding so that a clip lead can be attached for testing. Verify that the connections are correct by examining the underside of the board. Set the +20V part of your 280 Electronics Engineering Technician power supply to +9V and attach it to the +9V pin of the robot PC board between R14 and the power switch SW1. Attach the power supply common to one of the GND pins of the robot PC board. Turn SW1 on the check to see that your voltage regulator still works by measuring the voltage of the output pin with respect to ground. If the output voltage is not 5V, identify the problem and fix it. If you cannot rectify the problem ask your TA for help. 6.2 Shunt voltage Regulator Trans Fig. 6.3 Shunt Voltage Regulator Circuit Regulator Circuit Description In this case the Shunt Regulator shown is a DC to DC converter, with an Unregulated input DC voltage and a Regulated output DC voltage. The operational voltage of the shunt regulator will depend on the Zener diode used [CR1] and the transistor [Q1]. Transistor Q1 is a NPN transistor which needs to be able to work with the voltages used in the design. With the addition of Resistor Rs the reference iput is not connected to the reference of the output [the negative terminals]. 6.3 IC Voltage Regulators With the advet of micro-electronics, it is possible to incorporate the complete circuit on a monolithic silicon chip. This gives low cost, high reliability, reduction in size and excellent performance. Examples of monolithic regulators are 78 XX/79 XX series and 723 general purpose regulators. 78 XX series are three terminal, positive fixed voltage regulators. There are seven output voltage options available such as 5,6,8,12,15,18 and 24 V. In Paper - II Electronic Devices and Circuits 281 78 XX, the last two numbers (XX) indicate the output voltage. thus 7815 represents a 15V regulator. There are also available 79 XX series of fixed output, negative voltage regulators which are complements to the 78 XX series devices. There are two extra voltage options -2 V and -5.2 V available in 79 XX series. These regulators are available in two types of packages. Metal Package (TO - 3 type) Plastic Package (To - 220 type) Data Sheet Regulator: 7805 is a voltage regulator integrated circuit. It is a member of 78xx series of fixed linear voltage regulator ICs. The voltage source in a circuit may have fluctuations and would not give the fixed voltage output. The voltage regulator IC maintains the output voltage at a constant value. the xx in 78xx indicates the fixed output voltage it is designed to provide. 7805 provides +5V regulated power supply. Capacitors of suitable values can be connected at input and output, pind depending upon the respective voltage levels. Fig. 6.4 Pin Description 1 Input voltage (5V-18v) Input 2 Ground (0V) Ground 3 Regulated output, 5V (4.8V-5.2V) Output 282 Electronics Engineering Technician Figure 6.02 shows the standard repressentation of monolithic voltage regulator. A capacitor Ci (0.33 F) is usually connected betwen input terminal and ground to cancel the inductive effects due to long distribution leads. The output capacitor C0 (1)F) improves the transient reponse Unregulated input Vin Regulated output Vo Fig. 6.5 Standard representation of a three terminal positive monolithic regulator National Semiconductor also produces three terminal voltage regulators in LM317 series. There are three series available for different operating temperature ranges; LM 100 series -550C to +1250C LM 200 series -250C to +850C LM 300 series -00C +700C to The popular series are LM 340 positive regulators and LM 320 negative regulators with output ratings comparable to 78 XX/79 XX series. Characteristics There are four characteristics of three terminal IC regulators which must be mentioned. 1. V0 : The regulated output voltage is fixed at a value as specified by the manufacturer. There are a number of modesl available for different output voltages, for example 78 XX series has output voltage at 5,6,8 etc. 2. |Vin | >= |V0| + 2 volts: The unregulated input voltage must be atleast 2 V more than the regulated outpt voltage. For example, if V0 = 5 V, then Vin = 7 V. 3. I0 (max) : The load current may vary from 0 to rated maximum output current. The IC is usually provided with a heat sink, otherwise it may not provide Paper - II Electronic Devices and Circuits 283 the rated maximum output current. 4. Thermal shutdown: The IC has a temperature sensor (built-in) which turns off the IC when it becomes too hot(usually 1250 C to 1500 C). the output current will drop and remains there until the IC has cooled significantly. The electrical characteristics 7805 voltage regulator and the connection diagram of packages available. Some of the important performance parameters listed in the data sheet are explained as follows: Line/Input Regulation It is defined as the percentage change in the output voltage for a change in the input voltage. It is usually expressed in millivolts or as a percentage of the output voltage. Typical value of line regulation from the data sheet of 7805 is 3 mV. Absolute Maximum Ratings Input Voltage (5 V through 18 V) 35 V (24 V) 40 V Internal Power Dissipation Internally limited Storage Temperature Range -650C to +1500C Operating Junction Temperature Range (symbol)A7800 (symbol)A7800C -550C to +1500C -00C to +1250C Electrical Characteristics VIN = 10 V, IOUT = 500 mA, 00C <= Tj <= 1250C, CIN = 0.33 (symbol)F, COUT = 0.1 f , unless otherwise specified. The reference voltage is typically 7.15V. So the output voltage V0 is Vo = 7.15 x R2 / R1 + R2 which will always be less than 7.15V. So in the circuit of Fig. 6.06 is used as low voltage (<7V) 723 regulator. If it is desired to produce regulated output voltage greater than 7V, then the circuit of Fig. 6.07 can be used. The NI terminal is connected directly to Vref through R2. So the voltage at the NI terminal is Vref. The error amplifier operates as a non-inverting amplifier with a voltage gain of 284 Electronics Engineering Technician Av = 1 + ( R1 / R2) So the output voltage for the circuit is Vo = 7.15 x (1+ R2 / R1) 6.3.1 Advantages of IC Regulators · Available source input voltages · Desired supply output voltage magnitudes · Ability to step-down or step-up output voltages, or both · DC-DC converter efficiency (POUT / PIN) · Output voltage ripple · Output load transient response · Solution complexity (one IC solution, # of passive components, controller and external FETs) · Switching frequency (for switch-mode regulators) Data Sheets The circuits of Fig.6.07 have no protection. If the load demands more current e.g. under abort circuit condition, the IC tries to provide it at a constant output voltage getting hotter all the time. This may ultimately burn the IC. The IC is, therefore, provided with a current limit facility. Currenet limiting refers in the ability of a regular to prevent the load current from increasing above a present value. The characteristic curve of a current limited power supply. The output voltage remains constant for load current below Ilimit. As current approaches to the limit, the output voltage drops. The current limit Ilimit is set by connecting an external resistor Rsc between the terminals CL and CS terminals. The CL terminal is also connected to the output terminal V0 and CS terminal to the load. LM341,LM78M05,LM78M12,LM78M15 LM341/LM78MXX Series 3-Terminal Positive Voltage Regulators Paper - II Electronic Devices and Circuits 6.4 Posotive Voltage I.C. Regulators78 XX Series Fig. 6.8 285 286 Electronics Engineering Technician Paper - II Electronic Devices and Circuits 287 288 Electronics Engineering Technician 6.4.1 Negative Voltage Regulator Circuit Diagram using 79xx Regulator IC DC - DC Converter, Power Supply, Using Regulator IC, Voltage Regulator This is a Negative Voltage Regulator Circuit Diagram. You want get best performance via electronic circuits its need fixed DC power supply, especially digital electronic circuits. This voltage regulator circuit designed using Fixed Negative Voltage Regulator IC. You can use 79xx series regulator ICs (Ex: 7905, 7908, 7912) for this circuit and supply voltage is this circuit -8V to -30V. Output voltage is indicating in last two numbers of IC. You can select regulator IC using this table. IC No Voltage IC No Voltage 7905 -5V 7910 -10V 7912 -12V 7915 -15V 7918 -18V 7906 -6V 7908 -8V 7924 -24V 7909 -9V You can select output current limits of regulator IC using this table. IC No Output Current (Amp) Package 79Lxx (Ex: 79L05) 100mA TO 92 79Mxx (Ex: 79M05) 500mA TO 220 79xx (Ex: 7905) 1A TO 220 79Sxx (Ex: 79S05) 2A TO 220 79Txx (Ex: 79T05) 3A TO 220 79Hxx (Ex: 79H05) 5A TO 3 If you use regulator IC maximum output current is over 100mA, don’ forget to installed proper heatsink with IC. Paper - II Electronic Devices and Circuits 289 290 Electronics Engineering Technician 6.5 Adjustable Voltage Regulator By Using LM 317 description/ordering information (Continued) In addition to having higher performance than fixed regulators, this device includes on-chip current limiting, thermal overload protection, and safe-operatingarea protection. All overload protection remains fully functional, even if the ADJUST terminal is disconnected. The LM317 is versatile in its applications, including uses in programmable output regulation and local on-card regulation. Or, by connecting a fixed resistor between the ADJUST and OUTPUT terminals, the LM317 can function as a precision current regulator. An optional output capacitor can be added to improve transient response. The ADJUST terminal can be bypassed to achieve very high ripple-rejection ratios, which are difficult to achieve with standard three-terminal regulators. Fig. 6.9 Adjustable voltage Regulator IC 317 Description/ordering information The LM317 is an adjustable three-terminal positive-voltage regulator capable of supplying more than 1.5 A over an output-voltage range of 1.25 V to 37 V. It is exceptionally easy to use and requires only two external resistors to set the output voltage. Furthermore, both line and load regulation are better than standard fixed regulators. Paper - II Electronic Devices and Circuits 291 Since the feedback current is proportional to the output voltage, this circuit is voltage shunt feedback amplifier. The transfer trans resistance Rmf = Vs / Is Since Is = If (because IB isvery very small) Therefore Rmf = Vs / If = 1/ B = Rf 6.6 The Differential Amplifier A differential amplifier serves to amplify the difference between two signals. A differential amplifiers forms the basic stage of an integrated op-amp with differential inputs. The circuit diagram of the emitter cuopled differential amplifier is shown in Fig. 6.12. Ituses two identical npn transistors.The transistors are connected in CE mode. The emitter bias is used here. The two inputs v1 and v2 and the output is v0. If the inputs are similar, the output of the amplifier is zero. The amplifier output is proportional to the difference of the two inputs v1 and v2. Therefore it is called as differential amplifier. The emitter coupled differential amplifier of Fig.6.12 posses the following properties. Fig. 6.12 Emitter coupled differential amplifier 1. Low drift. 2. Very high input resistance. 292 Electronics Engineering Technician 3. Cancels the effects of supply voltages. 4. High CMRR. 5. Very high stability. 6.7 Operational AmplifierWorking The operational amplifier is a basc analog building block common to a number of electronic functions performed in instrumentation, computation and control. Op-amp is basically a differential amplifier whose function is to amplify the difference between two input signals. Op-amp is available in IC form. Basic Concepts and Characteristics of Operational Amplifier The ideal operational amplifier is shown as Fig.6.13, its equivalent circuit 6.13. A signal appearing at the negative terminal v1 is inverted at the output, a signal at the positive terminal v2 appears at the output with no change in sigh. Hence the negative terminal is called the ‘inverting terminal’ and the positive terminal the ‘non inverting terminal.’ In general the output voltage is directly propertional to the difference of the input voltage. The constant of proportionality, - A; is the voltage gain of the amplifier. Fig. 6.13 Equivalent Circuit Paper - II Electronic Devices and Circuits 293 6.8 Ideal OP-AMP Characteristics The ideal op-amp has the following characteristics. 1. Infinite gain A = 2. I1 = I2 = 0 of infinite input impedance Zi = . 3. Zero output impedance Z0 = 0. 4. Zero output voltage for vd = 0 i.e., zero offset 5. Infinite bandwidth Bw = 6. Infinite common mode rejection ratio CMRR = Operational Amplifier Earlier we have used an ideal op-amp, and assumed that the op-amp responds equally well to both ac and dc input voltages. However, a practical op-amp does not behave this way. A practical op-amp has some dc voltage at the output even with both the inputs grounded. The factors responsible for this and the suitabe compensating techniques are discussed. Also, under ac conditions the characteristics of an op-amp are frequency dependent. The limitations of an op-amp under ac conditions and methods of compensation are discussed. DC Characteristics An ideal op-amp draws no current from the source and its response is also independent of temperature. However, a real op-amp does not work this way. Current is taken from the source into the op-amp inputs. Also the two inputs respond differently to current and voltage due to mismatch in transistors. A real op-amp also shifts its operation with temperature. These non-ideal dc characteristics that add error components to the dc output voltage are: Input bias current Input offset current Input offset current Thermal drift. Input Bias Current The op-amp’s input is a differential amplifier, which may be made of BJT or FET. In either case, the input transistors must be biased into their linear region by supplying currents into the bases by the external circuits. In an ideal op-amp, we assumed that no current is drawn from the input terminals. However, 294 Electronics Engineering Technician practically, input terminals do conduct a small value of dc current to bias the input transistors. The base currents entering into the inverting an non-inverting terminals are shown as I-B and I+B respectively. Even though both the transistors are identical, I-B and I+B are not exactly equal due to internal imbalances between the two inputs. Manufacturers specify input bias current IB as the average value of the base currents entering into the terminals of an op-amp. The various electrical parameters supplied in the data sheet as follows: Input offset voltage: It is the voltage that must be applied between the input terminals of an op-amp to nollify the output. Since this voltage could be positive or negative its absolute value is listed on the data sheet. For 741C, the maximum value is 6 mV. Input offset current: The algebraic difference between the currents into the () input and (+) input is referred to as input offset current. It is 200 nA maximum for 741C. Input bias current: The average of the currents entering into the (-) input and (+) input terminals of an op-amp is called input bias current. Its value is 500 nA for 741C. Input resistance: This is the differential input resistance as seen at either of the input terminals with the other terminal connected to ground. For the 741C,the input resistance is 2 M. Input capacitance : It is the equivalent capacitance that can be measure at either of the input terminal with other terminal connected to ground. Atypical value of Ci is 1.4 pF. Offset voltage adjustment range: A special feature of the 741 family opamp is the provision of offset voltage null capability. For 741C offset voltage adjustment range is +- 15 mV. Input voltage range : This is the common-mode voltage that can be applied to both input terminals without disturbing the performance of an op-amp. For the 741 C, the range of the input common-mode voltage is +-13 V. Commonmode configuration is used only for test purpose to determine the degree of matching between the inverting and non-inverting terminals. Common-mode rejection ratio: For 741C, CMRR is typically 90 dB. CMRR is usually measured under the test condition that the input source resistance R8 <= 10 k. The higher the value of CMRR, better is the matching between the two input terminals and smaller the output common-mode voltage. Paper - II Electronic Devices and Circuits 295 Supply voltage rejection ratio: The change in an op-amp’s input offset voltage due to variations in supply voltage is called the supply voltage rejection ration (SVRR). Some manufacturers use terms like power supply rejection ratio (PSRR) or power supply sensitivity (PSS). These terms are expressed in microvolts per volt or in decibels. For 741C, SVRR = 150 V/V. Obviously, lower the value of SVRR, better the op-amp. Large Signal Voltage Gain : An op-amp amplifies the difference voltage between the two input terminals and, therefore, its voltage gain is defined as Voltage gain = output voltage / differential input voltage Sincethe amplitude of the output signal is much larger than the input signal, the voltage gain is commonly referred as large signal voltage gain. For 741C, typical value is 2,00,000 under test conditions, RL >+ 2 k and V0 = +- 10 V. Output Voltage Swing: The output voltage swing indicates the value of positive and negative saturation voltages of an op-amp, and never exceeds the supply voltage V+ and V-. For 741C, the output voltage swing is guaranteed to be between +13 V and -13Vfor RL > = 2 k. Output Resistance: Output resistance R0 is the resistance measured between the output terminal of the op-amp and the ground. It is 75 for the 741Copamp. Output Short Circuit Current: This is the current that may flow if an op-amp gets shorted accidentally and is generally high. The op-amp must be provided with short circuit protection. The short circuit current Isc for 741C is 25 mA This means that the built-in short circuit protection is guaranteed to withstand 25 mA of current. 