EASA Part 66 - Module 4 - Electronic Fundamentals

Index

Index

1

1 SEMICONDUCTOR SEMICONDUCTOR DEVICES....DEVICES... 1-11-1 1.1

1.1 RECTIFIER DIODESRECTIFIER DIODES... 1-21-2

1.1.1

1.1.1 CircuiCircuit t SymboSymbols ls & & IdentiIdentificatioficationn ... 1-21-2 1.1.2

1.1.2 OperaOperating ting CharaCharacteristics...cteristics... 1-31-3 1.1.3

1.1.3 ParaParallel llel & & SeriaSerial l ArrangArrangementements s of of DiodeDiodess ... 1-41-4 1.1.4

1.1.4 RectifRectification...ication... 1-51-5 1.2

1.2 SIGNAL DIODESSIGNAL DIODES... 1-101-10

1.3

1.3 ZENER DIODESZENER DIODES... 1-101-10

1.4

1.4 LIGHT EMITTING DIODESLIGHT EMITTING DIODES... 1-111-11

1.5

1.5 PHOTOCELLSPHOTOCELLS... 1-111-11

1.5.1

1.5.1 PhotoPhotoconduconductive ctive Cells...Cells... 1-111-11 1.5.2

1.5.2 PhotoPhotovoltaic voltaic Cells...Cells... 1-121-12 1.6

1.6 PHOTODIODESPHOTODIODES... 1-121-12

1.7

1.7 VARACTOR DIODEVARACTOR DIODE... 1-121-12

1.8

1.8 SILICON CONTROLLED RECTIFIERSILICON CONTROLLED RECTIFIER... 1-131-13

1.9

1.9 TRANSISTORSTRANSISTORS... 1-131-13

1.9.1

1.9.1 NPN NPN TranTransistor...sistor... 1-141-14 1.9.2

1.9.2 PNP PNP TranTransistorsistor... 1-161-16 1.10

1.10 TESTING SEMICONDUCTOR DEVICESTESTING SEMICONDUCTOR DEVICES ... 1-181-18

1.10

1.10.1 .1 TestinTesting g DiodeDiodess ... 1-181-18 1.10

1.10.2 .2 TestiTesting ng TransiTransistors...stors... 1-191-19 2

2 OPERATIONAL OPERATIONAL AMPLIFIERS...AMPLIFIERS... 2-12-1 2.1

2.1 THE PERFECT AMPLIFIERTHE PERFECT AMPLIFIER ... 2-12-1

2.2

2.2 OP AMP SPECIFICATIONOP AMP SPECIFICATION... 2-12-1

2.3

2.3 POWER REQUIREMENTSPOWER REQUIREMENTS... 2-22-2

2.4

2.4 PIN OUTSPIN OUTS && CIRCUIT SYMBOL CIRCUIT SYMBOL ... 2-22-2

2.5

2.5 OPERATIONOPERATION... 2-32-3

2.5.1

2.5.1 NegaNegative tive FeedFeedbackback ... 2-32-3 2.6

2.6 OPOP-- AMP COMPARATOR AMP COMPARATOR ... 2-52-5

2.7

2.7 OP AMP SUMMING AMPOP AMP SUMMING AMP ... 2-62-6

3

3 PRINTED PRINTED CIRCUIT CIRCUIT BOARDS...BOARDS... 3-13-1 3.1

3.1 BASE MATERIALBASE MATERIAL... 3-23-2

3.2

3.2 CONDUCTOR MATERIALCONDUCTOR MATERIAL ... 3-23-2

3.3

3.3 BONDING OF CONDUCTOR MATERIALBONDING OF CONDUCTOR MATERIAL... 3-23-2

3.3.1

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3.5 CIRCUIT ARTWORKCIRCUIT ARTWORK... 3-43-4

3.6

3.6 PRINTING OF CIRCUITSPRINTING OF CIRCUITS ... 3-53-5

3.6.1

3.6.1 EtchiEtching ng ProcessProcess ... 3-53-5 3.6.2

3.6.2 AddiAdditive tive Process.Process... 3-63-6 3.6.3

3.6.3 InspeInspection...ction... 3-73-7 3.7

3.7 SOLDERING METHODSSOLDERING METHODS... 3-73-7

3.7.1

3.7.1 Hand Hand SoldeSolderingring ... 3-73-7 3.7.2

3.7.2 Mass Mass SoldeSolderingring ... 3-73-7 3.8

3.8 SOLDER SPECIFICATIONSOLDER SPECIFICATION... 3-93-9

3.9

3.9 FLUXESFLUXES&& THEIR APPLICATION THEIR APPLICATION... 3-93-9

3.10

3.10 SOLDER RESISTSSOLDER RESISTS... 3-103-10

3.11

3.11 PLATING OF PRINTED WIRING CIRCUITSPLATING OF PRINTED WIRING CIRCUITS ... 3-103-10

3.11

3.11.1 .1 ThroThrough-Hugh-Hole ole PlatinPlatingg ... 3-103-10 3.12

3.12 ORGANIC PROTECTIVE COATINGSORGANIC PROTECTIVE COATINGS... 3-113-11

3.13

3.13 FLEXIBLE PRINTED WIRING CIRCUITSFLEXIBLE PRINTED WIRING CIRCUITS... 3-113-11

3.14

3.14 HANDLING OF CIRCUIT BOARDSHANDLING OF CIRCUIT BOARDS ... 3-123-12

3.14.

3.14.1 1 ElectrElectrostatic ostatic DischaDischarge rge SensiSensitive tive DeviceDevicess ... 3-123-12 3.14.2

3.14.2 Removal Removal & & Installation Installation of of ESDS ESDS Printed Printed Circuit Circuit Boards Boards 3-153-15 3.14.3

