Electronic & Electrical
Fundamentals
Introduction to Semiconductor
Applications
(Intermediate 2)
5839
September 1999
HIGHER STILL
Electronic and
Electrical
Fundamentals
Introduction to Semiconductor
Applications
Intermediate 2
Support Materials
CONTENTS
Lecture’s / Teacher’s information and support material
SECTION
DETAILS
Section 1
The learning outcomes
Section 2
Teaching and learning advice
Section 3
Recommended entry
Section 4
Assessment procedures
Section 5
Resource list
Section 6
Laboratory requirements
Section 7
Safety
Section 8
Acknowledgements
Student information and support material
SECTION
CONTENTS
Section 1
The learning outcomes
Section 2
Instruments of assessment
Section 3
Required achievement standard
Section 4
Student guide
Section 5
safety
Section 6
Student notes
a)
Outcome 1
b)
Outcome 2
c)
Outcome 3
d)
Outcome 4
SECTION 1
The learning outcomes to be covered in this unit
Outcome 1
Interpret the operation of semiconductor diode circuits
Performance criteria
a. The identification of operational limits from manufacturer’s data sheets is correct.
b. The measurement and recording of diode circuit voltage levels are correct.
c. The explanation of the circuit operation is correct.
Evidence requirements
The student will be set tasks, which assess the ability to interpret the operation of two
different semiconductor diode circuits. The student will be required to take
measurements of diode circuit voltage levels and answer questions, which relate these
measurements to manufacturer’s data sheets. The measurements will be recorded in a
prespecified format. The student will be required to maintain a logbook, which
includes a brief explanation of the operation of the circuits.
Satisfactory achievement of the outcome will be based on the student attaining all of
the performance criteria and correctly answering the questions connected with PCs (b)
and (c).
Outcome 2
Outline the use of power control devices
Performance criteria
a. Power control devices are correctly identified from their symbols.
b. The actions of the power control devices are correctly described.
c. Applications for power control devices are correctly stated.
Evidence requirements
The student will be set a structured question to test understanding of the use of power
control devices.
The student will be given a circuit diagram(s) containing a thyristor, diac and triac,
which he/she will be required to identify from their symbols. The student will be
required to describe the action of the device and a typical application for each.
Satisfactory achievement of the outcome is based on all parts of the question being
correctly answered.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2)
Teacher/Lecturer material
1
Outcome 3
Interpret the operating conditions of a single-stage resistance loaded small signal
amplifier.
Performance criteria
a. The measurement and recording of circuit voltages levels are correct.
b. The measured voltage are correctly related to the biasing of the transistor
c. The gain of the circuit is correctly determined from the measurement of input and
output signals.
d. The circuit operation is described correctly.
Note on the range of the outcome
Amplifier: bipolar (common emitter) or FET (common source)
Evidence requirements
The student will be set a task, which assesses the ability to interpret the operating
conditions of a single-stage resistance, loaded small signal amplifier. The student will
be required to take measurements of circuit voltage levels and relate these
measurements to the biasing of the transistor and gain of the amplifier and describe
the overall circuit operation. The measurements will be recorded in a prespecified
format.
Satisfactory achievement of the outcome will be based on the student attaining all of
the performance criteria.
Outcome 4
Investigate operational amplifier circuits.
Performance criteria
a. Appropriate adjustments are made to the circuit to provide a voltage null at the
output.
b. The gain of inverting and non-inverting configurations is correctly calculated from
measurements taken and recorded.
c. The phase relationship between input and output signals in inverting and noninverting configurations is correctly recorded.
Evidence requirements
The student will complete practical exercises to demonstrate an ability to investigate
operational amplifier circuits and explain the requirement for offset null.
The student will be given two preconstructed units in which adjustments and
measurements are to be made and recorded.
Exercises will be carried out in conjunction with a suitably constructed observation
checklist.
Satisfactory achievement of the outcome will be based on the student attaining all of
the performance criteria.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2)
Teacher/Lecturer material
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SECTION 2
Teaching and learning advice including how to use the resource material
Teaching methods
In general the teaching and learning methods used are very dependent on the contents
of the unit and the facilities and expertise available at the delivering centre. By their
very nature, however, the units in the Intermediate 2 Electronic and Electrical
fundamentals Course suggest the following teaching methods:
•
•
•
•
•
Student handouts for basic component and circuit theories
Laboratory based learning activities
Use demonstration circuits where possible
Use student centred circuit testing and analysis
Place components used in a commercial context.