6.9 OP - AMP Specification The manufacturers supply data sheets for the IC’s they produce. These data sheets provide information regarding pin diagram, absolute maximum ratings, electrical characteristics, equivalent circuit of the devices etc.In this section, significance of the electrical parameters supplied in a typical op-amp data sheet is discussed. The data sheet for a Fairchild A741 op-amp, 741 series are available in models 741, 741A,741C and 741E. The schematic diagram and electrical parameters for all these models are the same with only the values of the parameters differing from one model to another. We will consider specifications for 741C op-amp. 296 Electronics Engineering Technician Paper - II Electronic Devices and Circuits 297 From the data sheet it can be seen that: 1. 741 is internally frequency compensated op-amp. 2. 741 is a monolithic IC fabricated using planar epitaxial process. 3. It is useful for integrator, summer, voltage follower and other feed back applications. 4. Absolute maximum ratings are specified for supply voltage, internal power dissipation, differential input voltage, input voltage, storage and operating temperature ranges, soldering pin temperature and output short circuit duration. 5. 741 is available in all three packages viz 8-pin metal can, 10-pin flat pack and 8 or 14 pin DIP. The pin diagrams for all these packages are shown in the data sheet. 6. For 741C, two sets of electrical specifications are available, one set is applicable at room temperature (250C) and other set applies to the commercial temperature range (00 to + 700C). As we are interested only in showing the significance of the parameters listed, we limit the discussion to only one model, that is 741C at 250C. 6.10 Operational Amplifier Pin Diagram Supply current: Supply current I8 is the current drawn by the op-amp from the power supply. It is 2.8 mA for 741C. Power consumption: This gives the amount of quiescent power (Vi = 0V) that must be consumed by the op-amp so as to operate properly. It is 85 mW for 741C. Transient response: The rise time and overshoot are the two characteristics of the transient response of any circuit. These parameters are of importance whenever selecting an op-amp for ac applications. The transient response test circuit is included in the data sheet. For 741C, rise time is 0.3 (symbol)s and overshoot is 5%. Slew rate: This is another parameter of importance whenever selecting an opamp for high frequency applications. Op-amp 741C has a low slew rate (0.5 V/ s) and therefore cannot be used for high frequency applications. CMRR: Input impedence: 298 Electronics Engineering Technician 6.11 Scale Changer/Inverter In the basic inverting amplifier of 6.15, if the ratio Rf/R1 = K, where K is a real constant, then the closed loop gain ACL = -K. The circuit thus could be used to multiply by a constant factor if Rf and R1 , ACL = -1 and the circuit is called an inverter, i.e., the outpu is 1800 out of phase with respect to input through the magnitudes are same. Fig. 6.15 Scale Changer Summing Amplifier Op-amp may be used to design a circuit whose output is the sum of several input signals. Such a circuit is called a summing amplifier or a summer. An inverting summer or a non-inverting summer may be obtained as disccussed now. Invertor Summing Amplifier A typical summing amplifier with three input voltages V1, V2 and V3, three input resistors R1,R2 ,R4 and a feedback resistor Rf is shown in Fig.6.16. The following analysis is carried out assuming that the op-amp is an ideal one, that is, AOL = (symbol) and Ri = (symbol). Since the input bias current is assumed to be zero, there is no voltage drop across the resistor Rcomp and hence the noninverting input terminal is at ground potential. The voltage at node ‘a’ is zero as the non-inverting input terminal is grounded. The nodal equation by KCL at node ‘a’ is (V1 / R1 ) + (V2 / R2) + (V3 / R3) + (Vo / Rf ) = 0 Paper - II Electronic Devices and Circuits 299 Fig 6.16 Inverting Summing amplifier or Vo = - Rt Rt Rt --- V1 + --- V2 + --- V3 R2 R1 R3 Thus the output is an inverted, weighted sum of the inputs. In the special case, when R1 = R2 = R3 = Rfwe have V0 = -(V1 + V2 + V3) in which case the output V0 is the inverted sum of the input signals. we may also set R1 = R2 = R3 = 3Rf in which case Vo= -(V1+V2+V3) Thus the output is the average of the input signals (inverted). In a practical circuit, input bias current compensating resistor Rcomp should be provided. to find Rcomp make all inputs V1 = V2 = V3 = 0. So the effective input resistance R1 = R1 || R2 || R3. Therefore, Rcomp = R1|| Rf = R1 || R2 || R3 || Rf. AC Voltage Follower The circuit of a practical ac voltage follower is shown in Fig.6.17. The circuit is used as a buffer to connect a high impedance signal source to a low impedance load which may even be capacitive. The capacitor C1 and C2are chosen high so that they are short circuit at all frequencies of operation. Resistors 300 Electronics Engineering Technician R1 and R2 provide a path for dc input current into the non-inverting terminal. C2 acts as a bootstrapping capacitor and connects the resistance R1 to the output terminal for ac operation. Hence the input resistance that the source sees is approximately R1/(1 - ACL) [from Miller’s theorem] where ACLis the gain of the voltage follower which is close to unity (0.9997). Thus very high input impedance can be obtained. Fig. 6.17 AC Voltage follower 1. Voltage to Current Converter (Transconductance Amplifier) 2. Current to Voltage Converter In many applications, one may have to convert a voltage signal to a proportional output current. For this, there are two types of circuits possible. V-I Converter with floating load V-I Converter with grounded load Figure 6.18 shows a voltage to current converter in which load ZL is floating. Since voltage at node ‘a’ is vi, therefore, vi = iL R1 (I-B = 0) or iL=Vi / R1 That is the input voltage vi is converted into an output current of vi /R1. It may be see that the same current flows through the signal source and load and, therefore, signal source should be capable of providing this load current. A voltage-to-current converter with grounded loud is shown in Fig.6.18. Let vi be the voltage at node ‘a’. Writing KVL, we get Paper - II Electronic Devices and Circuits 301 i1 + i2 = iL or (vi - v1 / R) + (vo - v1 / R) = iL or vi - v1 - 2 v1 = iLR Therefore v1 = v1 + v0 - iLR / 2 Fig. 6.18 Voltage to current converter with (a) floating load, (b) Grounded load Since the op-amp is used in non-inverting mode, the gain of the circuit is 1 + R/R = 2. The output voltage is, v0 = 2 v1 = v1 + v0 - iLR that is, vi = iLR or, iL = vi /R As the input impedance of a non-inverting amplifier is very high, this circuit has the advantage of drawing very little current time from the source. A voltage to current converter is used for low votage dc and ac voltmeter, LED and zener diode tester. Current to Voltage Converter (Transresistance Amplifier) Photocell, photodiode and photovoltaic cell give an output current that is proportional to an incident radiant energy or light. The current through these devices can be converted to voltage by using a current-to-voltage converter and thereby the amount of light or radiant energy incident on the photo-device can be measured. 302 Electronics Engineering Technician Fig. 6.19 Current to Voltage converter Figure. 6.19. shows an op-amp used as I to V converter. Since the (-) input terminal is at virtual ground, no current flows through R8 and current i8 flows through the feedback resistor Rf. Thus the output voltage v0 = i8Rf. It may be pointed out that the lowest current that this circuit can measure will depend upon the bias current IB of the op-amp. This means that A741(IB = 3 nA) can be used to detect lower currents. The resistor Rf is sometimes shunted with a capacitor Cf to reduce high frequency noise and the possibility of oscillations. Finding Square Roots A divider circuit can be used to find square roots by connecting both the inputs of the multiplier tothe output of an op-amp as shown in Fig.6.20. VA = V2o / Vref and VA = -Vin So Vo2 = -Vin Vref Fig. 6.20 Finding Square roots Paper - II Electronic Devices and Circuits 303 Thus, output V0 is proportional to square root of magnitude of Vin. Note the Vinmust be negative orelse op-amp will saturate. The rangeof Vin lies between -1 and -10 V. Differentiator One of the simplest of the op-amp circuits that contain capacitor is the differetiating amplifier, or differentiator. As the same suggests, the circuit performs the mathematical operation of differentiation, that is, the output waveform is the derivative of input waveform. A differentiator circuit is shown in Fig.6.21. Analysis The node N is at virtual ground potential i.e., vN = 0. The current iC through the capacitor is, iC = C1 d/dt (vi-vN)=C1 dvi./dt The current if through the feedback amplifier is v0 / Rf and there is no current into the op-amp. Therefore, the nodal equation at node N is, C1 dvi / dt + vo/RF = 0 from which we have vo = RFC1 dvi / dt Fig. 6.21 Op-amp differentiator For a square wave input, say 1V peak and 1 KHz, the output waveform will consist of positive and negative spikes of mangitude Vsat which is approximately 13V for +- 15V op-amp power supply. During the time periods for which input is constant at +- 1V, the differentiated output will be zero. However, when input transits between +- 1V levels, gets clipped to about +- 13 V for a +- 15V op-amp power supply as shown in Fig. 6.22 304 Electronics Engineering Technician Fig. 6.23 (a) Sine-wave input and cosine output (b) Square wave input and spike output Integrator If we interchange the resistor and capacitor of the differentiator, we have the circuit of Fig. 6.23 which as we will see, is an integrator. The nodal equation at node N is, (vi / R1)+ Cf (dvo / dt) = 0 or, dvo / dt = -(1 / R1Cf ) vi Integrating on both sides, Fig. 6.23 (a) Op-amp integrator where v0 (0) is the initial output voltage. The circuit, thus provides an output voltage which is proportional to the time integral of the input and R1CF is the time constant of the integrator. It may Paper - II Electronic Devices and Circuits 305 be noted that there is a negative sign in the output voltage, and therefore, this integrator is also known as an inverting integrator. A resistance Rcomp = R1 is usually connected to the (+) input terminal to minimize the effect of input bias current. A simple low pass RC circuit can also work as an integrator when time contant is very large. This requires very large values of R and C. The components R and C cannot be made infinitely large because of practical limitations. However, in the op-amp integrator of Fig.6.24, by Miller’s theorem, the effective input capacitance becomes CF (1 - Av) where Av is the gain of the op-Amp.The gain Av is infinite for an ideal op-Amp, so the effective time constant of the op-Amp integrator becomes very large which results in perfect integration. The operation of the integrator can also be studied in the frequency domain. In phase notation, Equation can be written as Vo(8) = - 1 / sR1CF) Vi(s) In steady state, put s = jw and we get Vo(jw) = - 1 /jwR1CF) Vi(jw) So, the magnitude of the gain or integrator transfer function is The frequency response (or Bode Plot) of this basic integrator is shown n fig.6.23. The Bode Plot is a straight line of slope -6B/octave (or equavalently 20 dB/decade). The frequency fb in Fig 6.24 is the frequency at which the gain of the integrator is 0 dB and is given by fb = 1 / 2 R1CF Fig. 6.24 Frequency response of a basic and lossy integrator 306 Electronics Engineering Technician It can further be seen from that at w = 0, the magnitude of the integrator transfer function is infinite. At dc, the capacitor CF behaves as an open circuit and there is no negative feedback. The op-amp thus operates in open loop, resulting in an infinite gain. In practice, of course, output never becomes infinite, rather the output of the amplifier saturates at a voltage close to the op-Amp positive or negative power supply depending on the polarity of the input dc signal. As the gain of the integrator decreases with increasing frequency, the integrator circuit does not have any frequency problem as faced in a differentiator. However, at low frequencies suhc as at dc (=0), the gain becomes infinite (or saturates). The solution to the problem is discussed in the following. OP - AMP Comparitor The challenge sounds simple enough - take a 60 Hz (or 50 Hz) sinewave from the AC power line and convert it to a square wave. This signal will serve as a clock to drive counters for a 24 hour time clock. So you hook up an op-Amp as a comparator to do the job. But your surprised to see the clock running too fast! With oscilloscope in hand you discover the AC line is noisy! And to your horror, you see glitches (additional edges) at the comparator’s output, causing the counters to advance too quickly. What you need is a better comparator, immune to the noise swinging above and below the comparator’s threshold. Fig. OP - Amp Comparitor Paper - II Electronic Devices and Circuits 307 6.12 Block Diagram of IC 555 In industrial applications, the word timer isused more commonly. The timer is basically a small subsystem as the control element of a much larger electrical power system. Energy Source Timer Controlled Device Fig. 6.27 Block diagram of Timer System Fig.6.27 illustrates the block diagram of a timer controlling the operation of a device. A timer is basically as small unit of a control element of a system. To perform particular operation or to operate a machine for a particular time, manually it is difficult. The quality and rate of product gets affected if operated manually. Therefore the introduction of timer became must and overcome all these problems. Usually times is a small in size and often permanently connected to the machine. Classification of Timers Timers may be classified in the following ways. 1. According to the function performed by it. They are (a) Delay timers (b) Interval timers (c) Repeat cycle timer (d) Reset timer 2. According to the techique used to achieve the industrial timing (a) Thermal timers (b) Electromechnical timers (c) Electrochemical timers (d) Mechanical timers (e) Pneumatic timers (f) Electronic timers 308 Electronics Engineering Technician Applications of Timers 1. Control systems 2. Industrial use 3. Computers 4. Communication system 5. Measuring systems 6. Clock circuits 7. Motor starters etc. IC 555 TIMER (ELECTRONIC TIMER) Fig 6.28 gives the functional block diagram of 555 IC timer. The three equal resistors R1, R2 and R3 serves as internal voltage divider for the source voltage. Thus one third of source voltage VCC appears across each resistor. The voltages at point P1 and P2 serves as reference voltages for the comparators. The reference voltage for comparator 2 is + 1/3 VCC. If a trigger pulse is applied at the negative input of the comparator drops below + 1/3 VCC it causes a change in state. Comparator 1 is reference at voltage + 2/3 VCC . The output of each comparator is fed to the input termints of the flip-flop. Fig. 6.28 Block diagram of IC 555 Timer The flip-flop changes states according to the the voltage values of its input. Thus if the voltage at the threshold terminal rises above +2/3 VCC it causes comparator 1 to cause flip-flop to change its state. On the other hand, if the trigger voltage falls below +1/3 VCC it causes comparator 2 to change its state and hance cause flip-flop to change its states. Thus the output of the flip-flop is Paper - II Electronic Devices and Circuits 309 controlled by the voltages of the two comparators. A change in state occurs when he threshold voltage rises above +2/3 VCC or when the trigger voltage drops below +1/3 VCC . The output of the flip-flop is used to drive the discharge transistor and the output voltage. A high flip-flop output turns ON both the discharge transistor and the output stage.The discharge transistor becomes conductive and behaves as a low resistance