3.14.3 Removal Removal & & Installation Installation of of Metal-Encased Metal-Encased ESDS ESDS LRU's LRU's 3-163-16 4

4 SYNCHRONOUS SYNCHRONOUS DATA DATA TRANSMISSIONTRANSMISSION... 4-14-1 4.1

4.1 DESYNN SYSTEMDESYNN SYSTEM... 4-14-1

4.1.1

4.1.1 The The Basic Basic DesynDesynnn ... 4-14-1 4.1.2

4.1.2 Slab Slab Desynn...Desynn... 4-44-4 4.2

4.2 SYNCHRO SYSTEMSSYNCHRO SYSTEMS ... 4-44-4

4.2.1

4.2.1 SynchSynchro ro TypeTypess ... 4-54-5 4.2.2

4.2.2 SynchSynchro ro SchemaSchematics...tics... 4-74-7 4.2.3

4.2.3 XYZ XYZ SynchSynchro ro system...system... 4-94-9 4.2.4

4.2.4 SynchSynchro ro SuppSupplieslies ... 4-94-9 4.2.5

4.2.5 TorquTorque e SynchrSynchro o System...System... 4-104-10 4.2.6

4.2.6 ElectrElectrical ical ZeroZero ... 4-134-13 4.2.7

4.2.7 Differential Differential Torque Torque Synchro Synchro SystemSystem... 4-144-14 4.2.8

4.2.8 Control Control Synchro Synchro System...System... 4-164-16 4.2.9

4.2.9 Differential Differential Control Control Synchros...Synchros... 4-204-20 5

5 SERVSERVO O SYSTSYSTEMS...EMS... 5-15-1 5.1 CATEGORIES

5.1 CATEGORIES OFOF SERVOSERVO SYSTESYSTEMSMS ... 5-15-1 5.1.1

5.1.1 open open looploop ... 5-15-1 5.1.2

5.1.2 closed closed looploop ... 5-25-2 5.2

5.2 REMOTE POSITION CONTROL SERVOMECHANISMSREMOTE POSITION CONTROL SERVOMECHANISMS... 5-35-3

5.2.1

5.2.1 PositiPositional onal FeedFeedbackback ... 5-35-3 5.3 T

5.3.3

5.3.3 Accelerating Accelerating Input...Input... 5-55-5 5.4 S

5.4 SYSTEMYSTEMRRESPONSEESPONSE... 5-65-6

5.5 D

5.5 D AMPING AMPING... 5-75-7

5.5.1

5.5.1 Frictional Frictional Forces Forces which which Produce Produce DampingDamping ... 5-75-7 5.5.2

5.5.2 Velocity Velocity Feedback Feedback Damping...Damping... 5-95-9 5.6

5.6 VELOCITY CONTROL SERVOMECHANISMSVELOCITY CONTROL SERVOMECHANISMS ... 5-115-11

5.6.1

5.6.1 ResidResidual ual ErrorError ... 5-115-11 5.6.2

5.6.2 VelociVelocity ty Lag...Lag... 5-115-11 5.7

5.7  A A..CC.. SERVOMECHANISM COMPONENTS SERVOMECHANISM COMPONENTS... 5-125-12

5.7.1

5.7.1 E E & & I I Bar Bar Transducer...Transducer... 5-125-12 5.7.2

5.7.2 A.C. A.C. TachoTachogenegeneratorsrators ... 5-135-13 5.8

5.8 PRACTICAL SERVO SYSTEMSPRACTICAL SERVO SYSTEMS... 5-155-15

5.8.1

5.8.1 Direct Direct Servo Servo Current Current System...System... 5-155-15 5.8.2

5.8.2 Alternating Alternating Current Current Servo Servo SystemSystem ... 5-165-16 6

6 OTHEOTHER R TRANTRANSDUCESDUCERSRS ... 6-16-1 6.1

6.1 LINEAR VARIABLE DIFFERENTIAL TRANSFORMERLINEAR VARIABLE DIFFERENTIAL TRANSFORMER ... 6-16-1

6.2

6.2 ROTARY VARIABLE TRANSFORMERROTARY VARIABLE TRANSFORMER... 6-26-2

6.3

6.3 INDUCTIVE TYPE TRANSDUCERSINDUCTIVE TYPE TRANSDUCERS... 6-26-2

6.3.1

6.3.1 InducInduced ed EMF EMF TypeType... 6-26-2 6.3.2

FUNDAMENTALS FUNDAMENTALS

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1

1

SEMICONDUCTOR

SEMICONDUCTOR DEVICES

DEVICES

The early discoveries in the field of electricity made by Volta, Amp

The early discoveries in the field of electricity made by Volta, AmpHata! Yer iHata! Yer işşaretiareti

tan

tanıımlanmammlanmamıışş..ere, Gains, Faraday, Hertz and others raised fundamentalere, Gains, Faraday, Hertz and others raised fundamental

problems concerning the nature of matter. problems concerning the nature of matter.

The first breakthrough came in 1897, when Sir J.J

The first breakthrough came in 1897, when Sir J.J . Thompson discovered the. Thompson discovered the electron, a discovery soon

electron, a discovery soon verified by other verified by other investigators. investigators. In 1913 BoIn 1913 Bohr evolved thehr evolved the basic theory of atomic s

basic theory of atomic structure, and that theory has been developed to our presenttructure, and that theory has been developed to our present concept of the nature of matter.

concept of the nature of matter. The electrical characteristics of

The electrical characteristics of an atom are determined by how tightly the nucleusan atom are determined by how tightly the nucleus holds on to its outer ele

holds on to its outer electrons. ctrons. If the outer electrons are eaIf the outer electrons are easily removed from thesily removed from the atom, the material will cond

atom, the material will conduct easily and is known as a uct easily and is known as a conductor. conductor. If the outerIf the outer electrons are difficult to dislodge from their orbits, the material is known as an electrons are difficult to dislodge from their orbits, the material is known as an insulator.

insulator.

The material used in diodes and transistors is

The material used in diodes and transistors is known as 'semi-conductor' material.known as 'semi-conductor' material. One of the attributes of this

One of the attributes of this material is that the number of material is that the number of free electrons in any givenfree electrons in any given area can be fixed during the manufacturing process.

area can be fixed during the manufacturing process. Interest in semi-conductors began in 1873, when it

Interest in semi-conductors began in 1873, when it was discovered that thewas discovered that the resistance of rods and wires of selen

resistance of rods and wires of selenium decreased as they weium decreased as they were heated. re heated. This wasThis was surprising because the resistance of metals

surprising because the resistance of metals normally increased with an increase innormally increased with an increase in temperature.

temperature. Furthermore, some lowering Furthermore, some lowering of resistance was noted wheof resistance was noted when the rodsn the rods were exposed to

were exposed to light. light. Later investigations founLater investigations found similar effects in d similar effects in other materials,other materials, but the change in resistance was so

but the change in resistance was so small that no practical applications could besmall that no practical applications could be found.

found.

By 1906 a number of crystalline semi-conductors

By 1906 a number of crystalline semi-conductors were being used as radio signalwere being used as radio signal detectors, but the introduction of

detectors, but the introduction of thermionic valves put an end thermionic valves put an end to them. to them. The valvesThe valves were more reliable and had the advantage of being able to amplify the signal as were more reliable and had the advantage of being able to amplify the signal as wellwell as detect it.

as detect it.

During the development of radar systems in WW

During the development of radar systems in WWΙΙΙΙ, it was discovered that valve type, it was discovered that valve type mixers would not operate

mixers would not operate at the high frequenat the high frequencies being used. cies being used. Research turned toResearch turned to semi-conductor type mixers, an

semi-conductor type mixers, and silicon proved d silicon proved the most successful. the most successful. After the war,After the war, the peculiar properties of Germanium and Silicon were rigorously investigated, and the peculiar properties of Germanium and Silicon were rigorously investigated, and a germanium diode detector was made and used extensively in radio and t

a germanium diode detector was made and used extensively in radio and t elevision.elevision. During development of the Germanium detectors an important discovery was

During development of the Germanium detectors an important discovery was made.made. It was found that when two very c

It was found that when two very close contacts are made with a lose contacts are made with a piece of germanium,piece of germanium, the current flow through one of the contacts

the current flow through one of the contacts affects the amount of current flowaffects the amount of current flow through the other.

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Bell Telephone Laboratories latched onto this phenomena and eventually in 1948phenomena and eventually in 1948 they announced the manufacture of the first solid-state

they announced the manufacture of the first solid-state amplifying device, theamplifying device, the transistor.

transistor. This triggered This triggered renewed interest in renewed interest in semi-conductor diodes, resulting semi-conductor diodes, resulting in thein the development of a huge variety of semi-conductor devices

development of a huge variety of semi-conductor devices that we now take forthat we now take for granted.

granted. 1.1

1.1 RECTIFIER RECTIFIER DIODESDIODES

 A rectifier diode is the electrical equivalent of a one way valve, it is a semiconductor  A rectifier diode is the electrical equivalent of a one way valve, it is a semiconductor

device which allows current to f

device which allows current to flow in one direction but not in the low in one direction but not in the other.other. When conducting, the diode is said to be

When conducting, the diode is said to be 'forward biased''forward biased'. Under these conditions. Under these conditions the diode offers little resistance to current flow.

the diode offers little resistance to current flow. When opposing current flow, the diode is said t

When opposing current flow, the diode is said t o beo be 'reverse biased''reverse biased'. Under reverse. Under reverse biased conditions the diode has a high resistance.

biased conditions the diode has a high resistance.

1.1.1

1.1.1 CIRCUIT SYMBOLS & IDENTIFICATIONCIRCUIT SYMBOLS & IDENTIFICATION

The various symbols used for diodes are

The various symbols used for diodes are shown below.shown below.

Whether the triangles are filled or unfilled depends only on t

Whether the triangles are filled or unfilled depends only on t he drawing officehe drawing office preference.

preference. Where it is considered necessary, it is poWhere it is considered necessary, it is possible to show that one of thssible to show that one of thee electrodes is connected to the case

electrodes is connected to the case of the device by adding a dot tof the device by adding a dot to the symbol, buto the symbol, but this is

this is not often not often used. used. In every In every symbol,symbol, the arrow indicates the direction ofthe arrow indicates the direction of conventional current flow

conventional current flow..

The base of the triangle is the end where conventional current

The base of the triangle is the end where conventional current enters the diode, thisenters the diode, this end is called the ano

end is called the anode. de. The end througThe end through which current leaves the dh which current leaves the diode is theiode is the cathode.

cathode. In some cases the arrow symbol In some cases the arrow symbol is marked on the diode, is marked on the diode, where it is not,where it is not, the cathode is identified by a band or

the cathode is identified by a band or distinctive shape as shown above.distinctive shape as shown above. Two identification codes are

Two identification codes are used for diodes. used for diodes. In the American system the codeIn the American system the code always starts with 1N and is followe

always starts with 1N and is followed by a serial numbed by a serial number, i.e. 1N4001. r, i.e. 1N4001. In theIn the continental system, the first letter gives the semiconductor material; A for continental system, the first letter gives the semiconductor material; A for germanium; B for silicon, and the second letter

germanium; B for silicon, and the second letter identifies the use; A - identifies the use; A - signal diode; Ysignal diode; Y - rectifier diode and Z for

Most semiconductor diodes are made from silicon or

Most semiconductor diodes are made from silicon or germanium, these twogermanium, these two materials have different operating characteristics, although the principle

materials have different operating characteristics, although the principle of operationof operation and circuit symbols are both the same.

and circuit symbols are both the same.

1.1.2.1 Biasing 1.1.2.1 Biasing

 A diode is said to be 'biased' when a voltage is applied between th

 A diode is said to be 'biased' when a voltage is applied between the terminals suche terminals such that the diode operates as required.

that the diode operates as required.

 An external voltage applied so that the anode is positive and the catho

 An external voltage applied so that the anode is positive and the cathode negative isde negative is called '

called 'forward biasforward bias'. '. There are There are many ways of many ways of achieving this, for achieving this, for example:example:

•• Connect Connect the the anode anode to to +3V +3V and and the the cathode cathode to 0to 0V.V.

•• Connect Connect the the anode anode to to +1V +1V and and the the cathode cathode to -1to -1V.V.

•• Connect Connect the the anode anode to -50to -50V V and and the the cathode cathode to to -52V.-52V. So far as the diode is

So far as the diode is concerned, it is the voltage of concerned, it is the voltage of the anode with respect to thethe anode with respect to the cathode which determines the bias.

cathode which determines the bias. If the voltage is applied so

If the voltage is applied so that the anode is negative with respect to that the anode is negative with respect to the cathode,the cathode, the diode is ‘

the diode is ‘reverse biasedreverse biased’, again, there are many ways of achieving this.’, again, there are many ways of achieving this.

The forward voltage required to make the diode conduct depends on the material it The forward voltage required to make the diode conduct depends on the material it is made from.

is made from. Germanium diodes reqGermanium diodes require a voltage of auire a voltage of approximately 0.1 to 0.2 voltspproximately 0.1 to 0.2 volts and silicon diodes 0.6 to 0.7 volts.

and silicon diodes 0.6 to 0.7 volts.

1.1.2.2

1.1.2.2 Forward Forward Voltage Voltage DropDrop

Ideally a diode should have zero resistance when conducting and should cause no Ideally a diode should have zero resistance when conducting and should cause no voltage drop, unfortunately this do

voltage drop, unfortunately this does not happen. es not happen. Germanium diodes create aGermanium diodes create a voltage drop of approximately 0.6V and silicon diodes a

voltage drop of approximately 0.6V and silicon diodes a drop of approximately 1.1V.drop of approximately 1.1V. This needs to be taken into account when doing circ

This needs to be taken into account when doing circ uit calculations.uit calculations.

1.1.2.3

1.1.2.3 Reverse Reverse Leakage Leakage CurrentCurrent

When a diode is reverse biased, it

When a diode is reverse biased, it should ideally have infinite resistance and noshould ideally have infinite resistance and no current should flow.

current should flow. Unfortunately when a Unfortunately when a diode is reverse biased, a smadiode is reverse biased, a small currentll current called 'reverse leakage current' flows, generally this is too small to be of

called 'reverse leakage current' flows, generally this is too small to be of significance, however, it should be noted that t

significance, however, it should be noted that t he value of this current increases he value of this current increases withwith an increase in diode

an increase in diode temperature. temperature. The reverse current of silicon diodeThe reverse current of silicon diodes is muchs is much smaller than that of germanium diodes, (approx. one thousandth), therefore silicon smaller than that of germanium diodes, (approx. one thousandth), therefore silicon diodes can be used more successfully

diodes can be used more successfully at high temperatures (150º - 200ºC) thanat high temperatures (150º - 200ºC) than germanium diodes (80º - 100ºC).

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1.1.2.4

1.1.2.4 Reverse Reverse Breakdown Breakdown VoltageVoltage

If the reverse bias v

If the reverse bias voltage is increased, eventually the diode breaks down andoltage is increased, eventually the diode breaks down and current flows in the wrong d

current flows in the wrong direction through the diodirection through the diode. e. This causes permanentThis causes permanent damage and the diode has to be replaced.

damage and the diode has to be replaced.

The breakdown voltage can have any value from a few volts

The breakdown voltage can have any value from a few volts , up to 1000V for silicon, up to 1000V for silicon diodes and 100V for germanium, depending on the construction and forms of

diodes and 100V for germanium, depending on the construction and forms of material used.

material used. The maximum re

The maximum reverse voltage is averse voltage is an important diode n important diode characteristic. characteristic. Under normalUnder normal conditions this value should not be exc

conditions this value should not be exceeded.eeded.

1.1.2.5

1.1.2.5 Graphical Graphical RepresentationRepresentation

Shown below is a graphical representation of the operating characteristics Shown below is a graphical representation of the operating characteristics of aof a typical silicon and germanium diode.

typical silicon and germanium diode.

1.1.3

1.1.3 PARALLEL & SERIAL ARRANGEMPARALLEL & SERIAL ARRANGEMENTS OF DIODESENTS OF DIODES

It is possible to operate silicon rectifier diodes in parallel or in series to provide It is possible to operate silicon rectifier diodes in parallel or in series to provide respectively, higher current or higher voltage capabilities.

respectively, higher current or higher voltage capabilities.

1.1.3.1

1.1.3.1 Parallel Parallel ArrangementArrangementss

In parallel arrangements used for higher currents, some method must be

In parallel arrangements used for higher currents, some method must be used toused to ensure that the current divides eq

ensure that the current divides equally through the individuaually through the individual diodes. l diodes. This is difficultThis is difficult to do.

Series arrangements can be used if the applied voltage is

Series arrangements can be used if the applied voltage is greater than thegreater than the maximum rated value of

maximum rated value of a single diode. a single diode. Some method must be Some method must be used to ensure theused to ensure the applied voltage

applied voltage divides equally amondivides equally among the individual g the individual diodes. diodes. Resistors or capacitorsResistors or capacitors in parallel can be used in an effort to achieve this.

in parallel can be used in an effort to achieve this.

1.1.4 RECTIFICATION 1.1.4 RECTIFICATION

Rectifier diodes are designed

Rectifier diodes are designed to convert ac to dc. to convert ac to dc. To do this effectively andTo do this effectively and efficiently they must have:

efficiently they must have:

•• Low Low resistance resistance to to current current flow flow in in the the forward forward direction.direction.