short circuit to ground. The output stage behaves similarity. When the flip-flop output assumes the low state, reverse action takes place. Thus the output stage is high. Thus the output and discharge transistor state depends on the voltage applied to the threshold and the trigger input terminals. IC 555 as Monostable Multivibrator Fig.6.29 shown 555 IC connected as a monostable multivibrator which uses only one resistors R and one capacitor C. Fig. 6.29 IC 555 as Monostable Multivibrator In monostable multivibrator, a simple output pulse is generated in response to one input trigger pulse. When a negative input going place is applied at the trigger input (pin 2) this result in outut (at pin 3) to go high to VCC . The trigger pulse cause the comparator 2 to drop below its reference voltage +1/3 VCC and this in turn causes the flip-flops to go its low state. A negative voltage to the discharge transistor causes its resistance to become infinite. This in turn removes the shunt to ground for capacitor C. Hence the voltage across C begins to rise in accordance witht he time constant RC when this voltage across C exceeds + 2/3 VCC . It causes the comparator to change states and result in discharge transistor again becoming conductive. Capacitor then discharges very quickly 310 Electronics Engineering Technician to ground through pin 7. The output voltage drop to its low or ground state. Thus the output stage follows the change in the trigger level. The time duration T of the output is given by T = 1.1 RC. 6.13 IC 555 Astable Multivibrator: The astable multivibrator using IC 555 is shown in Fig.6.30. During the charging up period transistor T1 is held open by the flip-flop and the capacitor charges through the series connected resistors RA and RB. Whe the voltage across the capacitor reaches the reference level of the upper comparator 2 VCC / 3, the comparator changes the state of the flip-flop and this terms the transistor T1 ON. The capacitor discharges through resistor RB until its voltage reaches the reference level of the lower comparator VCC /3. This comparator changes the state of flip-flop again, which in turn makes the transistor T1 OFF and the cycle repeats The charging time of the capacitor is determined by T1 = C (RA + RB) loge (VCC - VCC /3) / VCC - 2 VCC/3 The above equation immediately follows from the fact that the charging of capacitor starts from VCC /3 instead of zero. Further the charging continues upto 2 VCC / 3, after which the upper comparator changes state. The equation () simplifies to T1 = C (RA + RB) loge 2 (or) T1 = 0.7 (RA + RB) C Fig. 6.30 IC 555 Astable Multivibrator Paper - II Electronic Devices and Circuits 311 6.14 Phase locked Loop (PLL) A phase locked loop (PLL) is basically a closed loop feedback system. The action of PLL is to lock or synchronise the frequency of a controlled oscillator to that of an incoming signal. Basically, PLL consists of three functional blocks. 1. Phase detector 2. Voltage controlled oscillator (VCO) 3. Low pass filter The basic loop may also contain an amplifier Block diagram of basic PLL is shown in Fig.6.31. The phase detector exhibits a multiplier characteristics. With no input signal applied to the PLL, the output from the phase detector is zero. The error voltage applied as the control signal to V.C.O is also zero and VCO operates at its free running frequency f0. Fig. 6.31 Block diagram of Basic PLL This frequency is referred to as the centre frequency. If an input signal is applied to the loop the phase detector produces an output signal which contains components as the sum and differences frequencies i.e., fs + f0 and fs - f0. If f2 is significantly different from f0 then both components do not fall into the pass band of low pass filter, and hence, are attenuated. Under such condition, the frequency of VCO is not changed and the loop does not acquire a lock. If the input signal frequency fs has values such that the frequency component fs - f0 lies within the passband of the low pass filter, then this component is amplified and applied as a control signal to the VCO. This causes the VCO frequency to vary in a direction which reduces the frequency difference between fs and f0 . If fs is sufficiently close to fo the feedback action of the loop causes the VCO frequency is identical to that of the input signal and the component fs - fo is a direct voltage of magnitude proportional to cos (symbol) where (symbol) is the phase difference between the input signal and the VCO signal. The action of 312 Electronics Engineering Technician the loop is to cause (symbol) to take on just that value which is required to generate the d.c. control voltage necessary to change the frequency of the VCO from free running value to frequency of the input signal. This action allows the PLL to ‘track’ any frequency changes of the input signal once lock has been acquired. 6.15 Functional Block Description Of PLL Type LM -565 Functional block diagram of LM - 565 PLL is shown in Fig. 6.32 and pin diagram is shown in Fig.6.32. It is self-contained, adaptable filter and demodulator for the frequency range from 0.001 Hz to 500 kHz. The circuit consists of a VCO, a phase comparator, an amplifier and a low pass filter. R2, C2 form a low pass filter. The capacitor C2 is connected externally while R2 is an internal resistor of value 3.6 k(symbol). Here the free running frequency of the VCO is determined by the values of a external resistor R, connected between pin 8 and the positive supply line and an external capacitor C, connected between pin 9 and the negative supply line. A capacitor ofvalue typically 0.001mF is normally connected between pins 7 and 8 to eliminate possible oscillation in the VCO voltage controlled current source. The square wave output signal of VCO is available at pin 4 and in order to close the loop, pin 4 must be connected externally to the phase comparator input Pin 5. The amplified loop error voltage which is applied as the control signal tothe VCO is available at pin 7. This signal is referenced to the positive supply line. A reference voltage which is normally equal to the voltage at Pin 7 in available at Pin 6 and this allows differential stages to be both biased and driven by connecting them to Pin 6 and 7. The signal inputs to the phase comparator are differential at Pin 2 and Pin 3, and the d.c. level at these two Pins must be made the same. If dual power supplies are used it is simplest to bias 2 and 3 at the potential of the common power supply line. With single supply operation they should be biased to a level in the lower half of the total power supply voltage by means of a suitable potential divider. Paper - II Electronic Devices and Circuits 313 Fig. 6.32 LM 565 PLL 6.16 Operation of VCO LM566. LM566C Voltage Controlled Oscillator General Description The LM566CN is a general purpose voltage controlled oscillatorwhich may be used to generate square and triangular waves, the frequency of which is a very linear function of a control voltage. The frequency is also a function of an external resistor and capacitor. The LM566CN is specified for operation over the 0oC to a 70oC temperature range. Features • Wide supply voltage range: 10V to 24V • Very linear modulation characteristics • High temperature stability • Excellent supply voltage rejection • 10 to 1 frequency range with fixed capacitor 314 Electronics Engineering Technician Paper - II Electronic Devices and Circuits 315 316 Electronics Engineering Technician 6.17 PLL Lock Range The range of frequencies over which a PLL can maintain lock with a input signal is called the ‘lock range’ of the system. This is always longer than the band of frequencies over which the PLL can acquire lock with an incoming signal. The lock range is decreases as higher order odd harmonics are used to achieve lock. 6.18 PLL Capture RangeWorking The range of frequencies over which a PLL can acquire lock is known as the ‘capture range.’ The greatest capture range possible is equal to the lock range but in general the capture range is less than the lock range. The capture of an input signal does not takes place as soon as the signal is applied, but takes finite time called the ‘Pull-In’ time to establish lock. 6.19 Application of PLL 1. For FM demodulation 2. For AM demodulation 3. For frequency multiplication 4. For frequency shift keying 5. In modems 6. For frequency division 7. In Telemetry receivers. 6.20 Frequency Multiplier Using PLL Fig 6.33 shows a practical circuit for frequency multiplication using PLL. Frequency multiplication can be achieved in two different ways. 1. Locking to harmonic of the input signal (or) 2. Including a counter in the loop between the VCO and the phase comparator. Paper - II Electronic Devices and Circuits 317 Fig. 6.33 Frequency multiplier using PLL Fig. 6.33 shows the second method which can provide large multiplication of frequency. To set up the circuit, the frequency limits of the input signal are determined. The f0 is adjusted by means of R1 and C1 which ensures the output frequency of the divider midway between the input frequency limits. The value of C2 is selected large enough to eliminate variations in the demodulated output voltage at Pin 7 so that the VCO frequency is established. The output can now be taken as the VCO square wave output, and its fundamental will be the desired multiple of the input frequency as long as the loop is in lock. Summary 1. An op-amp can be used to perform mathematical operations such as scale changer, addition and subtraction. 2. An instrumentation amplifier is useful for amplifying low level signals which are obtained by sensing with a transducer in the measurement of physical quantities like temperature, water flow etc. 3. Op-Amps can be used for amplifying both ac and dc inputs. A capacitively coupled amplifier is used for amplifying ac signals only. 4. The V - to - I converters are useful in low voltage dc and ac voltmeters, LED and zener diode testers. 5. The I-to -V converters are used for testing photo devices. 6. A diode in the feedback loop of an op-amp behaves as a precission diode as its cut-in voltage gets divided by the open-loop gain of op-amp. 7. A precision diode may be used for half-wave rectification, full wave rectification, peak-value detector, clipper and clamper. 8. A sample and hold circuit samples an input signal and holds on to its last 318 Electronics Engineering Technician sampled value until the input is sampled again. Thie circuit is used in analog to digital interfacing and pulse modulating system. 9. Op-Amp may be used to perform functions such as In, log, antilog, multiply or divide signals. 10. The op-Amp integrator and differentiator are useful for signal wave shaping. 11. Integrators are preferred over differentiators for analog computers as the gain of integrator decreases with increasing frequency and hence signal to noise ratio of integrator is higher than that of differentiator. 12. Monolithic audio power amplifiers with built in heat sink are available. 13. The operational transconductance amplifier (OTA) outputs a current proportional to its input voltage. OTAs are used to build programmable gain voltage amplifiers, voltage controlled resistances, neural networks etc. Short Answer Type Questions 1. Mention the types voltage regulators. 2. Mention the types of IC regulators. 3. What are the applications of Differential amplifiers. ? 4. What are the IC numbers of positive/negative regulators? 5. What is adjustable voltage regulator ? 6. Define Op-Amp input impedence,open loop gain. 7. Define Op-Amps slew rate,CMRR. 8. Define Op-Amps input offset voltage,input offset current. 9. Mention the number of pins in IC741. 10. Write applications of Op-Amp. 11. What are the applications of IC555 ?. 12. What is PLL.? 13. What are applications of LM565 ? 14. What is lock range of PLL?. 15.What is capture range of PLL ? Paper - II Electronic Devices and Circuits 319 16.What are the applications of PLL ? Long Answer Type Questions 1. Explain the operation of transister series voltage regulator. 2. Explain the operation of shunt voltage regulator. 3. Explain the operation of positive/negative voltage regulator using IC 78XX. 4. Explain the operation of adjustable voltage regulator by using LM317. 5. Draw and explian working of an operational amplifier. 6. What are the specifications of ideal Op-Amp? 7. Draw and explain working of IC 741.Write applications. 8. Draw and explain working of IC555. 9. Explain working of astable multivibrator using 555ic. 10. Draw and explain working of PLL-LM565. 11. Explain the operation of LM566. 12. Explain the frequency multifier and FM demodulater using PLL. Practical/OJT Questions 1. Study the operation of series/shunt voltage regulators. 2. Study the operation of IC regulators. 3. Study the operation of Op-Amp and its applications 4. Study the operation of differential amplifier and its applications. 5. Study the operation of 555IC. 6. Study the operation of LM 565,LM566 and its applications. 320 Electronics Engineering Technician UNIT 7 Power Electronic Devices Learning Objectives • Study of power electronics devices SCR,DIAC,TRIAC,GTOs,PUT, QUADRACS, SIDACS, SLRs, UJT, RCT, MCT, IGBT, etc,.symbols. • Study of construction and working of SCR. • Study the construction and working of DIAC and TRIAC. • Study the triggering of SCR by using UJT. • Study the operation and working of Thtrister. 7.0 Introduction Thyristers are a family of semiconductor devices that exibits bi-stable currentvoltage characteristics and can be switched between a high impedence, low current of state and a low impedance, high current on state. 7.1 The Thyrister Family Devices are (1) DIACS (2) Gate Turn - off (GTO) Thyristers (3) Programmable Unijunction Transisters (4) Quadracs Paper - II Electronic Devices and Circuits 321 (5) Sidacs (6) Silicon Controlled Rectifies SLR (7) Thyrister surge suppressors (8) Thyristors (9) TRIACS (10) Unijunction Transistors The diode acts as a switch during forward bias condition. The characteristic curve of the PNPN diode is shown in the figure. 7.2 Thyristor Family Devices: The impartent multilayer devices of the Thyristor family as follows, 322 Electronics Engineering Technician 7.3 SCR ISI Specification (Silicon Controlled Rectifier) The basic structure and circuit symbol of SCR is shown in fig. It is a four layer three terminal device in which the end P-layer acts as anode, the end Nlayer acts as cathode and P-layer nearer to cathode acts as gate. As leakage current in silicon is very small compared to Germanium, SCRs are made of Silicon and not Germanium. Fig. 7.2 Basic Structure and Circuit Symbol of SCR Constructional Details of SCR The internal equivalent circuit of SCR is two transistors PNP,NPN are arranged back to back,the feedback occures because collector of Q1 drives the base of Q2 and vice-versa.Assuming normal transistor common emitter phas reversal and producing from Q2 to Q1 we can observe that positive going voltage at the base of Q2 is inverted as its collector.This is common with Q1 s base.Transistor Q1 also inverts ,and signal at its collector is then in phase with Paper - II Electronic Devices and Circuits 323 the original input voltage.Thae overall current gain of the two transistor arrangement is equal to 1*2.The total anode to cathode current is given by Ia=Ic1+Ic2 consider the gate voltage,if Vgk is zero or negative the NPN trasistor Q2 is biased off and therefore Q1 is never turned on.Thus Ia equals the sum of the leackage currents of Q1 and Q2.Tresistance from anode to cathode is very high and the anod to cathode voltage drop is high.Therefore the switch is open.If Vgk is sufficiently positive,Q2 is forward biased.Therfore Ic2 increases and Q1 is tured ON. Thus the becomes regenerative. Both transistors saturate.This reduses the forward resistance and voltage drop drastically,so the switch is closed .Once the SCR is on ,there is no need for gate current.This is because Ic1 is sufficient to maintain Ib2 and keep Q2 ON. The only way to turn off the SCR is reduce Ia below some minimum holding current.This sometimes disadvantage with SCR. Due to this ON-OFF action SCR can be triggered from the open or blocking state to the closed or low resistance high conducting stage. 