•• High High resistance resistance to to current current flow flow in in the the reverse reverse direction.direction.

 Almost all semiconductor rectifier diodes are silicon, junction types.

 Almost all semiconductor rectifier diodes are silicon, junction types. The symbolThe symbol used in circuit diagrams can be any

used in circuit diagrams can be any of those shown earlier in the notes.of those shown earlier in the notes.

1.1.4.1

1.1.4.1 Basic Basic Rectifier Rectifier CircuitCircuit

 A basic rectifier circuit is shown below.

 A basic rectifier circuit is shown below. The diode is inserted in series between theThe diode is inserted in series between the a.c. supply and the load.

a.c. supply and the load.

The diode only p

The diode only passes current when forward biaseasses current when forward biased. d. Thus when an aThus when an a.c. signal is.c. signal is applied, pulses of uni-directional (d.c.) v

applied, pulses of uni-directional (d.c.) voltage are developed across the output loadoltage are developed across the output load resistance.

resistance.

Note from the diagrams that the d.c.

Note from the diagrams that the d.c. polarity can be reversed by reversing polarity can be reversed by reversing the diodethe diode connections.

connections.

If the average value of ½ wave

If the average value of ½ wave rectified a.c. is calculated irectified a.c. is calculated it will be found to be 32%t will be found to be 32% of the peak value of the output voltage.

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1.1.4.2

1.1.4.2 Centre Centre Tap Tap Full Full Wave Wave RectifierRectifier

In full wave rectification, both halves of every cycle of input voltage produce current In full wave rectification, both halves of every cycle of input voltage produce current pulses through the load resistor.

pulses through the load resistor.

In the circuit shown above, two

In the circuit shown above, two diodes Ddiodes D11 and D and D22 and a transformer with a centre- and a transformer with a

centre-tapped secondary are used. tapped secondary are used.

During the positive half cycle of the input waveform, A is positive with respect to O During the positive half cycle of the input waveform, A is positive with respect to O and D

and D11 conducts, the current flowing top to  conducts, the current flowing top to bottom through the bottom through the load resistor. load resistor. DuringDuring

this time diode D

this time diode D22 is reversed biased and does not  is reversed biased and does not conduct.conduct.

During the negative half cycle of the input waveform, B is positive with respect to O During the negative half cycle of the input waveform, B is positive with respect to O and D

and D22 conducts, the current again flowing top  conducts, the current again flowing top to bottom through the load resistor.to bottom through the load resistor.

During this time diode D

During this time diode D11 is reverse biased and does not conduct. is reverse biased and does not conduct.

In effect, the circuit consists of two half wave rectifiers working into the same load on In effect, the circuit consists of two half wave rectifiers working into the same load on alternate half cycles of the input.

alternate half cycles of the input. The current through R The current through R is in the same directionis in the same direction during both half cycles and a fluctuating d.c. is created across R.

during both half cycles and a fluctuating d.c. is created across R. The average value of this full wave

The average value of this full wave rectified a.c. is 64% of rectified a.c. is 64% of the peak value of thethe peak value of the voltage across the load resistor R.

voltage across the load resistor R.

The output frequency is double that of the input frequency. The output frequency is double that of the input frequency.

1.1.4.3

1.1.4.3 Full Full Wave Wave Bridge Bridge RectifierRectifier

The circuit of a Full Wave B

The circuit of a Full Wave Bridge rectifier is shown below. ridge rectifier is shown below. The rectifier has 4 dThe rectifier has 4 diodesiodes as opposed to 2 and does not have a c

as opposed to 2 and does not have a centre tapped transformer.entre tapped transformer.

During the positive half cycle diodes D1

During the positive half cycle diodes D1 and D2 conduct, the current flowing top toand D2 conduct, the current flowing top to bottom through the load R

bottom through the load RLL. . During the neDuring the negative half cycle D3 gative half cycle D3 and D4 conand D4 conduct, theduct, the

current again flowing top

current again flowing top to bottom through the to bottom through the load. load. The output from this rectifier isThe output from this rectifier is the same as that obtai

the same as that obtained from the centred ned from the centred tapped transformer type. tapped transformer type. The averageThe average value again being 64% of the peak voltage across the

value again being 64% of the peak voltage across the load resistor.load resistor. It should be noted that in this rectifier, the peak voltage across R

It should be noted that in this rectifier, the peak voltage across RLL is equal to the is equal to the

whole of the secondary transformer output voltage, whereas in t

whole of the secondary transformer output voltage, whereas in t he previous rectifier,he previous rectifier, the peak voltage across R

the peak voltage across RLL is only half the transformer secondary voltage. is only half the transformer secondary voltage.

1.1.4.4 Smoothing 1.1.4.4 Smoothing

The rectifier

The rectifier circuits previously discussed circuits previously discussed produce pulsating produce pulsating d.c. outputs. d.c. outputs. AA smoothing circuit changes these outputs into a s

smoothing circuit changes these outputs into a s teady d.c. voltage level.teady d.c. voltage level.

1.1.4.4.1

1.1.4.4.1 Half Half Wave Wave RectifierRectifier

The diagram below shows a simple half wave rectifier

The diagram below shows a simple half wave rectifier with a reservoir capacitor, C,with a reservoir capacitor, C, connected in parallel with the load R

connected in parallel with the load RLL. . The capThe capacitor charges acitor charges towards the towards the peakpeak

value of the input voltage whenever the input voltage is

value of the input voltage whenever the input voltage is greater than Vgreater than VCC and the and the

diode is condu

diode is conducting. cting. When the inWhen the input voltage is leput voltage is less than Vss than VCC the diode cuts-off and the diode cuts-off and

the capacitor discharges through the load. the capacitor discharges through the load.

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This results in a mean d.c. output level less than the peak of output level less than the peak of the input, with a ripplethe input, with a ripple superimposed at the input frequency.

superimposed at the input frequency.

1.1.4.4.2

1.1.4.4.2 Full Full Wave Wave RectifierRectifier

The diagram above shows a centre tapped full wave rectif

The diagram above shows a centre tapped full wave rectif ier with a reservoirier with a reservoir capacitor.

capacitor. The charge is now The charge is now topped up twice dutopped up twice during each cycle of the inpring each cycle of the inputut waveform which results in:

waveform which results in:

•• A lower A lower amplitude amplitude ripple, at ripple, at twice the twice the frequency of frequency of that from that from the hathe half wavelf wave rectifier.

 A measure of the amount of ripple present at the output of a d.c. supply is given by  A measure of the amount of ripple present at the output of a d.c. supply is given by

the ripple factor, which is

the ripple factor, which is usually expressed as a percentage and defined as:usually expressed as a percentage and defined as: Ripple

Ripple factor factor == Hata!Hata! ××  100%  100%

1.1.4.6

1.1.4.6 Peak Peak Inverse Inverse VoltageVoltage

The peak voltage across a rectifier

The peak voltage across a rectifier diode in the reverse direction is known as diode in the reverse direction is known as thethe 'peak inverse voltage'.

'peak inverse voltage'. In a half In a half wave rectifier with wave rectifier with a reservoir capacitor, the a reservoir capacitor, the peakpeak inverse voltage is twice the amplitud

inverse voltage is twice the amplitude of the peak voltage e of the peak voltage across the load. across the load. i.e. meani.e. mean d.c. level to maximum negative pe

d.c. level to maximum negative peak. ak. The diode must bThe diode must be able to withstand the able to withstand thisis voltage without breaking down.

voltage without breaking down.

1.1.4.7

1.1.4.7 Voltage Voltage RegulationRegulation

Voltage regulation is a measure of the ability of

Voltage regulation is a measure of the ability of a power supply to provide ana power supply to provide an increased load without a fall in output voltage.

increased load without a fall in output voltage. Regulation =

Regulation = Hata!Hata! ××  100%  100%

1.1.4.8

1.1.4.8 Filter Filter CircuitsCircuits

Smaller ripple factors and improved voltage regulation is obtained by using R-C Smaller ripple factors and improved voltage regulation is obtained by using R-C andand L-C filter circuits across the output of the rectifier.

L-C filter circuits across the output of the rectifier.

1.1.4.9

1.1.4.9 Ripple Ripple FrequencyFrequency

The ripple frequency on the d.c. output from

The ripple frequency on the d.c. output from a half wave rectifier is a half wave rectifier is equal to theequal to the supply frequency.

supply frequency. For a full wave rectifier, the rippFor a full wave rectifier, the ripple frequency is double le frequency is double the supplythe supply frequency.

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1.2

1.2 SIGNAL SIGNAL DIODESDIODES

Signal diodes are used to detect radio signals (a process similar to rectification in Signal diodes are used to detect radio signals (a process similar to rectification in which radio frequency a.c. is converted to d.c.), because of their very low

which radio frequency a.c. is converted to d.c.), because of their very low capacitance.

capacitance. A capacitor passes a.c. The higA capacitor passes a.c. The higher the frequency of the her the frequency of the a.c. and thea.c. and the greater the capacitance, the le

greater the capacitance, the less opposition it offers. ss opposition it offers. At radio frequencies, a nAt radio frequencies, a normalormal diode would be

diode would be of little use as a detector bof little use as a detector because of its large junction areecause of its large junction area. a. TheThe large junction area resulting in a large capacitance v

large junction area resulting in a large capacitance v alue and little opposition toalue and little opposition to current flow.

current flow.

 A point diode type signal diode has a very small junction area resulting in a low  A point diode type signal diode has a very small junction area resulting in a low

value of capacitance and a large opposition to current f value of capacitance and a large opposition to current f low.low. Germanium is used for signal diodes since it

Germanium is used for signal diodes since it has a lower 'turn-on' voltage thanhas a lower 'turn-on' voltage than silicon, and so lower signal

silicon, and so lower signal voltages start it conducting in the fvoltages start it conducting in the forward direction.orward direction. 1.3

1.3 ZENER ZENER DIODESDIODES

In an ordinary diode, if the reverse

In an ordinary diode, if the reverse bias is increased, the diode breaks down and thebias is increased, the diode breaks down and the diode suffers permanent d

diode suffers permanent damage. amage. A zener diode is designeA zener diode is designed to be used in thd to be used in thee breakdown region.

breakdown region. The zener diode The zener diode looks like a rectifier diode, the catholooks like a rectifier diode, the cathode oftende often being marked

being marked by a bandby a band. . Its symbol is shown Its symbol is shown above.above. From the characteristic graph, it

From the characteristic graph, it can be seen that the reverse current can be seen that the reverse current is negligible asis negligible as the reverse bias is increased until

the reverse bias is increased until the breakdown voltage is reached, then itthe breakdown voltage is reached, then it suddenly increases.

suddenly increases. The breakdown The breakdown voltage is called thvoltage is called the zener or ree zener or referenceference voltage.

over a wide range o

over a wide range of reverse currents. f reverse currents. It is this property of a zener dIt is this property of a zener diode that makesiode that makes it useful in stabilised power supplies.

it useful in stabilised power supplies. To limit the reverse current

To limit the reverse current at breakdown and prevent overheating, the power ratingat breakdown and prevent overheating, the power rating of the diode mu

of the diode must not be exceeded. st not be exceeded. This is achieved by using a resistor in seriesThis is achieved by using a resistor in series with the diode.

with the diode. 1.4

1.4 LIGHT LIGHT EMITTING EMITTING DIODESDIODES

 A light emitting diode is a specially constructed and doped diode type device which  A light emitting diode is a specially constructed and doped diode type device which

emits light when operated

emits light when operated in the forward bias conin the forward bias condition. dition. The colour of light emittedThe colour of light emitted depends on the semi-conductor material used.

depends on the semi-conductor material used. Gallium

Gallium arsenide arsenide phosphide phosphide - - red red lightlight Gallium

Gallium phosphide phosphide - - green green lightlight Symbols used are similar to the

Symbols used are similar to the photodiode.photodiode.

Unless an LED is of t

Unless an LED is of the constant current type, which incorporates an integratedhe constant current type, which incorporates an integrated circuit regulator, it must have an external resistor connected in series to limit the circuit regulator, it must have an external resistor connected in series to limit the forward current which typically may only be

forward current which typically may only be 10mA. 10mA. The voltage drop aThe voltage drop across across a conducting LED is about 1 to7 volts.

conducting LED is about 1 to7 volts.

In seven segment LED displays, each segment is

In seven segment LED displays, each segment is a separate LED and depending ona separate LED and depending on which segments are en

which segments are energised, the display lights up the ergised, the display lights up the number 0 to 9. number 0 to 9. SuchSuch displays are usually designed to operate from a

displays are usually designed to operate from a 5V supply - each segment needs a5V supply - each segment needs a separate current limiting resistor and all

separate current limiting resistor and all the cathodes or anodes are joined togetherthe cathodes or anodes are joined together to form a common connection.

to form a common connection. 1.5 PHOTOCELLS

1.5 PHOTOCELLS Photocells change

Photocells change light into electrical signalight into electrical signals. ls. There are There are two basic types,two basic types, Photoconductive cells and Photovoltaic cells.

Photoconductive cells and Photovoltaic cells.

1.5.1

1.5.1 PHOTOCONDUCPHOTOCONDUCTIVE CELLSTIVE CELLS

The resistance of certain semiconductors decreases as the intensity The resistance of certain semiconductors decreases as the intensity of light falling o

of light falling on them increases. n them increases. They are therefoThey are therefore light sensitivere light sensitive resistors and sometimes referred to as light dependent resistors. resistors and sometimes referred to as light dependent resistors.

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1.5.2

1.5.2 PHOTOVOLTAIC CELLSPHOTOVOLTAIC CELLS

When illuminated,

When illuminated, a photovoltaic cell pa photovoltaic cell produces a voltage. roduces a voltage. If anIf an external circuit is connected

external circuit is connected to the cell, currento the cell, current flows through t flows through it. it. TheThe source of energy is the light.

source of energy is the light.

The voltage available depends on the material used, the intensity of

The voltage available depends on the material used, the intensity of the light and thethe light and the amount of current drawn

amount of current drawn from the cell. from the cell. For a silicon cell in full sunlighFor a silicon cell in full sunlight the voltaget the voltage on open circuit is 0.45V.

on open circuit is 0.45V. With a maximum current of 3With a maximum current of 35mA for each square cm o5mA for each square cm off cell.

cell. Only about 1Only about 10% of the 0% of the light is turned into light is turned into electrical energy.electrical energy. 1.6 PHOTODIODES

1.6 PHOTODIODES

Photodiodes are

Photodiodes are operated undeoperated under reverse bias conditionr reverse bias conditions. s. TheThe leakage current increasing in proportion to the amount of light

leakage current increasing in proportion to the amount of light fallingfalling on the d

on the device. evice. Photodiodes are Photodiodes are used as fast couused as fast counters and lightnters and light meters.

meters.

1.7

1.7 VARACTOR VARACTOR DIODEDIODE

 A varactor diode is a special type of diode constructed to act as a voltage controlled  A varactor diode is a special type of diode constructed to act as a voltage controlled

capacitor.

capacitor. It is also It is also known as known as a varicap a varicap diode. diode. The diode The diode is operated is operated under reverseunder reverse bias conditions, with an increase in

bias conditions, with an increase in bias decreasing the value bias decreasing the value of capacitance. of capacitance. TheThe circuit symbols are as shown below.

circuit symbols are as shown below.