7.4 Volt-Ampere, Charactrtistics of SCR This is a graph between Ia anode current versus V ak voltage for different gate currents.When the anode voltage is negative the SCR works as two revrese biased PN junctions.A small leackage current flows .When VAK exceeds the reverse break down voltage Fig.7.3 SCR VI Characteristics 324 Electronics Engineering Technician The characteristics of SCR are shown in Fig. SCR acts as a switch when it is forward biased. When the gate is kept open, i.e. gate current IG = 0, Operation of SCR is similar to PNPN diode. When IG < 0, the amount of reverse bais applied to J2 is increased. So the breakover voltage VBO is increased. When IG >0, the amount of reverse bias applied to J2 is decreased thereby decreasing the breakover voltage. With very learge positive gate current breakover may occur at a very low voltage such that the characteristics of SCR is similar to that of ordinary PN diode. As the voltage at which SCR is switched ‘ON’ can be controlled by varying the gate current IG . It is commonly called as controlled switch. Once SCR is turned ON, the gate loses control i.e. the gate cannot be used to switch the device OFF. One way to turn the device OFF is by lowering the anode current below the holding current IH by reducing the supply voltage below holding voltage VH, keeping the gate open. SCR is used in relay control, motor control, phase cotrol,heater control, battery chargers inverters, regulated power supplies and as static switches. Two transitor version of SCR The operation of SCR can be explained in a very simple way by considering it in terms of two transistors, called as the two transistor version of SCR. As shown in fig 7.4., an SCR can be split in to two parts and displayed machanically from one another but connected electrically. Thus the device may be considered to be constituted by two transistors. T1 (PNP) and T2 (NPN) connected back to back. Ib1=IA = Ie1 = IA - 1IA =(I - 1) IA (7.01) Also, from the Fig 7.03, it is clear that Ib1=Ic2 (7.02) and Ic2=2Ik (7.03) Substituting the values given in Eqs (7.02) and (7.03) in Eq. (7.01) we get (1-1) IA = 2IK Paper - II Electronic Devices and Circuits 325 Fig. 7.4 Two Transistor Version of SCR We know that IK - IA + 1g. (7.03) Substituting Eq. (7.03) in Eq (7.02) we obtain (7.04) Equation(7.04) indicates that (1+ 2) = I then IA = infinity, i.e. the anode current IA suddenly reaches a very high value approaching infinity. Therefore, the device suddenly triggers into ON state from the original OFF state. This characteristics of the device is known as its regenerative action. The value of (1+ 2) can be made almost equal to unity by giving a proper value of positive current Ig for a short duration. This signal Ig applied at the gate which is the base of T2 will cause a flow of collector current IC2 by transferring T2 to its ON state. As IC2 = Ib1, the transistor T1 will also be switched ON. Now the action is regenerative since each of the transistors would supply base current to the other. At this point even if the gate signal is removed, the device keeps on conducting, till the current level is maintained to a minimum value of holding current. 7.5 SCR Ratings The following are the ratings of SCR 1. Current at break over point 326 Electronics Engineering Technician 2. Off state fotward current 3. Holding current 4. Gate Current 5. Latching current 6. Reverse Current 7. On State current 8. On state forward average current 9. On state forward RMS current 10. Repetitive peak forward current 11. Maximum surge forward current 12. Power Dissipation 13. Gate power dissipation 14. Blocking resistance 15. Dynamic forward resistance 16. Thermal resistance 17. Delay time 18. Junction recovery time 19. Turn on time 20. Turn off time 21. Raised Time 22. Recovery time 23. Forward blocking RMS voltage 24. Forward brake over voltage 25. Reverse breakover voltage 26. Repetative Peak off straight forward voltage 27. Non repitative off stage forward voltage 28. Gate voltage Paper - II Electronic Devices and Circuits 29. Gate voltage 30. Repitative peak reverse voltage 31. On State forward voltage drop. 7.6 Constructional Details of DIAC and TRIAC. 327 328 Electronics Engineering Technician Paper - II Electronic Devices and Circuits 329 330 Electronics Engineering Technician Paper - II Electronic Devices and Circuits 331 332 Electronics Engineering Technician Fig. 7.6 Paper - II Electronic Devices and Circuits 333 7.7 DIAC, TRIAC Volt-Ampere Characteristics Forward and Reverse BIAS Construction – Working – Characteristics – Diac as bi-directional switch. (for learning activity )DIAC symbol: Fig. 7.5(a) Diac Symbol Fig. 7.5(b) Diac VI Characteristics Construction: The DIAC is basically a two terminal parellel-inverse combination of semiconductor layers that permits triggering in either direction. The basic arrangement of the semiconductor layers of the diac is shown in the figure, along with its graphical symbol. Nore that either terminal is referred as the cathode. Instead, there is an anode 1 and an anode 2. When the anode 1 is positive with respect to anode 2, the semiconductor Operation: DIAC circuits use the fact that a DIAC only conducts current only after a certain breakdown voltage has been exceeded. The actual breakdown voltage will depend upon the specification for the particular component type. When the diac break down voltage occurs, the resistance of the component decreases abruptly and this leads to a sharp decrease in the voltage drop across the diac, and a corresponding increase in current. The DIAC will remain in its conducing state until the current flow through it drops below a particular value known as the holding current. When the current falls below the holding current, the DIAC switches back to its high resistance, or non-conducting state. 334 Electronics Engineering Technician DIAC are widely used in AC applications and it is found that the device is “reset” to its non-conducting state, each time the voltage on the cycle falls so that the current falls below the holding current. As the behaviour of the device is approximately equal in both directions, it can provide a method of providing equal switching for both halves of an AC cycle, e.g for triacs. Most DIAC s have a breakdown voltage of around 30 volts, although the exact specifications will depend upon the particular type of device.. Interestingly their behaviour is somewhat similar to that of a neon lamp, although they offer a far more precise switch on voltage and thereby provide a far better degree of switching equalisation. TRIAC Symbol Fig. 7.6 TRIAC Symbol The structure of a TRIAC may be considered as a p-n-p-n structure and the triac may be considered to consist of two conventional SCRs fabricated in an inverse parallel configuration. In operation, when terminal A2 is positive with respect to A1, then a positive gate voltage will give rise to a current that will trigger the part of the triac consisting of P1 N1 P2 N2 and it will have an identical characteristic to an SCR. When terminal A2 is negative with respect to A1 a negative current will trigger the part of the triac consisting of P2 N1 P1 N3. In this way conduction on the TRIAC occurs over both halves an alternating cycle. TRIAC structure Triacs do not fire symmetrically as a result of slight differences between the two halves of the device. This results in harmonics being generated, and the less symmetrical the triac fires, the greater the level of harmonics produced. It is generally undesirable to have high levels of harmonics in a power system and as a result TRIACS are not favoured for high power systems. Instead two thyristors may be used as it is easier to control their firing. Paper - II Electronic Devices and Circuits 335 To help in overcoming this problem, a device known as a diac (diode AC switch) is often placed in series with the gate. This device helps make the switching more even for both halves of the cycle. Fig. 7.7 Structural Symbol of TRIAC This results from the fact that the diac switching characteristic is far more even than that of the TRIAC. Since the diac prevents any gate current flowing until the trigger voltage has reached a certain voltage in either direction, this makes the firing point of the TRIAC more even in both directions. Fig. 7.8 V.I Characterstics of TRIAC 336 Electronics Engineering Technician 7.8 Different Modes of TRIAC Triggering TRIAC Triggering modes are as follows a. First quadrant operation. b. Second quadrant operation c. Third quadrant operation d. Fourth quadrant operation 7.9 SCR Circuit Triggered by UJT One common application of the Uni Juntion Transistor is the triggering of the other devices such as the SCR, TRIAC etc. The basic elements of such a triggering circuit are shown in figure. The resistor RE is chosen so that the load line determined by RE passes through the device characteristic in the negative resistance region, that is, to the right of the peak point but to the left of the valley point, as shown in figure. If the load line does not pass to the right of the peak point P, the device cannot turn on. For ensuring turn-on of UJT RE < VBB – VP­ / IP V BB Motor lamp heater or some other device UJT Triggering of an SCR Peak Point P R1 (Load Line) Negative Resistance Region Valley Point Fig. 7.9 How to trigger TRIAC by using UJT Paper - II Electronic Devices and Circuits 337 This can be established as below Consider the peak point at which IRE = Ip and VE = VP (the equality IRE = IP is valid because the charging current of capacitor, at this instant is zero, that is, the capacitor, at this particular instant, is changing from a charging state to a discharging state). Then VE = VBB – IRE RE So, RE(MAX) = VBB – VE­ / IRE = VBB – Vp­ / IP at the peak point. At the valley point, V IE = IV and VE = VV so that VE = VBB – IRE RE So RE(MIN) = VBB – VE / IRE = VBB – VV / IV or for ensuring turn-off. RE > = VBB – VV / IV So, the range of resistor RE is given as VBB – VP / IP >RE > VBB – VV / IV The resistor R is chosen small enough so as to ensure that SCR is not turned on by voltage VR when emitter terminal E is open or IE = 0 The voltage VR = RVBB/R + RBB for open-emitter terminal. The capacitor C determines the time interval between triggering pulses and the time duration of each pulse. By varying RE, we can change the time constant RE C and alter the point at which the UJT fires. This allows us to control the conduction angle of the SCR, which means the control of load current. 7.10 Power Control Circuits of DIAC, TRIAC and SCR Power electronic converters can be found wherever there is a need to modify a form of electrical energy (i.e. change its voltage, current or frequency). The power range of these converters is from some milli Watts (as in a mobile phone) to hundreds of megawatts (e.g. in a HVDC transmission system). With “classical” electronics, electrical currents and voltage are used to carry information, whereas with power electronics, they carry power. Thus, the main metric of power electronics becomes the efficiency. The first very high power electronic devices were mercury-arc valves. In modern systems the conversion is performed with semiconductor switching devices such as diodes, thyristors and transistors, as pioneered by R. D. 338 Electronics Engineering Technician Middlebrook and others beginning in the 1950s. In contrast to electronic systems concerned with transmission and processing of signals and data, in power electronics substantial amounts of electrical energy are processed. An AC/DC converter (rectifier) is the most typical power electronics device found in many consumer electronic devices, e.g. television sets, personal computers, battery chargers, etc. The power range is typically from tens of watts to several hundred watts. In industry the most common application is the Variable Speed Drive (VSD) that is used to control an induction motor. The power range of VSDs start from a few hundred Watts and end at tens of mega Watts. The power conversion systems can be classified according to the type of the input and output power · AC to DC (rectifier) · DC to AC (inverter) · DC to DC (DC-to-DC converter) · AC to AC (AC-to-AC converter) Switching As efficiency is at a premium in a power electronic converter, the losses that a power electronic device generates should be as low as possible. The instantaneous dissipated power of a device is equal to the product of the voltage across the device and the current through it P= V x I. The losses of a power device are at a minimum when the voltage across it is zero (the device is on) or when no current flows through it (off). Power electronic converters are built around one (or more) device operating in switching mode. Practical devices have non-zero voltage drop and dissipate power when on, and take some time to pass through an active region until they reach the “on” or “off” state. These losses are a significant part of the total lost power in a converter. Devices The capabilities and economy of power electronics system are determined by the active devices that are available. Their characteristics and limitations are a key element in the design of power electronics systems. Formerly, the mercury arc valve, the high-vacuum and gas-filled diode thermionic rectifiers, and triggered devices such as the thyratron and ignitron were widely used in power electronics. As the ratings of solid-state devices improved in both voltage and current- Paper - II Electronic Devices and Circuits 339 handling capacity, vacuum devices have been nearly entirely replaced by solidstate equivalents, or by solid state devices that have no thermionic equivalent. Power electronic devices may be used as switches, or as amplifiers. [1] An ideal switch is either open or closed and so dissipates no power; it withstands an applied voltage and passes no current, or passes any amount of current with no voltage drop. Semiconductor devices used as switches can approximate this ideal property and so most power electronic applications rely on switching devices on and off, which makes systems very efficient as no power is wasted in the switching devices. By contrast, in the case of the amplifier, the current through the device varies continuously according to a controlled input. The voltage and current at the device terminals follow a load line, and the power dissipation inside the device is large compared with the power delivered to the load. Several attributes dictate how devices are used. Devices such as diodes conduct when a forward voltage is applied and have no external control of the start of conduction. Power devices such as silicon controlled rectifiers and thyristors (as well as the former mercury valve and thyratron) allow control of the start of conduction, but rely on periodice reversal of current flow to turn them off. Devices such as gate turn-off Thyristors, bipolar junction transistors. (BJT), and MOSFET transistors provide full switching control and can be turned on or off without regard to the current flow through them. Transistor devices also allow proportional amplificaton, but this is rarely used for systems rated more than a few hundred Watts. The control input characteristics of a device also greatly affect design; sometimes the control input is at a very high voltage with respect to ground and must be driven by an isolated source. Devices vary in switching speed. Some diodes and Thyristors are suited for relatively slow speed and are useful for power freqauency switching and control; certain thyristors are useful at a few KHz. Devices such as MOSFETS and BJTs can switch at tens of KHz up to a few megahertz in power applications, but with decreasing power levels. Very high power (hundreds of KW) at very high frequency (hundreds or thousands of megahertz) is still the area where vacuum tube devices predominate. The use of faster switching devices minimizes energy lost in the transitions from on to off and back, but may create problems with radiated electtromagnetic interference. Gate drive (or equivalent) circuits must be designed to supply sufficient drive current to achieve the full switching speed possible with a device. A device that doesn’t get sufficient drive to switch rapidly, may be destroyed by excess heating. Power handling and dissipation of devices is also a critical factor in design. Power electtronic devices may have to dissipate tens or hundreds of watts of waste heat, even switching as efficiently as possible between conducting and 340 Electronics Engineering Technician non-conducting states. In the switching mode, the power controlled is much larger than the power dissipated in the switch. The forward voltage drop in the conducting state translates into heat that must be dissipated. High power semiconductors require speicalized heat sinks or active cooling systems to keep their junction temperature from rising too high; exotic semicoductors such as Silicon carbide have an advantage over straight silicon in this respect, and Germanium, once the main-stay of solid-state electronics is now little used due to its unfavorable properties at high temperature. Semiconductor devices exist with ratings up to a few KV in a single device. Where very high voltage must be controlled, multiple devices must be used in series, with networks to equalize voltage across all devices. Again, switching speed is a critical factor since the slowest-switchind device will have to withstand a disproportionate share of the overall voltage. The former mercury valves were available with ratings to 100 KV in a single unit, simplifying their application in HVDC systems. The current rating of a semiconductor device is limited by the heat generated within the dies and the heat developed in the resistance of the interconnecting leads. Semiconductor devices must be designed so that current is evenly distributed within the device across its internal junctions (or channels); once a “hot spot” develops, breakdown effects can rapidly destroy the device. Certain SCRs are available with current ratings to 3000 Amperes in a single unit. Applications Power electronic systems are found in virtually every electronic device. For example: DC/DC converters are used in most mobile devices (mobile phones, PDA etc.) to maintain the voltage at a fixed value whatever the voltage level of the battery is. These converters are also used for electronic isolation and power factor correction. AC/DC converters (rectifiers) are used every time an electronic device is connected to the mains (computer, television etc.). These may simply change AC to DC or can also change the voltage level as part of their operation. AC/AC converters are used to change either the voltage level or the frequency (international power adapters, light dimmer). In power distribution networks AC/AC converters may be used to exchange power between utility frequency 50 Hz and 60 Hz power grids. Paper - II Electronic Devices and Circuits 341 DC/AC converters (inverters) are used primarily in UPS or renewable energy systems or emergency lighting systems. When mains power is available, it will charge the DC battery. If the mains fails, an inverter will be used to produce AC electricity at mains voltage from the DC battery. 