There are 3 main uses for varactor

There are 3 main uses for varactor diodes:diodes:

•• As As remotely remotely controlled controlled capacitors capacitors in in RF RF tuned tuned circuits.circuits.

•• As As variable variable capacitors capacitors in in amplifiers.amplifiers.

Silicon controlled rectifiers (SCR's) are now more commonly known as thyristors. Silicon controlled rectifiers (SCR's) are now more commonly known as thyristors. They are semiconductor devices which rectify

They are semiconductor devices which rectify a.c. and control the power supplied toa.c. and control the power supplied to a load in a wa

a load in a way that wastes very little energy. y that wastes very little energy. They are commonly used They are commonly used inin household lighting

household lighting dimmer switches. dimmer switches. The general The general symbol is shown besymbol is shown below, togetherlow, together with the symbol for 'P' and 'N' types.

with the symbol for 'P' and 'N' types.

SCR's normally block the flow of

SCR's normally block the flow of current in both directions, but can be tcurrent in both directions, but can be triggered soriggered so as to allow current to f

as to allow current to flow in the forward direction as in a low in the forward direction as in a normal diode, whilst stillnormal diode, whilst still blocking current flow

blocking current flow in the reverse in the reverse direction. direction. In the triggereIn the triggered condition thed condition the characteristics are similar to rectifier diodes.

characteristics are similar to rectifier diodes.

 An SCR will continue to conduct until the load current is reduced to zero, or until it is  An SCR will continue to conduct until the load current is reduced to zero, or until it is

reverse biased, when it automatically returns to

reverse biased, when it automatically returns to the blocking state.the blocking state. The SCR is triggered

The SCR is triggered by applying a pulse to by applying a pulse to a third terminal called the a third terminal called the gate. gate. TheThe duration of the pulses can be extremely

duration of the pulses can be extremely short.short. 1.9 TRANSISTORS

1.9 TRANSISTORS Transistors are the

Transistors are the most important device most important device in electronics today. in electronics today. Not only are Not only are theythey made as discrete components, but integrated circuits

made as discrete components, but integrated circuits may contain severalmay contain several thousands on a tiny slice o

thousands on a tiny slice of silicon. f silicon. They are 3 terminal They are 3 terminal devices used as amplifiersdevices used as amplifiers and as switches, and are classed as active devices.

and as switches, and are classed as active devices. Hundreds of differen

Hundreds of different transistors are available. t transistors are available. The same idThe same identification code is uentification code is usedsed as for diodes, but in the American system transistors always start with 2N followed as for diodes, but in the American system transistors always start with 2N followed by a numb

by a number. er. In the continenIn the continental system the first letter gtal system the first letter gives the semiconductorives the semiconductor material and the second letter gives the

material and the second letter gives the use:use:

•• C C indicates indicates an an audio audio frequency frequency device.device.

•• F F a a radio radio frequency frequency device.device.

•• S S a a switching switching transistor.transistor.

 An example being BC108, a silicon audio frequ

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The two basic types of transistors are:

•• The bipolaThe bipolar or junction r or junction transistor.transistor.

•• The unipoThe unipolar or field lar or field effect transistor.effect transistor. In this element of the course

In this element of the course we concentrate on bipolar transistors, of we concentrate on bipolar transistors, of which therewhich there are two basic types.

are two basic types. The NPN and The NPN and the PNP, both of whthe PNP, both of which are active devicesich are active devices having three terminals labelled; Base, Collector and Emitter.

having three terminals labelled; Base, Collector and Emitter.

1.9.1

1.9.1 NPN TRANSISTORNPN TRANSISTOR

NPN transistors are made from 3 pieces

NPN transistors are made from 3 pieces of semi-conductor material joined togetherof semi-conductor material joined together in a

in a manner similar manner similar to two to two diodes, as diodes, as shown in shown in the diagram the diagram below. below. Also shown Also shown isis the circuit diagram with each t

the circuit diagram with each terminal identified.erminal identified.

If the base is made positive with respect to the collector, the diode, or

If the base is made positive with respect to the collector, the diode, or junction junction as it as it is called, is f

is called, is forward biased and current flows (conventional current flows frorward biased and current flows (conventional current flows fr om baseom base to collector).

to collector).

If the base is made positive

If the base is made positive with respect to the emitter, with respect to the emitter, again the junction (diode) isagain the junction (diode) is forward biased and conventional current flows from base to

forward biased and conventional current flows from base to emitter.emitter.

If the collector is made positive with respect to the emitter, or the emitter is made If the collector is made positive with respect to the emitter, or the emitter is made positive with respect to the collector no current will flow, because in either direction positive with respect to the collector no current will flow, because in either direction one of the junctions (diodes) is reverse biased and will prevent current flow.

one of the junctions (diodes) is reverse biased and will prevent current flow. The last three paragraphs should be noted, as their contents

The last three paragraphs should be noted, as their contents is invaluable when itis invaluable when it comes to determinin

comes to determining the terminals g the terminals and testing transistors. and testing transistors. This will be This will be discusseddiscussed later.

1.9.1.1

1.9.1.1 NPN NPN Transistor Transistor as as a a switchswitch

If the NPN transistor is If the NPN transistor is

connected as shown, it can be connected as shown, it can be used as a switch.

used as a switch.

In this diagram the transistor is In this diagram the transistor is being used to turn on a lamp, it being used to turn on a lamp, it could however be used to

could however be used to

operate any type of d.c. device operate any type of d.c. device such

such as a as a relay, solerelay, solenoid,noid, another transistor or an LED. another transistor or an LED.

When the base is made positive with respect

When the base is made positive with respect to the emitter, the junction is fto the emitter, the junction is forwardorward biased and current flows in

biased and current flows in through the base athrough the base and out of the emitter. nd out of the emitter. The flow ofThe flow of current from base to emitter creates a reaction in the transistor that causes the current from base to emitter creates a reaction in the transistor that causes the reverse biased collector/base junction to b

reverse biased collector/base junction to break down and condreak down and conduct. uct. Current can thenCurrent can then flow from the battery positive terminal,

flow from the battery positive terminal, through the lamp, through the reverse biasedthrough the lamp, through the reverse biased collector base junction, through the forward biased base emitter

collector base junction, through the forward biased base emitter junction and back tojunction and back to the battery, illuminating the lamp.

the battery, illuminating the lamp.

When the base is made sufficiently positive

When the base is made sufficiently positive with respect to the emitter (with respect to the emitter (approx. 0.6Vapprox. 0.6V for silicon, 0.2V for germanium) so that current flows from collector to emitter

for silicon, 0.2V for germanium) so that current flows from collector to emitter through the transistor, the transistor is said to be switched or turned 'ON'. through the transistor, the transistor is said to be switched or turned 'ON'. If the base / emitter

If the base / emitter potential is reduced below the switch 'ON' potential is reduced below the switch 'ON' potential, or removedpotential, or removed totally, the collector / base junction will return to its reverse bias condition and will totally, the collector / base junction will return to its reverse bias condition and will prevent current flowing around

prevent current flowing around the circuit through the lamthe circuit through the lamp. p. Under these conditionsUnder these conditions the transistor is said to be switched or turned 'OFF'.

the transistor is said to be switched or turned 'OFF'. If should be noted that it may

If should be noted that it may be necessary to limit tbe necessary to limit the current through the transistorhe current through the transistor when it is switched on, this can be achieved by a series resistor as in the LED

when it is switched on, this can be achieved by a series resistor as in the LED circuit.

circuit.

1.9.1.2

1.9.1.2 NPN NPN Transistor Transistor as as an an amplifieramplifier

When the base is made positive with respect to the emitter so that the transistor is When the base is made positive with respect to the emitter so that the transistor is switched 'ON', the amount of b

switched 'ON', the amount of base emitter current required is vease emitter current required is very small. ry small. If the baseIf the base / emitter current is increased slightly, by increasing the base emitter voltage, the / emitter current is increased slightly, by increasing the base emitter voltage, the transistor will turn 'ON' more, its effective resistance will decrease and the collector / transistor will turn 'ON' more, its effective resistance will decrease and the collector / emitter current will increase.

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If the base / emitter current is decreased If the base / emitter current is decreased slightly by reducing the base / emitter slightly by reducing the base / emitter voltage, the transistor will turn 'OFF' more, voltage, the transistor will turn 'OFF' more, it's effective resistance will increase and the it's effective resistance will increase and the collector / emitter current will decrease. collector / emitter current will decrease.

The transistor can therefore b

The transistor can therefore be likened to a variable ree likened to a variable resistor. sistor. As the base / emitterAs the base / emitter bias increases, the resistance of the transistor effectively decreases and more bias increases, the resistance of the transistor effectively decreases and more current flows from collector

current flows from collector to emitter. to emitter. The change The change in current and in current and resistance causesresistance causes the output voltage to decrease.

the output voltage to decrease.

 As the base / emitter bias decreases, the effective resistance of the transistor  As the base / emitter bias decreases, the effective resistance of the transistor

increases and less

increases and less current flows from collector current flows from collector to emitter. to emitter. The change The change in currentin current and resistance now causes the output voltage to

and resistance now causes the output voltage to increase.increase.

When set up correctly, millivolt changes across the base / emitter junction produce When set up correctly, millivolt changes across the base / emitter junction produce changes at the output of 10's or even

changes at the output of 10's or even 100's of volts, depending on the collector100's of volts, depending on the collector voltage.

voltage.

If a small sinusoidal a.c. signal is applied to the base / emitter junction, the bias will If a small sinusoidal a.c. signal is applied to the base / emitter junction, the bias will vary sinusoidally as will the resistance of the transistor and the output voltage, vary sinusoidally as will the resistance of the transistor and the output voltage,

however the output voltage will vary sinusoidally 10's of volts for millivolt changes in however the output voltage will vary sinusoidally 10's of volts for millivolt changes in the input signa

the input signal. l. (Using the ex(Using the example voltage in ample voltage in the diagram).the diagram).

It should be noted, that although the changes in output voltage are

It should be noted, that although the changes in output voltage are much greatermuch greater than the changes in input voltage,

than the changes in input voltage, the bipolar transistor is a the bipolar transistor is a current devicecurrent device.. Small changes in base / emitter current result in large changes in collector / emitter Small changes in base / emitter current result in large changes in collector / emitter current.

current. It is these changes in collect / emIt is these changes in collect / emitter current that produce the laitter current that produce the large outputrge output voltage swings.

voltage swings.

1.9.2

1.9.2 PNP TRANSISTORPNP TRANSISTOR

PNP transistors are made in a similar manner to NPN transistors, except the PNP transistors are made in a similar manner to NPN transistors, except the direction of the junctions is reversed.

direction of the junctions is reversed.

If the base is made negative with respect

biased and current flows. biased and current flows.

Current cannot flow between collector and emitter, because irrespective

Current cannot flow between collector and emitter, because irrespective of the biasof the bias applied, one junction will be reverse biased.

applied, one junction will be reverse biased.

 Again these three statements are worth remembering wh

 Again these three statements are worth remembering when it comes to determiningen it comes to determining the terminals and testing transistors.

the terminals and testing transistors.

1.9.2.1

1.9.2.1 PNP PNP Transistor Transistor as as a a switchswitch

When connected as shown, the PNP When connected as shown, the PNP transistor can also be used as a transistor can also be used as a switch, however, for the transistor to switch, however, for the transistor to be tuned 'ON', the base must be be tuned 'ON', the base must be made negative with respect to the made negative with respect to the emitter.

emitter. For a For a silicon transistor silicon transistor thethe base needs to be about 0.6V

base needs to be about 0.6V

negative with respect to the emitter, negative with respect to the emitter, for a germanium transistor 0.2V for a germanium transistor 0.2V negative.

negative.

Once turned 'ON', conventional current flows from

Once turned 'ON', conventional current flows from the emitter to the collector, the emitter to the collector, whichwhich is in the opposite direction to that in the NPN transistor.

is in the opposite direction to that in the NPN transistor.

1.9.2.2

1.9.2.2 PNP PNP Transistor Transistor as as an an amplifieramplifier

The PNP transistor can a

The PNP transistor can also be used an also be used an amplifier. mplifier. It operates in a similar manner toIt operates in a similar manner to the NPN transistor except the t

the NPN transistor except the transistor must be turned 'ON' by ransistor must be turned 'ON' by making the basemaking the base negative with respect to the

negative with respect to the emitter, as seen above. emitter, as seen above. If the base / emitter potenIf the base / emitter potential istial is increased by making the base more negative with r

increased by making the base more negative with r espect to the emitter, theespect to the emitter, the transistor turns 'ON' more, its effective resistance decreases and more emitter / transistor turns 'ON' more, its effective resistance decreases and more emitter / collector current flows.

collector current flows. If the bias If the bias potential is decreased, potential is decreased, by making the by making the base lessbase less negative with respect to the emitter, the transistor turns 'OFF' slightly, the effective negative with respect to the emitter, the transistor turns 'OFF' slightly, the effective resistance increases and less emitter / collector current flows.

resistance increases and less emitter / collector current flows.

 A small sinusoidal signal applied to the base will vary the effective resistance of the  A small sinusoidal signal applied to the base will vary the effective resistance of the

transistor and produce much larger changes in the

transistor and produce much larger changes in the output voltage as with the NPNoutput voltage as with the NPN transistor.

transistor. Again Again it must it must be reabe realised that lised that the transistor the transistor is a is a current device. current device. TheThe small changes in base emitter bias potential cr

small changes in base emitter bias potential cr eated by the input signal results ineated by the input signal results in small changes in base emitter current, resulting in large changes in collector / small changes in base emitter current, resulting in large changes in collector / emitter current.

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1.10 TESTING SEMICONDUCTESTING SEMICONDUCTOR DEVICESTOR DEVICES

1.10.1 TESTING DIODES 1.10.1 TESTING DIODES

Diodes only conduct in one direction,

Diodes only conduct in one direction, it is therefore relatively easy it is therefore relatively easy to determine theto determine the terminals and serviceability using a multimeter,

terminals and serviceability using a multimeter, however, 2 points need noting:however, 2 points need noting:

•• When AVWhen AVO's are O's are used on used on a resistance a resistance range, the range, the black terminal black terminal is positive is positive withwith respect to the red terminal.

respect to the red terminal.

•• The poThe potential difference tential difference between between the red the red and blaand black terminals ck terminals of a of a digital meterdigital meter may be insufficient to forward bias

may be insufficient to forward bias a silicon diode (remember: requires 0.6V).a silicon diode (remember: requires 0.6V). This would indicate that the diode was non conducting in both direction

This would indicate that the diode was non conducting in both direction leading toleading to the false assumption that the diode is unservic

the false assumption that the diode is unservic eable.eable.

1.10.1.1

1.10.1.1 Determining Determining the the TerminalsTerminals

When forward biased, a diode has a resistance of

When forward biased, a diode has a resistance of approx. 1kapprox. 1kΩΩ. . When When reversereverse biased the resistance is in the

biased the resistance is in the order of megohms. order of megohms. To determine the terminaTo determine the terminals of als of a diode, it is simply a matter of connecting the meter across the diode to see if it will diode, it is simply a matter of connecting the meter across the diode to see if it will conduct, if it will

conduct, if it will not, the terminals should be reversed to confirm not, the terminals should be reversed to confirm conduction andconduction and serviceability.

serviceability. When conducting, the blaWhen conducting, the black terminal of an AVO, ock terminal of an AVO, or the red terminal ofr the red terminal of a digital meter, is connected to

a digital meter, is connected to the anode (flat end of symbol).the anode (flat end of symbol).