7.11 Working of Reverse Conducting Thyristor Reverse conducting Thyristor A reverse conducting thyristor (RCT) has an integrated reverse diode, so is not capable of reverse blocking. These devices are advantageous where a reverse or freewheel diode must be used. Because the SCR and diode never conduct at the same time they do not produce heat simultaneously and can easily be integrated and cooled together. Reverse conducting Thyristors are often used in frequency changers and inverters. 7.11(a) Thristor (RCT) The thyristor is a four-layered, three terminal semiconducting device, with each layer consisting of alternately N-type or P-type material, for example PN-P-N. The main terminals, labelled anode and cathode, are across the full four layers, and the control terminal, called the gate, is attached to P-type material near to the cathode. (A variant called an SCS - Silicon Controlled Switch brings all four layers out to terminals.) The operation of a thyristor can be understood in terms of a pair of tightly coupled bipolar junction transistors, arranged to cause the self-latching action: Structure on the physical and electronic level, and the Thyristor symbol. Thyristors have three states: 1. Reverse blocking mode - Voltage is applied in the direction that would be blocked by a diode 2. Forward blocking mode - Voltage is applied in the direction that would cause a diode to conduct, but the Thyristor has not yet been triggered into conduction 3. Forward conducting mode - The Thyristor has been triggered into conduction and will remain conducting until the forward current drops below a threshold value known as the “holding current” 7.11(b)Function of the gate terminal The thyristor has three P-N junctions (serially named J1, J2, J3 from the anode). 342 Electronics Engineering Technician When the anode is at a positive potential VAK with respect to the cathode with no voltage applied at the gate, junctions J1 and J3 are forward biased, while junction J2 is reverse biased. As J2 is reverse biased, no conduction takes place (Off state). Now if VAK is increased beyond the breakdown voltage VBO of the thyristor, avalanche breakdown of J2 takes place and the thyristor starts conducting (On state). I f a positive potential VG is applied at the gate terminal with respect to the cathode, the breakdown of the junction J2 occurs at a lower value of VAK. By selecting an appropriate value of VG, the thyristor can be switched into the on state suddenly. Once avalanche breakdown has occurred, the thyristor continues to conduct, irrespective of the gate voltage, until: (a) the potential VAK is removed or (b) the current through the device (anode”cathode) is less than the holding current specified by the manufacturer. Hence VG can be a voltage pulse, such as the voltage output from a UJT relaxation oscillator. These gate pulses are characterized in terms of gate trigger voltage (VGT) and gate trigger current (IGT). Gate trigger current varies inversely with gate pulse width in such a way that it is evident that there is a minimum gate charge required to trigger the thyristor. Switching characteristics V - I Characteristics. In a conventional thyristor, once it has been switched on by the gate terminal, the device remains latched in the on-state (i.e. does not need a continuous supply of gate current to conduct), providing the anode current has exceeded the latching current (IL). As long as the anode remains positively biased, it cannot be switched off until the anode current falls below the holding current (IH). A thyristor can be switched off if the external circuit causes the anode to become negatively biased. In some applications this is done by switching a second thyristor to discharge a capacitor into the cathode of the first thyristor. This method is called forced commutation. After a thyristor has been switched off by forced commutation, a finite time delay must have elapsed before the anode can again be positively biased and retain the thyristor in the off-state. This minimum delay is called the circuit commutated turn off time (tQ). Attempting to positively bias the anode within this time causes the Thyristor to be self-triggered by the remaining charge carriers (holes and electrons) that have not yet recombined. Paper - II Electronic Devices and Circuits 343 For applications with frequencies higher than the domestic AC mains supply (e.g. 50 Hz or 60 Hz), thyristors with lower values of tQ are required. Such fast thyristors are made by diffusing into the silicon heavy metals ions such as gold or platinum which act as charge combination centres. Alternatively, fast thyristors may be made by neutron irradiation of the silicon. 7.11 (c) Insulated Gate BipoalarTransistor (IGBT) Widely used in any accession where need amplify and drive at grids gate, realize the safety electrical isolation between the power semiconductor device and control circuit by using opto-coupler. The switching frequency high up to 20K Hz, with short protection and output fault, output soft off when over-current, timing and reset function, etc. This series include QP series ( with isolated power) and hot sale QC series. • QP series is built with isolated power, with high reliability and layout and very easy to use. • QC series can cross lots of competitors’ item. • QA series is the assistant driving power of QC series Application: Inverter, uninterrupted power supply (UPS,), servo drive, welding machine and other occasion with high power IGBT. 344 Electronics Engineering Technician Paper - II Electronic Devices and Circuits 345 346 Electronics Engineering Technician 7.11 (d) Bipolar Junction Transistor (BJT) BJT has three terminals: Fig. 7.10 Signal Level Transistor Structure Power transistor of npn types are easy to manufacture and cheaper. Used in high-voltage and high current application. Working The base-emitter diode (forward) acts as a switch Base Collector Base Terminal Terminal Current Emitter Terminal Fig. 7.11 When v1>0.7 it lets the electrons flow toward collector, so we can control our output current (Ic) with the input current (Ib) by using transistors. Vertical Cross Section Paper - II Electronic Devices and Circuits 347 Fig. 7.12 Steady State Characteristics of Signal Level BJT IB versus VBE input characteristics Fig. 7.13 IC versus VCE output characteristics Fig. 7.14 Steady State Characteristics of Power Transistor In transistor, base current is effectively the input current and collector current is output current. 348 Electronics Engineering Technician Fig. 7.15 Output Characteristics Curve I ? IB=0 Curve II? IB?0 Initial part of the curve II, characterized by VCE ? called saturation region. In this region transistor acts like switch. Flat part of the curve ?with increasing VCE, almost IC is constant ? called active region. In this region transistor acts as amplifier. Almost vertically rising curve is the breakdown region, which must be avoided at all cost. The load line IC=(VCC-VCE)/RC. The line joining A and B. When transistor is ON, VCE=0, the IC=VCC/RC. This collector current is shown by point A When transistor is OFF, or in cut-off, VCC appears across collector-emitter and there is no collector current. This value is indicated by point B. Transfer Characteristics Forward current gain a=IC/IE The ratio of collector current (O/P) IC and base current IB (I/P) called current gain. Paper - II Electronic Devices and Circuits Fig. 7.16 Working Transistors work in 3 regions Active: Always on —IC=BIb Saturation :Ic=Isaturation On as a switch Off :Ic=0 Off as a switch Transistors have three terminals: Fig. 7.17 349 350 Electronics Engineering Technician Transistors can be used as switches. Transistors can either conduct or not conduct current. Fig. 7.18 Transistor Switching Example When VBE is less than 0.7V the transistor is off and the lamp does not light. When VBE is greater than 0.7V the transistor is on and the lamp lights. Transistor operation as switch means that transistor operates either in saturation region or in cut-off region and nowhere else on the load line. As an ideal switch operate at A. At point B in cut-off state as an open switch. Fig. 7.19 Large base current will cause the transistor work in saturation region at point A’ with small saturation voltage VCES. When the control or base is reduce to 0, the transistor is turn-off and its operation is shift to B’ in the cut-off region. A small leakage current ICEO flow in the collector circuit when the transistor is off. Paper - II Electronic Devices and Circuits 351 When the control or base is reduce to 0, the transistor is turn-off and its operation is shift to B’ in the cut-off region. A small leakage current ICEO flow in the collector circuit when the transistor is off. If VCE(S) is the collector –emitter saturation voltage, then the collector current ICS is:The ratio of ICS to IB is called forced current gain and less than ß. the time during which collector current rises from 0.1 Ics to 0.9Ics. This shows the total turn on time ton=td+tr. Safe-Operating Area The safe operating area of a power transistor specifies the safe-operating limits of collector current versus collector emitter voltage. For reliable operation of the power transistor, the collector current and voltage must always lie within this area. Two types of safe-operating areas are specified by manufacturer: FBSOA (Forward-biased safe-operating area) RBSOA (Reverse-biased safe-operating area) FBSOA ? belongs to the transistor operation when base-emitter junction is forward biased to turn-on the transistor. Forward Biased Safe Operating Area (FBSOA) DC AS WELL AS SINGLE PULSE OPERATION FBSOA INCREASES ?PULSE-WIDTH DECRESASES Reverse biased safe operating area (RBSOA) Reverse biased safe operating area (RBSOA) 352 Electronics Engineering Technician Fig. 7.20 During turn-off, a transistor is subjected to high-current and high voltage with BJT reverse biased. Safe-operating area of transistor during turn-off is specified as RBSOA. RBSOA specifies the limit of transistor operation at turn-off when the base current is zero or when the base emitter junction is reversed biased (with –ve base current). With increasing reverse bias, area RBSOA decreases in size. Advantages of BJTs • Have high switching frequencies. • Turn-on losses are small. • Controlled turn-on & turn-off characteristics. • No commutation circuit required. Disadvantages of BJTs • Drive circuit is complex. • Has the problem of charge storage. • Has the problem of second breakdown. • Cannot be used in parallel • Problems of negative temperature coefficients. Paper - II Electronic Devices and Circuits 353 7.12 Manufacturer data Sheet of Power Electronic Devices 354 Electronics Engineering Technician Paper - II Electronic Devices and Circuits 355 356 Electronics Engineering Technician Paper - II Electronic Devices and Circuits 357 7.13 Applications of all Power Electronic Devices Introduction Power electronic converters can be found wherever there is a need to modify a form of electrical energy (i.e. change its voltage, current or frequency). The power range of these converters is from some milliwatts (as in a mobile phone) to hundreds of megawatts (e.g. in a HVDC transmission system). With “classical” electronics, electrical currents and voltage are used to carry information, whereas with power electronics, they carry power. Thus, the main metric of power electronics becomes the efficiency. The first very high power electronic devices were mercury-arc valves. In modern systems the conversion is performed with semiconductor switching devices such as diodes, thyristors and transistors, as pioneered by R. D. Middlebrook and others beginning in the 1950s. In contrast to electronic systems concerned with transmission and processing of signals and data, in power electronics substantial amounts of electrical energy are processed. An AC/DC converter (rectifier) is the most typical power electronics device found in many consumer electronic devices, e.g. television sets, personal computers, battery chargers, etc. The power range is typically from tens of watts to several hundred watts. In industry the most common application is the variable speed drive (VSD) that is used to control an induction motor. The power range of VSDs start from a few hundred watts and end at tens of megawatts. The power conversion systems can be classified according to the type of the input and output power · AC to DC (rectifier) · DC to AC (inverter) · DC to DC (DC-to-DC converter) · AC to AC (AC-to-AC converter) Switching As efficiency is at a premium in a power electronic converter, the losses that a power electronic device generates should be as low as possible. The instantaneous dissipated power of a device is equal to the product of the voltage across the device and the current through it. The losses of a power device are at a minimum when the voltage across it is zero (the device is on) or when no current flows through it (off). Power electronic converters are built around one (or more) device operating in switching mode. 358 Electronics Engineering Technician Practical devices have non-zero voltage drop and dissipate power when on, and take some time to pass through an active region until they reach the “on” or “off” state. These losses are a significant part of the total lost power in a converter. Devices The capabilities and economy of power electronics system are determined by the active devices that are available. Their characteristics and limitations are a key element in the design of power electronics systems. Formerly, the mercury arc valve, the high-vacuum and gas-filled diode thermionic rectifiers, and triggered devices such as the thyratron and ignitron were widely used in power electronics. As the ratings of solid-state devices improved in both voltage and currenthandling capacity, vacuum devices have been nearly entirely replaced by solidstate equivalents, or by solid state devices that have no thermionic equivalent. Power electronic devices may be used as switches, or as amplifiers. [1] An ideal switch is either open or closed and so dissipates no power; it withstands an applied voltage and passes no current, or passes any amount of current with no voltage drop. Semiconductor devices used as switches can approximate this ideal property and so most power electronic applications rely on switching devices on and off, which makes systems very efficient as no power is wasted in the switching devices. By contrast, in the case of the amplifier, the current through the device varies continuously according to a controlled input. The voltage and current at the device terminals follow a load line, and the power dissipation inside the device is large compared with the power delivered to the load. Several attributes dictate how devices are used. Devices such as diodes conduct when a forward voltage is applied and have no external control of the start of conduction. Power devices such as silicon controlled rectifiers and thyristors (as well as the former mercury valve and thyratron) allow control of the start of conduction, but rely on periodice reversal of current flow to turn them off. Devices such as gate turn-off thyristors, bipolar junction transistors (BJT), and MOSFET transistors provide full switching control and can be turned on or off without regard to the current flow through them. Transistor devices also allow proportional amplificaton, but this is rarely used for systems rated more than a few hundred watts. The control input characteristics of a device also greatly affect design; sometimes the control input is at a very high voltage with respect to ground and must be driven by an isolated source. Devices vary in switching speed. Some diodes and thyristors are suited for relatively slow speed and are useful for power freqauency switching and control; certain thyristors are useful at a few KHz. Devices such as MOSFETS and Paper - II Electronic Devices and Circuits 359 BJTs can switch at tens of KHz up to a few MHz in power applications, but with decreasing power levels. Very high power (hundreds of kilowatts) at very high frequency (hundreds or thousands of MHz) is still the area where vacuum tube devices predominate. The use of faster switching devices minimizes energy lost in the transitions from on to off and back, but may create problems with radiated electtromagnetic interference. Gate drive (or equivalent) circuits must be designed to supply sufficient drive current to achieve the full switching speed possible with a device. A device that doesn’t get sufficient drive to switch rapidly, may be destroyed by excess heating. Power handling and dissipation of devices is also a critical factor in design. Power electtronic devices may have to dissipate tens or hundreds of watts of waste heat, even switching as efficiently as possible between conducting and non-conducting states. In the switching mode, the power controlled is much larger than the power dissipated in the switch. The forward voltage drop in the conducting state translates into heat that must be dissipated. High power semiconductors require speicalized heat sinks or active cooling systems to keep their junction temperature from rising too high; exotic semicoductors such as silicon carbide have an advantage over straight silicon in this respect, and Germanium, once the main-stay of solid-state electronics is now little used due to its unfavorable properties at high temperature. Semiconductor devices exist with ratings up to a few kilovolts in a single device. Where very high voltage must be controlled, multiple devices must be used in series, with networks to equalize voltage across all devices. Again, switching speed is a critical factor since the slowest-switchind device will have to withstand a disproportionate share of the overall voltage. The former mercury valves were available with ratings to 100 KV in a single unit, simplifying their application in HVDC systems. The current rating of a semiconductor device is limited by the heat generated within the dies and the heat developed in the resistance of the inter connecting leads. Semiconductor devices must be designed so that current is evenly distributed within the device across its internal junctions (or channels); once a “hot spot” develops, breakdown effects can rapidly destroy the device. Certain SCRs are available with current ratings to 3000 Amperes in a single unit. Applications Power electronic systems are found in virtually every electronic device. For example: · DC/DC converters are used in most mobile devices (mobile phones, PDA etc.) to maintain the voltage at a fixed value whatever the voltage level of 360 Electronics Engineering Technician the battery is. These converters are also used for electronic isolation and power factor correction. · AC/DC converters (rectifiers) are used every time an electronic device is connected to the mains (computer, television etc.). These may simply change AC to DC or can also change the voltage level as part of their operation. · AC/AC converters are used to change either the voltage level or the frequency (international power adapters, light dimmer). In power distribution networks AC/AC converters may be used to exchange power between utility frequency 50 Hz and 60 Hz power grids. · DC/AC converters (inverters) are used primarily in UPS or renewable energy systems or emergency lighting systems. When mains power is available, it will charge the DC battery. If the mains fails, an inverter will be used to produce AC electricity at mains voltage from the DC battery. 7.14 Power Control Schematic Power supplies and control schematics · +9V *and* -9V from one battery · 0-14 volt, 0-2 amp current limited variable power supply regulator · 12 Vdc - 120 Vac Inverter Schematic · 12 volt battery monitor · 12 Volt Gel Cell Charger · 12 volt power supply · 12 Volt Switching Power Supply circuit diagram and PCB layout · 12V 30A power supply · 12V Lead-Acid Battery Monitor using LM3914 · 12V to 120V Inverter · 12V, 4-AA Cell Differential Temperature Charger · 13.8V 30-40A Power Supply (PDF) · 1A Variable Regulated Power Supply · 200 Watt Modified PC Power Supply 13.5 Volt 14 Amp · 3.3V / 5V Regulated Power Supply Circuit Paper - II Electronic Devices and Circuits 361 · 3rd harmonic distortion meter for measuring the quality of AC supply · 5 volt power supply · 500W low cost 12V to 220V inverter · 6V to 12V Converter · 6V to 12V Converter · AC Power Meter · Active Power Zener · Adjustable power supply using LM317 · Adjustable Voltage Regulator using a 7805 or other fixed linear voltage regulator instead on LM317 · Advanced High Voltage PSU circuit · Alkaline battery charger · Alternative power source for Magellan GPS receivers · Amplified zener regulator · Assorted power source and control circuits · Automatic 12V Lead Acid Battery Charger · Automatic 9V NiCad battery charger · Back And Forth - Bidirectional Bipolar Stepper Motor Driver · Basic 78xx series regulator mains power supply circuit diagram · Basic Power Supply · Basic Solid State Relays · Basic UPS Power Supply · Battery Characterizer · Battery Charger Ideas · Battery Charger, Current and Voltage Regulated for Sealed Lead Acid batteries · Battery Low Voltage Beeper · Battery Low Voltage Beeper 362 Electronics Engineering Technician · Battery voltage monitor · Bench power supply that allows a number of varying output voltages to be preset. Includes PCB layout · Breadboard supply - very low dropout adjustable power supply · Build A 10 Amp 13.8 Volt Power Supply · Build a breadboard power module for integrated circuits · Build A High Performance Voltage Regulator From Discrete Components · Build A Simple Rechargeable CMOS Battery · Car Ignition Coil Driver from 110V AC · Car Ignition Coil Driver from 12V DC - Can be used as an electric fence · Charge Monitor for 12V lead acid battery · Charger for gel lead acid batteries · Cockcroft-Walton voltage multipliers (PDF) · Compressor-mate power protection for refrigerators, freezers and air conditioners · Controller for hybrid (photovoltaic- wind turbine and diesel engine) power plant · Current booster for 78nn series voltage regulators · DC to AC inverter using a 555 timer · DC Voltage and Current Source · Digital bench power supply based on a PIC16F870 · Dual (postive and negative) 12V power supply · Dual Polarity Power Supply · Dual Polarity Power Supply · Dual Polarity Unregulated PSU For High-End Audio Amps · Dual power supply · Dual regulated power supply Paper - II Electronic Devices and Circuits 363 · Dynamo Current and Voltage Regulator · Efficient unipolar stepper motor driver (only uses power when it makes a step) · Emergency power system · Expanded Scale Battery Volt Meter · Expanded Scale Battery Volt Meter · Fast NiMH / NiCd Battery charger · Filtering PC bus POWER · Fixed Voltage Power Supply · Fixed Voltage Power Supply · Flyback transformer driver · Fuse blown indicator · Fuse monitor / alarm · General purpose portable DC power supply using rechargeable C cells · Generating -5VDC from +5VDC · Gyrator circuit · High Current Power Supply · High Current Power Supply · High current regulated power supply · High Side Current Monitors (LM358, Zetex - ZXCT-1009) · High Voltage Converter: 90V From 1.5V · High voltage DC generator · High Voltage High Current Power Supply · High Voltage High Current Power Supply · High-Voltage Pulse Generator · HV supply: 12VDC in, 12KV out · Inverter, 12 volt unit, MOSFET design 364 Electronics Engineering Technician · Inverter, A 12 volt unit, Very Basic type · Lead acid battery charger with float · Lead/acid battery charger · Lead-Acid Battery Monitor · LED battery voltage monitor. A fuel gage for your gel cell battery. · Lithium Battery Rejuvinator · Lithium Ion Battery Charger based on a PIC micro including circuit diagram and source code · LM311 Thermostat circuit diagrams · LM317 Regulator Circuit · LM3914 battery monitor · Low Battery Voltage Cutout Circuits · Low Power LED Voltmeter · Low Power LED Voltmeter · Low Voltage Alarm for batteries and other volatile DC power sources · Low-dropout 12V regulator (LM324) · Machine power loss beepter (PDF) · Multiple voltage power supply · N.O. Magnetic Reed Switch ON /OFF Circuit (SCR equivalent) · Nagative voltage generation using 555 timer · Negative Supply from single positive Supply using 555 timer · Negative voltage generator · Nicad battery charger · NiCad Discharger for Tx & Rx Packs · NiCd Cell Charger · Nine Volt Battery Eliminator · One 9V battery gives +18, +25, +33V Paper - II Electronic Devices and Circuits 365 · Op-Amp Current Source with Floating Load including SPICE simulation · Power reminder beeper (PDF) · Power supply metering circuits for measuring both voltage and current · Power supply provides +5VDC regulated, +10VDC unregulated and 7.5VAC · Preselect Twin Coil Switch Machine Circuit · Pulse Charger for reviving tired Lead Acid batteries · PWM DC Motor Speed Control · PWM Motor Speed Controller / DC Light Dimmer · PWM Motor/Light Controller · PWM Motor/Light Controller · PWM Motor/Light Controller variants · Regulated 12V supply · Regulated Power Supply Circuits · Simple +5V power supply circuit · Simple Capacitance Multiplier Power Supply For Class-A Amplifiers · Simple constant current source · Simple DC Adapter Power Supply · Simple NiCad battery charger using LM317 · Simple switching power supply · Simple switching power supply (mains operated) · Simple switching regulator (experimental) · Simple voltage booster based on Linear Technologies LT1372, includes PCB design · Single to 3-phase power conversion · Small battery-powered USB charger including circuit diagram and PCB layout · Snowmobile GPS power adapter 366 Electronics Engineering Technician · Solid state relay circuit · Solid State Tesla Coil/High Voltage Generator · Student DC power supply · Team digital - SCR16 - Twin Coil Switch Machine Adapter · Temperature Controlled Nicad Charger · Temperature Controlled NICD Charger · Tesla coil / HV generator · Transformer Secondary Voltage Reduction · Transformerless Power Supply · Transformerless Power Supply · TTL power supply with crowbar protection · Unplugged power cord alarm · Unregulated power supply · USB charger · Using Pass Transistors Beef Up Voltage Regulator current output · Variable Dual Lab Power Supply · Variable power supply · Voltage and current regulated power supply · Voltage doubler · Voltage Inverter · Voltage Inverter · Voltage inverter · Voltage Inverter using 555 Timer · Voltage Inverter using LM380 audio amplifier IC · Voltage Monitor using UA741 operational amplifier · Voltage monitor with LED indicator · Windmill DIY Analog MPPT (maximum power point tracker) Circuit Paper - II Electronic Devices and Circuits Short Answer Type Questions 1.Mention the names of thyristor family devices. 2.Write ISI symbols of power devices. 3.Draw the symbols of DIAC,TRIAC,SCR. 4.Write applications of SCR . 5Write applications of DIAC. 6.Write applications of TRIAC. 7.Draw the symbol of UJT. 8.Write applications of UJT. Long Answer Type Questions 1. Explain construction working V I characteristics of SCR. 2.Explain construction and working of DIAC and TRIAC. 3.Explainconstruction and working of UJT. 4.Write applications of power electronic devices. Practical/OJT Questions . • Study the V I characteristics of SCR. • Study the V I characteristics of DIAC and TRIAC. • Study the V I characteristics of UJT. 367 Electronics Engineering Technician 368 UNIT 8 Opto Electronic Devices Learning Objectives • Study of Opto electronic devices. • Study of classification of opto eletronic devices. • Study of operation of LDR. • Study of operation and working of LED applications. • Study of construction and working of LCD applications. • Study of consrtuction and working of Photo diode. • Study of construction and working of Photo transister. • Study of construction and working of Opto coupler. • Study of working of Photo conductive cells. 8.0 Introduction of OPTO Electronic Devices In semiconductor material, the process of conduction is achieved by using different techniques of liberating the valence elections. The liberation of these electrons can be achieved by impartly some external energy viz heat, light, photon, Paper - II Electronic Devices and Circuits 369 bombardment etc. Out of this light is one of the common sources used for imparting external energy. The device especially made to change their properties with light are called as Optoelectronic devices; the optoelectronic devices are product of a technology that combines optics with electronics. A phase locked loop (PLL) is basically a closed loop feedback system. The action of PLL is to lock or synchronise the frequency of a controlled oscillator to that of an incoming signal. The implementation of PLL with discrete components involves circuits of considerable cost and complexity. For the reason, the use of PLL in the past has been limited to specialized measurements. The development of integrated circuits PP now makes it highly economical as well as reliable. In this chapter we will study the application of 555IC timer and description of face locked loop. In this chapter we will also study the different opto electronic devices. 8.1 Classification of OPTO Electronic Devices Opto electronic devices are basically classified into 1. Sensors 2. Emitters 3. Couples of insulators Opto electronic devices are of three categories 1. Photo conductive device Example of Photo conductive device are (a) Photo bodies (b) Photo transistor (c) Light Dependent Resistor. 2. Photo Emissive Devices Example of photo emissive devices are (a) Light Emitting Device or (LED) (b) Liquid Crystal Display (LCD) (c) Photo Tube (d) Photo multiplier (e) Light Activated Simulating Emitter Radiation(LASER) Electronics Engineering Technician 370 3. Photo voltaic devices Example of photo voltaic devices are (a) Solar cells (b) Light detectors Opto Electronics Devices Sensors Proconductive Emitters Solar Cells Photo diode Photo Emissive Photo voltaic Photo Transistor Light detector LCD LED Laser PhotoTube Light Dependent Resistor (LDR) Fig. 8.1 8.2 Light Dependent Resistor (LDR) A Light dependent resistor (LDR) or photo resistor is made from semiconductor. Materials whose resistance various with the amount of light energy imparted to it. The LDR made with cadmium sulphide (CdS) cadmium solenoid (CdSe) and lead simple (PbS). Resistance of LDR is very high when kept in total darkness and is very low and kept in well illuminated area. The resistance (R) is indirectly proportional to the amount of light energy (E) falling in its surface. Working Principle of LDR When energy (E) is imparted to the materials like CdS, the liberation of valence electronic generate electronic whole pairs within the material. Those pairs act as charge which initiate the conduction process. The resistance R of the materials is reduced according as AE –a Where A =constant and a = lies between 0.7and0.9 for CdS .the greater the amount of light falling on the surface, lower will be the value of resistance of the material and vice-versa. Paper - II Electronic Devices and Circuits 371 Working and VI Characteristic: (a) Construction (b) Circuit Symbol Fig. 8.2 Light Dependent Resistor Fig 8.2 shows Photo Conducting Cell and its symbol. The photo resistor is deposited on top ceramic substrate. After fabrication it has translucent to and hermetically sealed. Ratings and performance of the devices are characterized by the value of current flowing through the device at a given voltage and the amount flux. In the presence of light, it poses a little reactance in the circuit giving ON stage. In darken it poses a very high resistance which causes almost no flow of current thus resulting in OFF state of the switch. Hence the device acts like an automatic switch whose ON and OFF states are dependent on total illumination and dark condition respectively. The characteristic of CdS LD is shown figure 8.3. Fig. 8.3 Ilumination Characteristics of LDR Advantages and disadvantages of LDR Advantages: 1. Low cost Electronics Engineering Technician 372 2. Easy Operation 3. High photo sensitivity Disadvantages: 1. Effect of light intensity 2. Poor temperature stability 3. Narrow spectral response Application of LDR 1. Automatic street lighting 2. Burglar alarm 3. Relay circuits 4. Light meters 8.3 Light Emitting Diode The LED is basically a device which convert input electrical energy into output optical radiation in the visible or infrared portion of spectrum depending on the semi conductor material used. LEDs have replaced incandescent lamps in much application because of low voltage long life and fast ON-OFF switching. The material used in manufacturing the LEDs are 1.Arsenide phosphide (GaAsP): it provides either red light or yellow light 2.Galliun phosphide (GaP) :it provide red or green