1.10.1.2

1.10.1.2 Confirming Confirming ServiceabilityServiceability

The serviceability of a diode is

The serviceability of a diode is determined by ensuring it has a resistance determined by ensuring it has a resistance in thein the order of 1K

order of 1KΩΩ in one direction and a resistance i in one direction and a resistance in the order of megohms in then the order of megohms in the opposite direction.

 As we have seen, transistors basically comprise 2 back-to-back diodes, therefore  As we have seen, transistors basically comprise 2 back-to-back diodes, therefore

the process of confirming the serviceability and determining the terminals is similar the process of confirming the serviceability and determining the terminals is similar to that used for diodes.

to that used for diodes.

1.10.2.1

1.10.2.1 Determining Determining the the BaseBase

The base of the transistor can be

The base of the transistor can be found by considering the transistor as two found by considering the transistor as two back-to- back-to-back diodes, and using a multimeter set on

back diodes, and using a multimeter set on ohms.ohms.

1.10.2.1.1

1.10.2.1.1 NPN NPN TransistorsTransistors

Connect the positive terminal of the meter

Connect the positive terminal of the meter to one of the three transistor terminals.to one of the three transistor terminals. Measure the resistance betw

Measure the resistance between this terminal and theen this terminal and the other two. e other two. If both indicate aIf both indicate a low resistance then the po

low resistance then the positive terminal is connected to the basitive terminal is connected to the base. se. If the resistanceIf the resistance to the other two terminals is

to the other two terminals is not low, the positive terminal is not low, the positive terminal is not connected to thenot connected to the base.

base. Connect the positive terminal of the Connect the positive terminal of the meter to another terminal anmeter to another terminal and repeat thed repeat the process until the base is

process until the base is determined.determined.

1.10.2.1.2

1.10.2.1.2 PNP PNP TransistorsTransistors

The procedure used to identify the base of a

The procedure used to identify the base of a PNP transistor is the sPNP transistor is the same as thatame as that used to determined the base of the NPN transistor,

used to determined the base of the NPN transistor, except that the negative terminalexcept that the negative terminal of the meter is connected to each transistor terminal in turn, and it is this negative of the meter is connected to each transistor terminal in turn, and it is this negative terminal that indicated the base.

terminal that indicated the base.

1.10.2.2

1.10.2.2 Confirming Confirming the the ServiceabilityServiceability

Both types of transistor are serviceability tested by confirming that each forward Both types of transistor are serviceability tested by confirming that each forward biased junction (Diode) has a low resistance, and each

biased junction (Diode) has a low resistance, and each reverse biased junction areverse biased junction a high resistance.

high resistance. The high resistance betwThe high resistance between collector and emitter should een collector and emitter should also bealso be confirmed.

confirmed. Remember the poinRemember the points made about AVOts made about AVO's and Digital meters, otherwise's and Digital meters, otherwise incorrect conclusions may be drawn from

FUNDAMENTALS FUNDAMENTALS

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NPN Base Base to to Emitter Emitter - - forward forward biased biased - - low low resistanceresistance Base

Base to to Collector Collector - forward - forward biased - low biased - low resistanceresistance Emitter

Emitter to to Collector Collector - - reverse reverse biased biased - - high high resistanceresistance PNP

PNP Emitter Emitter to to Base Base - - forward forward biased biased - - low low resistanceresistance Collector

Collector to to Base Base - forward - forward biased - low biased - low resistanceresistance Emitter

2

2

OPERATIONAL

OPERATIONAL AMPLIFIERS

AMPLIFIERS

Operational amplifiers are integrated circuit devices designed to be

Operational amplifiers are integrated circuit devices designed to be a closea close approximation to the perfect amplifier.

approximation to the perfect amplifier. 2.1

2.1 THE THE PERFECT PERFECT AMPLIFIERAMPLIFIER

 Although a theoretical device, the specification of a perfect amplifier would be as  Although a theoretical device, the specification of a perfect amplifier would be as

follows: follows:

•• GainGain infinitely high.  infinitely high. This has to be controlled This has to be controlled in some way otherwise thein some way otherwise the smallest input would result in maximum output.

smallest input would result in maximum output.

•• Input impedance. Input impedance. Infinitely high Infinitely high so as not so as not to load to load the source.the source.

•• Output impedance. Output impedance. Zero, so that Zero, so that the amplifier can the amplifier can be connected be connected to any loadto any load without the output voltage being affected.

without the output voltage being affected.

•• BandwidthBandwidth. . Infinite, so that signals Infinite, so that signals from d.c. to from d.c. to infinite frequency are infinite frequency are allall amplified by the same amount.

amplified by the same amount.

•• Supply voltage. Supply voltage. The amplifier shoThe amplifier should be unauld be unaffected by variations in ffected by variations in the powerthe power supply voltage.

supply voltage. 2.2

2.2 OP OP AMP AMP SPECIFICATIONSPECIFICATION The following specification is for a

The following specification is for a SN72741 operationSN72741 operational amplifier. al amplifier. This is a veryThis is a very popular operational amplifier generally simply referred to as a

popular operational amplifier generally simply referred to as a 741 op-amp.741 op-amp.

•• GainGain - - 200 000 200 000 voltage gain voltage gain (106db a(106db approx.)pprox.)

•• Input impedance - Input impedance - 2M2MΩΩ..

•• Output impedance - Output impedance - 7575ΩΩ •• BandwidthBandwidth - - d.c. d.c. to to 1MHz.1MHz.

•• Supply voltage - Supply voltage - The op-amp The op-amp will operate with a suppwill operate with a supply of plus and minus 5 toly of plus and minus 5 to 15 volts, and take a qu

15 volts, and take a quiescent current of about 2mAiescent current of about 2mA. . The output voltage The output voltage willwill change less than 150

change less than 150μμV per volt change in supply V per volt change in supply voltage.voltage. It can be seen that the

It can be seen that the 741 Op Amp approximates the specification of a perfect741 Op Amp approximates the specification of a perfect amplifier.

engineering

engineering

2.3 POWER POWER REQUIREMENTSREQUIREMENTS

Operation is most convenient from a dual balanced d.c.

Operation is most convenient from a dual balanced d.c. power supply giving equalpower supply giving equal positive and negative voltag

positive and negative voltages (+ Vs) in the range +5V es (+ Vs) in the range +5V to +15V. to +15V. The centre point ofThe centre point of the power supply, i.e. 0V is

the power supply, i.e. 0V is common to input and output and is taken as tcommon to input and output and is taken as t heir voltageheir voltage reference.

reference.

The input signs on the circuit symbol for an Op Amp should not be confused The input signs on the circuit symbol for an Op Amp should not be confused with those for the supply polarities.

with those for the supply polarities.

 An op-amp can be operated from a single p

 An op-amp can be operated from a single power supply. ower supply. The voltage differenceThe voltage difference available from, for example, a 0V to

available from, for example, a 0V to 18V supply is the same as 18V supply is the same as that from a +9V tothat from a +9V to 0V to -9V one, however, if a

0V to -9V one, however, if a single power supply is used, extra single power supply is used, extra components arecomponents are required.

required. 2.4

2.4 PIN PIN OUTS OUTS & & CIRCUIT CIRCUIT SYMBOLSYMBOL The circuit symbol and pin outs

The circuit symbol and pin outs of a typical operation amplifier are shown below.of a typical operation amplifier are shown below.

Most of the terminals are self-explanatory or

Most of the terminals are self-explanatory or will be explained in the course of will be explained in the course of thesethese notes.

notes. Terminals 1 and 5, the Terminals 1 and 5, the offset null terminals however reoffset null terminals however require furtherquire further explanation.