light 3.Galliun Arsenide (GaAss):it provide infrared radiation Principle : When a PN junction is forward baised, charge carrier recombination take place at a junction as electrons cross then-side and combine with holes on the P-side. Free electrons are in the conduction band of energy levels while holes are in the valence band. Therefore electrons are at higher energy level then holes. When recombination take place, some of its energy given up in the form of heat and light. If the semiconductor material is translucent, the light will be emitted and junction becomes light source, which is called as Light Emitting Diode (LED) Paper - II Electronic Devices and Circuits 373 Construction and Working : Diffused P-type Epitaxial a-type (b) Circuit Diagram Gold Film Cathode Connection (a) Cross Section view of LED Fig. 8.4 LED Construction and Working of LED The cross sectional view of a typical LED is shown in Fig 8.4 (a) the semiconductor material employed is Gallium Arsenate or Gallium Arsenatic Phosphate or Gallium phosphate. An N type bar is grown up on a substract and P – region is created by diffusion since the charge carrier recombination occur in the p – region it must be kept upper most . The P – region therefore becomes the surface of the device and the metal film anode connection must be patterned to a allows most of the light to be emitted. A gold film is applied to the bottom of substrate to reflect as much as possible of the light toward the surface of the device and to provide cathode connection FIG 8.4(b) shows the circuit Symbol of LED. The outward arrow indicates emission of light .Fig 8.5 shows the V-I characteristic of LDR. From V-I characteristic one can expect a typical operating current of 10mA operating voltage range of LED is from 1.7 V to 3.0 V. Opto Electronic Devices, Timers & Phase Locked Loops Advantages and Disadvantages of LED Advantages : 1. Low working Voltage and current 2. Less Power Consumption 3. No warm up time Electronics Engineering Technician 374 4. Very fast action 5. Emission of monochromatic light 6. Small size less in weight 7. No effect of mechanical vibrations 8. Less fragile than glass 9. Extremely long life Fig. 8.5 Characteristics of LED Disadvantages : 1. Sensitive to damage by over voltage or over current 2. Wide optical band width compared to LASERS 3. Temperature dependent of radiant output power and wave length4 4. Theoretical overall efficiency is not achieved except in special cases or pulsed conditions. Applications of LED 1.Calculators 2. Multi meters 3. Picture phones Paper - II Electronic Devices and Circuits 375 4. Burgl alaram system 5. Digital meters 6. Microprocessors 7. Digital computers 8. Electronic telephone exchange 9. Intercomes 10. Digital watches 11. Electronic panels 12. Sold state video display 13. Optical communication system. 8.4 Generation of Different Colour LED’S As part of the National Instruments Introduction to NI ELVIS II, NI Multisim, and NI LabVIEW courseware, the labs introduces students to the capabilities of the NI ELVIS II educational design and prototyping platform. Students can explore how NI ELVIS II allows for an easy transition from design, simulation to prototype as it interfaces with both NI Multisim and LabVIEW software. The courseware includes 11 lab experiments starting with the an introduction to the NI ELVIS environment and steps you through AC circuits to communications. The labs are designed as a starting point for your own curriculum design, demonstrations in the classroom, and method to inspire students to be imaginative and creative in their design projects. View all of the labs for the Introduction to NI ELVIS II, NI Multisim and NI LabVIEW courseware. Goal This lab focuses on using NI ELVIS II to illuminate diode properties, diode test methods, bit patterns for a two-way stoplight intersection, and the use of NI ELVIS II APIs in a LabVIEW program to run the stoplights automatically. A Multisim challenge encourages the reader to design a two-way stoplight intersection using discrete transistor-transistor logic (TTL) ICs. Electronics Engineering Technician 376 Exercise 8.1: Testing Diodes and Determining Their Polarity A semiconductor junction diode is a polar device with a band on one end which indicates the cathode. The other end is called the anode . While there are many ways to indicate this polarity in the packaging of a diode, one thing is always the same – a positive voltage applied to the anode results in the diode being forward-biased so that current can flow. You can use NI ELVIS II to determine the diode polarity. Complete the following steps to set up NI ELVIS II for diode and polarity tests: 1. Launch the NI ELVIS II Instrument Launcher strip and select DMM. 2. Click on the diode test button . 3. Connect one of the LEDs to the workstation banana sockets DMM (VÙ ) and (COM). When you apply a positive voltage to the cathode, the diode blocks the current. The display, which reads the same value as it does when no diode is connected (open circuit), shows the word OPEN (see Figure 7.1). Fig 8.6. Reverse-biased Diode Reading When you apply the positive voltage to the anode, the diode allows current to flow. The display reads a low voltage level. Paper - II Electronic Devices and Circuits 377 Fig. 8 .7 Forward-biased Diode Reading For example, a silicon rectifying diode in the forward-bias direction displays a voltage ~0.6 V. In the reverse-bias direction, the display shows the word OPEN. NOTE: You can use this simple test to determine the polarity of a colored LED. Connect a red LED to your test leads. In one direction, you see light (forward-biased) and, in the other direction, no light (reverse-biased). The DMM display does not change, but there is enough current to produce some light. Check closely “ the LED is dimly lit and may be difficult to see with bright lights in the room. When the LED is lit, the red lead connection is the anode. The way this works is that the display shows the voltage required to generate a small current flow of about 1 mA. In the forward-bias region, this voltage level is usually smaller than the open circuit voltage. In the reverse-bias direction, no current flows and the tester displays the word “open”. For LEDs, the voltage threshold is often larger than the open circuit voltage. The 1 mA test is not Electronics Engineering Technician 378 sufficient to discern the forward-bias test (GOOD), but it is enough to generate a low light intensity. Exercise 8.2 Characteristic Curve of a Diode The characteristic curve of a diode, that is, a plot of the current flowing through the device as a function of the voltage across the diode, best displays the diode’s electronic properties. Complete the following steps to display the characteristic curve of a diode: 1. Place a silicon diode across the DMM/Impedance Analyzer pin sockets DUT+ and DUT-. The anode diode pin goes to the + input. For reference, the flat side of the LED is the cathode. 2. Launch the NI ELVIS II Instrument Launcher strip and select the TwoWire Current-Voltage Analyzer (2-Wire). A new SFP opens so you can display the characteristic (I-V) curve for the device under test. This SFP applies a test voltage to the diode from a starting voltage level to an ending level in incremental voltage steps, all of which you can select. 3. For a silicon diode, set the following parameters: Start: -2 V Stop: +2.0 V Increment: 0.05 V 4. Set the maximum current in either direction to ensure the diode does not operate in a current region where damage may occur. Check the diode specifications. In the reverse-bias direction, the current should be very small (mA) and negative. In the forward-bias direction, you should see that above a threshold voltage, the current rises exponentially to the maximum current limit. 6. Change the Display buttons [Linear/Log] to see the curve plotted on a different scale. 7. Try the Cursor operation. It gives the (I,V) coordinate values as you move the cursor along the trace. The threshold voltage is related to the semiconductor material of the diode. For silicon diodes, the threshold voltage is about 0.6 V, and for germanium diodes, it is about 0.3 V. One way to estimate the threshold voltage is to fit a Paper - II Electronic Devices and Circuits 379 tangent line in the forward-bias region near the maximum current (refer to Figure 7.8). The point where the tangent intersects the voltage axis defines the threshold voltage. Observe the (I,V) characteristic curve for a light emitting diode. For this LED, the threshold voltage given by the intersection of the tangent with the voltage axis is about 1.56 V. Fig. 8 .8 Current-Voltage Characteristic Curve of a Silicon Diode 8. Using the Two-Wire Current-Voltage Analyzer, determine the threshold voltage for a red, yellow, and green LED, and complete the chart below. Red LED ____________ V Yellow LED ____________ V Green LED ____________ V Exercise 8.3: Manual Testing and Control of a Two-Way Stoplight Intersection Complete the following steps to build and manually test and control a twoway stoplight intersection. Electronics Engineering Technician 380 1. Install two each of red, yellow, and green LEDs on the NI ELVIS II protoboard, positioned as a two-way stoplight intersection. Fig. 8.9 LED layout of a Two-way Stoplight Intersection 2. Connect the pin socket DIO <0> to the anode of the red LED in the North-South (Up-Down) direction. 3. Connect the other end of the LED through a 220 W resistor to digital ground (not pictured). NOTE: The resistor is used to limit the current through the LED. 4. Connect the remaining colored LEDs in a similar fashion. Here is the complete mapping scheme. DIO <0> Red N-S direction DIO <4> Red E-W direction DIO <1> Yellow N-S direction DIO <5> Yellow E-W direction DIO <2> Green N-S direction DIO <6> Green E-W direction 5. From the NI ELVIS II Instrument Launcher strip, select Digital Writer (DigOut). 6. Using the vertical slide switches, select any 8-bit pattern and output that pattern to the NI ELVIS II digital lines. Recall that Bit 0 is connected to the pin socket on the protoboard labeled DIO <0>. Paper - II Electronic Devices and Circuits 381 7. Set the Generation Mode to (Run Continuous) and Pattern to (Manual), 8. To activate the port, click on the Run button. Fig. 8.10 Digital Writer for Testing LEDs When all switches (Bits 0-2 and 4-6) are HI, all the LEDs should be lit. When all these switches are LO, all the LEDs should be off. You can now use these switches to find out which 8-bit codes are necessary to control the various cycles of a stoplight intersection. Here are some clues for an intersection. The basic operation of a stoplight is based on a 60-second time interval with 30 seconds for red, followed by 25 seconds for green, followed by 5 seconds for yellow. For example, in a twoway intersection, the yellow light in the North-South direction is on while the red Electronics Engineering Technician 382 light in the East-West direction is on. This modifies the 30-second red timing interval to two timing intervals: a 25-second cycle followed by a 5-second cycle. There are four timing periods (T1, T2, T3, and T4) for two-way stoplight intersection operation. 9. Study the following chart to see how a two-way stoplight intersection works. Direction N-S E-W Lights RYG RYG 8-Bit Code Decimal Value Bit # 012 456 T1 25 s 0 0 1 1 0 0 00001100 12 T2 5 s 0 1 0 1 0 0 ________ ___________ T3 25 s 1 0 0 0 0 1 ________ ___________ T4 5 s 1 0 0 0 1 0 ________ ___________ 10. Use the Digital Writer to determine which 8-bit codes need to be written to the digital port to control the stoplights in each of the four timing intervals. For example, timing period 1 requires the code 00101000. Computers read the bits in the reverse order (least significant bit on the right). This code then becomes 00010100. In the white box above the Manual Pattern Line switches display, you can read the radix of the switch pattern in binary {00010100}, decimal {20}, or hexadecimal {14}. 11. Click on the black ^ to left of the white display box to change the radix. You can use this feature to determine the numeric codes for the other timing intervals T2, T3, and T4. If you output the 8-bit code for each of the timing intervals in sequence, you can manually operate the stoplights. NOTE: You can also change the radix in the Line States display by clicking on the white x beside the Numeric Value display. Repeating this four-cycle sequence automates your intersection. Exercise 8.4: Automatic Operation of the Two-Way Stop light Intersection Complete the following steps to automate the timing cycle on the stop light circuit. 1. Close NI ELVIS II SFPs and launch LabVIEW. Paper - II Electronic Devices and Circuits 383 2. Open the program StopLightsMx.vi. There is only one control on the front panel “ a Boolean switch used to stop the operation of the stoplights. NOTE: This LabVIEW program is configured to connect to”Dev1" for your NI ELVIS workstation. If your device is configured to another device name, you need to rename your NI ELVIS workstation to “Dev1,” in Measurement & Automation Explorer (MAX) or modify the LabVIEW programs to your current device name. 3. Switch to the block diagram (Window»Show Block Diagram). 4. Observe the four-cycle sequence generated by the for loop. The NI-ELVISmx Digital Writer API is the structure that outputs the light code to the stoplights. This API expects the input code to be an 8-bit Boolean array. For example, the first timing interval T1 requires the code 12 (twelve decimal). Its value is placed in the first element of an integer array labeledLights Pattern. You must transfer the other integer codes from the table in Exercise 7.3 into the three blank elements of the Lights Pattern array. Figure 8.11. Block Diagram for Automated Operation of a Two-way Stoplight Intersection 384 Electronics Engineering Technician In operation, one of the elements of the Lights Pattern array is selected on the boundary of the for loop (inner loop) and converted into an 8-bit Boolean array. In a similar way, the appropriate time delay is selected at the for loop boundary and passed to the Wait function. The timing intervals are stored in the four elements of the Time Delay array. To speed up operation, the 25-second time interval is reduced to 5 seconds and the 5-second time interval is reduced to 1 second. What’s Cool! LEDs are amazing devices. If you multiply the threshold voltage, VT, times the electronic charge, e, the product is energy that is close to the band gap energy of the semiconductor material used to manufacture the semiconductor diode. Further, in the forward-biased region, the light from the LED has an energy of hc/ë, where h is Planck’s constant, c is the speed of light, and ë is the wavelength of the center of the energy distribution. Conservation of energy yields the equation: From the LED specifications, you can determine the wavelength or the LED color. For example, red LEDs have a wavelength of about 560 nm. From the I-V characteristic curve of the LED , you can measure the threshold voltage VT. If you plot VT versus 1/ë for the three different colored LEDs, you find a straight line with a slope approximately equal to (hc/e), a mixture of three fundamental constants of nature. Multisim Challenge: Design a Control Circuit for a Two-Way Stoplight Intersection Modern-day stoplights use a cluster of red, yellow, or green LEDs to produce the stoplight signals. In this lab, you have learned about the electrical and optical characteristics of visible LEDs. You have used colored LEDs to form a simple two-way stoplight intersection and a LabVIEW program to control the light sequences. With Multisim, you can design a stoplight controller using discreet logic ICs. A stoplight program requires a shift register and variable delays. Recall that the red light is on for (25 + 5) seconds, the green light for 25 seconds, and the yellow light for 5 seconds. Load the Multisim program called Stop Light Timing. Study the operation carefully. This program uses two 7474 Dual D edge-triggered flip-flop ICs to form a 4-bit shift register. It uses a special clock circuit to generate the timing sequence 25, 5, 25, 5 seconds. This program controls only one set of red, yellow, and green stoplights. Your challenge is to modify the program so that it can control two sets of stoplights in a two-way stoplight intersection. Paper - II Electronic Devices and Circuits 385 8.5 Liquid Crystal Display (LCD): LCD are passive display devices characterized by very low power consumption and good contrast ratio the liquid crystal display does not emit light or generate light. It require an external or internal light source. Although LED give off light but LCDs are not light sources but controls light.LCD a reflect part of surrounding light, while the other parts of the display absorb light. Principle: The molecules in ordinary liquids normally have random orientation. In liquid crystals, the molecules are oriented in a definite crystal pattern. when an electric field is applied to the crystal, the molecules, which are approximately cigar shaped, tend to align themselves perpendicular to the field charge carrier flowing through the liquid disrupt the molecular alignment and cause a turbulence within the liquid. This is illustrated in Fig 8.12. (a) Molecules in Liquid crystal when no current is flowing (b) Change carrier flow through liquid crystal disturbs molecular alignment and causes turbance Fig. 8.12 Molecules in Liquid Crystals When not activated, the liquid crystal is transparent. When activated, the molecular turbulence causes the light to be scattered in all directions, so that the activated areas appear bright. The phenomenon is known as dynamic scattering and is shown in fig 8.13. The actual liquid crystal material may be one of several organic compounds which exhibit the optical properties of a solid while retaining the fluidity of a liquid. Example of such compounds are 1.Cholestaryl nonanoate 2. p- azoxy anisole Electronics Engineering Technician 386 Constructing and working The constructional features consists of a layer of liquid crystal material sandwiched between the glass sheets with transparent electrons deposited in the inside surface and is shown in Fig. 8.13 Liquid Crystal Transparent Electrodes Spacer Fig. 8.13 Construction of LCD With both glass sheets transparent the cell is known as “transmitive type cell”. When only one glass sheet is transparent and other had a reflective coating the cell is termed as reflective type the liquid crystal cells do not generate light but transmit or reflect light from external sources thus only energy required by the cell is to produce :dynamic scattering effect”. LCDs are usually seven segment type or dot matrix type displays. In these displays LCDs are activated by applying voltage between the segments and common electrode. Segments on the LCD are driven by low frequency a.c typical driving voltage of 5 V rms. When segments is not activated, the transmitive type cell will simply transmit rear or edge lighting through the segment in straight line. In this condition the corresponding segment will not appear bright. In reflective type the light is reflected in usual way from mirror surface and corresponding segment will not appear bright. When the segment is activated the incident light is diffusely scattered forward and the corresponding segment appears bright Advantages and Disadvantages of LCD Advantages 1. Low power consumption 2. Small voltage requirement 3. They are economical 4. Good contrast is display Paper - II Electronic Devices and Circuits 387 Disadvantages 1.Some time the output of the LCD is not visible clearly 2.They are not very reliable 3.Slow operation 4.Occupies large area 5.They are very sensitive to damage by over voltage or over current 6.They are limited temperature range. 7.In LCDs a.c Square wave drive of frequency less then 50Hz is employed. Application of LCD 1. Display unit in calculators 2. Display unit in higher end CRO 3. Display unit in watches 4. Display unit in computer 5. They are used in televisions 6. Used as the slide in a projection system to obtain on enlarged image. 7. Used in all portable instrument display. 8.6 Comparision Between LED and LCD Displays The displays, used in electronic instruments, equipments etc are 1. Seven segment display 2. Dot matrix display Electronics Engineering Technician 388 S.No. LED 1. Consumes more power 10250 mw power per digit. 2. Because of high power requirement it requires external interface circuitry when driven from ICs. LCD Essentially act as a capacitor and consume very less power requires 10-200 mirco W power per digit. Can be driven directly from IC chips 3. Good brigtness level Moderate brightness level 4. Operating temperature range - 40oC to 85oC. Temperature range limited to 20oC to 60oC 5. LED Life is around 1,00,000 hours Lifetime is limited to 50,000 hours due to chemical degradation. 6. Emits light in red, orange, yellow, greeen and white Invisible in darkness - requires external illumintion.. 7. Operating voltage range is 1.5 to 5V dc Operating voltage range is 3 to 20 V dc. 8. Response time is 50 to 500 ns Has a slow decay time - response time is 50 to 200 ns. 9. Viewing angle 150o Viewing angle is 100o Paper - II Electronic Devices and Circuits 389 8.6.1 Seven Segment Display For conventional Seven segment display (including the decimal point i:e the segment) the wiring pattern is simplified by making the terminal of LED or LCD common to all other segments. In LRDs the terminals can be common anode (CA)form or common cathode (CC) form. Fig 8.15(a)shows a Seven segment display. in this circuit the anodes of all the diodes are connected together to the positive terminals of the d.c voltage source. The cathodes are connected to the external resistor. By grounding the external resistor we can form any decimal digit from 0 to 9. (a) 7-Segments (b) Schematic Digit (c) Digit 2 in seven segment display Fig. 8.15 Seven Segment display For example by grounding a,b,g,e and d we can form the digit “2”.In similar manner by grounding f,g,b,and c can form the digit “4” and so on. A Seven segment display can also display the capital letters A,C,E, and F besides this, it can also display the lower case letter b and d. The Seven segment display are used in digital clocks calculators, Stereo tuners, microwave ovens, digital multi meters etc. 8.6.2 DOT Matrix Display The Seven segment display is not commonly used technology and is also the easiest to implement electronically. However it is limited to displaying numeric and a small range of alphabetic information. the dot matrix can display a wide range of numeric, alphabetic information and other characters. The dot matrix display is a method of generating characters with a matrix of dots. The commonly used dot matrix for the display of prominent characters are 5 x 7, 5 x 8 and 7 x 9. The 5 x 7 dot matrix display is shown in Fig 8.16 The two wiring patterns of dot matrix display are as follows. Electronics Engineering Technician 390 (a) Dot Matrix (b) Bit 4 in Dot Matrix Fig. 8.16 Dot Matrix Display 1.Common anode or common cathode connection 2. X_Y array connection (Economical and can be extended vertically or horizontally using a minimum number of wires). 8.7 Photo Diode Construction and Working: Photo Diode is a opto device which is designed to respond to photo absorption. Under illumination, the carriers conduction is directly proportional to the injected carrier generation. This device when operated with reverse voltage applied, functions as a photo conductive cell and when operated without reverse voltage, functions as a photo voltaic cell Principle: When a pn junction is reversed baised a reverse saturation current I flows due to thermally generated holes and electrons crosses the junction as a minority carriers. Increasing the junction temperature more electron hole pairs are generated and so the minority carrier current will be increased. Light Waves Fig. 8.17 Principle of Working of Photodiode Paper - II Electronic Devices and Circuits 391 The same effect occurs if the junction is illuminated. Hole and electron pairs are also generated by the incident light energy and minority charge carriers cross the junction. the diode specially made to follow this phenomenon is called Photo Diode. Construction and Working: The Photo Diode is made of semi conductor or P – N junction kept in sealed plastic or glass casing. The cover is so designed the light rays are allowed to fall on one surface across the junction the remaining sides of the casing are painted to restrict the penetiation of light rays. A lens permits light to fall on junction When light falls on the reverse baised PN Photo Diode junction hole – electrons pairs are created. The movement of these electron hole pairs in a property connected circuit results Fig. 8.18 Photo Diode In current flow. The magnitude of photo current depends on the number of charge carriers generated and hence, on the illumination on the diode element. This current is also affected by the frequency of the light falling on the junction of the Photo Diode. Fig, 8.18 Shows the circuit symbol. The inward arrows in circuit symbol represent the incoming light, illustrate the V- I characteristics of Photo Diode for different intensity levels. When there is no light or the applied illumination is zero, the current flowing through the device is known as dark current. As the light becomes brighter or illumination increases, the magnitude if the reverse current also increases. It may observe that the current will become zero only with positive applied bias. Electronics Engineering Technician 392 Advantages and Disadvantages of Photo Diode Advantages 1. Low noise 2. Very good spectral response 3. Fastest photo detector Disadvantages 1. Sensitive device 2. Current increased with temperature 3. Could not exceed the working temperature limit specially by the manufacturers Application of Photo Diode 1. Light decetor 2. Demodulators 3. Encoders 4. Optical communication spectrum 5. High speed counting and switching circuits 6. Computers card punching and tapes 7. Light operated Switches 8. Sound tracks Films 9. Electrionic control circuits 8.8 OPTO Transistor (Photo Duo Diode): A transistor is similar to an ordinary bipolar junction transistor except that no base is provided. Instead of base current the input to the transistor is provided in the of light. Principle: Contain ordinary transistor with its base terminals open circuited. the collector base age current ICBO will act base current. In transistor ICBO is increased when the collector base junction is illuminated. where is increased the collector current is also increased therefore for a given Paper - II Electronic Devices and Circuits 393 amount illumination on a very small area of the photo transistor provides much largest output current. Therefore the photo transistor is a light detector which combines a photo diode and a transistor amplifier. Construction and Working: The Construction of a photo transistor is just like a conventional NPN transistor with a little hole made on the surface near to collector base junction. A small lens is fixed on the hole for allowing a focused light beam to concentrate on the collector – base junction. In the modern methods of fabrication highly light effective materials are used instead of making a hole and fixing a lens on it. From fig. 8.19 is clear that emitter base junction JE is forward biased, here as the collector base junction JC is reversed baised. when the transistor is kept in darkness there will be very few minority charge carriers (Thermally generated which will cause the flow of reverse saturation collector current .This current for obvious reasons, will be negligible small. On light being focused at the collector base junction additional photo generated minority charge carriers will be available which will add to the reverse saturation current thus as soon as the light source is applied the transistor starts conducting and amplified current starts flowing through the reverse biased junction. thus owing to the transistor amplifier action, the current caused by the luminous flux will increase a lost. Fig 8.19 (b) shows circuit symbol of photo transistor. Fig 8.19 shows the characteristics of photo transistor drawn for Ic verses Vce as a function of illumination H. The current is a photo transistor is dependent mainly on the intensity of light entering the lens and is less affected by the voltage applied to the external circuit. Housing Light Lens (b) Symbol (a) Construction Fig. 8.19 : Photo Transistor Electronics Engineering Technician 394 Fig. 8.20 Characteristics of Phototransistor Application of Photo Transistor 1. High speed reading of computer punched cards and tapes. 2. Light detection system 3. Light operated switches. 4. Realin of film sound track 5. Production line counting of objects which interrupt a light beam. 8.9 Applications of Photo Transistor 1. High speed reading of computer punched cards and tapes. 2. Light detection system. 3. Light operated switches. 4. Realin of film sound tracks. 5. Production line counting of objects which interrupt a light beem. 8.10 OPTO Coupler (Opto isolator): Opto coupler is a solid state component in which light emitter, the light path and light detector are all enclosed within the component and cannot be altered externally. Usually infrared emitting Diode (IRED) can be used as a light emitter. photo transistor can be used as a light detector. Paper - II Electronic Devices and Circuits 395 Principle: There is one way of transfer of electrical signal from LED(IRED) to photo transistor without any electrical connection between the input and output circuitry. The degree of isolation between input and output depends on the kind of materials used in the light path and on the distance between the light emitter and light detector. If the distance between emitter and detector is greater than & the isolation is better but current transfer ratio i:e,ratio of detector to emitter current is lower. LED Photo Transister Fig. 8.21 Opto Coupler Working : Fig 8.21, shows the symbol of opto coupler. The coupler may be operated as a switch in which cause both the LED and photo transistor are normally OFF.A pulse of current through the LED (IRED) causes the transistor to be switched ON for the duration of the pulse. Operation as a linear coupler is also possibly by setting usp a bias current in the LED. The signal is then capacitively coupled to the LED and causes its brightness to increase or decrease thus the photo transistor receives a light signal which increases and decreases linearly around the constant bias level. Advantages of Opto Coupler 1. The electrical isolation can be superior to that of a transformer isolation. the charge less photons are not influenced by electrostatic or electromagnetic fields. 2. The conditions of load changing will not affect the input as the signal transfer is unilateral. 3. These are faster than isolation transformer Electronics Engineering Technician 396 4. In switching or chopping application, the inherent mechanical problems are all eliminated by contactless operation of these isolators. Application of Opto coupler 1. Opto couplers are used where the electronic circuit isolations required. 2. To eliminate common ground connection. 3. To reduce common mode noise. 4. In fibre optic communication 5. In switching applications. 8.11 Photo Conductive Cell The Photo Conductive Cell is a two terminal semiconductor device whose terminals resistance will vary linearly with the intensify of the incident light. For obvious reason, it is frequently called a photo resistive device. Construction and Working: Light Sensitive material Resistance (k 100 Top View 10 1 Side View (b) Circuit Symbol 0.1 (a) Construction 10 100 1000 10,000 (c) Illunination Characteristics Fig. 8.15 Photo Conductive Cell The photo conductive material most frequently used include Cadmium Sulphide (CdS) and Cadmium Selenide (CdSe). Both materials respond rather slowly to changes in light intensity. The essential element of a photo conductive materials, metallic electrodes to connect the device in to a moisture resistance enclosure. The circuit symbol and construction of a typical photo conductive cell are shown in Fig. 8.22 Paper - II Electronic Devices and Circuits 397 Light sensitive material is arranged in the form of a long strip zigzagged across a disc shape base with protective sides for added protection, a glass or plastic cover may be included. The two ends of the strip are brought out to connecting pins below the base. The illumination characteristic of a typical photo conductive cell are shown in Fig4.15 (c) when cell is not illuminated its resistance may be more than 1K this resistance is called the dark current. when cell is illuminated the resistance may fall to few hundred ohms. Note that the plot is drawn in log scale. Applications of Photo Conductive Cell 1.ON – OFF 2.Relay Circuits 3.Light Meters. Short Answer Type Questions 1.Mention the names optoelectronic devices. 2.Write applications of LDR. 3.Write working principle of LED. 4.Write applications of LED. 5.Write any specifications of LED. 6.Write working principle of LCD. 7.Write applications of LCD. 8.Write working principle of photo diode. 9.Write applications of photo diode. 10.Write working principle of photo transistor. 11.Write applications of photo transistor. 12.Write applications of opto couplers. Long Answer Type Questions 1.Write construction and working of LDR. 2.Draw and explain construction working of LED. 3.Explain construction and working of LCD with neat diagram. Electronics Engineering Technician 398 4.Write specifications of LCD and LED. 5.Write construction and working of photo diode. 6.Write construction and working of photo transistor. 7.Explain working of opto coupler with neat diagram. 8.Explain working of photo conductive cells. Practical/OJT Questions • Study the working of LDR. • Study the working of LED and LCD. • Study the working of photo diode/transistor. • Study the opto coupler/photo conductive cell.