Introduction to Semiconductor Applications Unit Delivery
MAIN TOPICS
SUGGESTED DELIVERY
OUTCOME 1
Semiconductor theory
Explain basic technology, handout and tutorial
examples
PN junction
Explain concept of depletion layer , forward and
reverse biasing, tutorial examples
Diodes
Laboratory based practical exercises covering
rectification, clipping, clamping and voltage
stabilisation. Use could be made of suitable
software, such as ‘Electronic Workbench’ for
further investigations.
OUTCOME 2
Power control devices
Explain basic technology, handout and tutorial
examples
Demonstration of working circuit(s) including
waveforms on oscilloscope should be used as
part of the lesson.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2)
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MAIN TOPICS
OUTCOME 3
SUGGESTED DELIVERY
Bipolar transistor amplifier
Explain basic transistor operation followed by
biasing conditions required for small signal
amplification.
Bipolar transistor amplifier(cont’d)
Simple demonstrations and practical exercises
to illustrate the correct biasing conditions,
reduction of negative feedback using a bypass
capacitor. Calculation of gain.
FET small signal amplifier
Explain basic FET operation followed by
biasing conditions required for small signal
amplification.
Simple demonstrations and practical exercises
to illustrate the correct biasing conditions.
Calculation of gain.
Use could be made of a computer software
package to aid the investigation of different
biasing conditions.
Operational amplifier circuits
Explain basic technology, tutorial examples.
Demonstration of offset null adjustments
followed by student based exercises.
Practical laboratory based exercises using
inverting and non-inverting amplifier
configurations.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2)
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SECTION3
Recommended entry for this unit is Mathematics and either Technological Studies or
Physics at Standard Grade 3 or above. The following gives a guide to the areas that
the student should be familiar with before starting this unit.
Mathematics
Drawing and interpretation of graphs
Transposition of formulae
Electrical
Having covered the work for Electrical Fundamentals prior to starting on Introduction
to Semiconductor Applications the student would have covered all the electrical
concepts required for this unit.
Electronics
Identification of electronic components, and the interpretation of schematic diagrams
and their relationship with wiring diagrams should be covered before starting this
unit. Use of electronic test instruments, such as digital multimeters, signal generators
and oscilloscopes should also have been covered before attempting this unit.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2)
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SECTION 4
Assessment procedures showing what is to be assessed, when it is to be assessed
and result recording methods.
Using the instrument of assessment
The unit requires four assessment tasks as follows
The following table shows how assessment tasks are related to their learning
outcomes. It also lists the evidence, which should be collected.
ASSESSMENT
TASK
OUTCOME
EVIDENCE
1
Interpret the operation of two semiconductor
diode circuits
Laboratory assignment reports
2
Outline the use of Power Control Devices
Structured question answers
3
Interpret the operation of a single stage
resistance loaded small-signal amplifier
Laboratory assignment reports
and calculations
4
Investigate operational amplifier circuits
(inverting and non-inverting configurations
only)
Laboratory investigation reports
and checklist
The assessments including both assignments and the practical exercises are carried
out individually and under closed book conditions. Data sheets should be supplied.
Timing and duration
In principle, there are no time limits for the completion of each instrument of
assessment but it is likely that in practice a maximum time will be allocated for the
completion of each task. It is expected that the average student will complete the task
well within the maximum time allowed. The table below indicates the maximum time
that could be allocated to each instrument of assessment.
OUTCOME
SUGGESTED MAXIMUM TIME
1
60 minutes
2
45minutes
3
75 minutes
4
75 minutes
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2)
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Reassessment
Time is allowed within the units for assessment and reassessment of outcomes. Where
a candidate has not attained the standard necessary to pass a particular outcome or
outcomes, they should have the opportunity to be reassessed.
Reassessment should focus on the outcome(s) concerned and as a general rule, should
be offered on a maximum of one occasions, once shortly after the initial assessment.
In some cases candidates will be required to resubmit original work which has been
revised to take account of earlier weaknesses. In other cases, candidates will be
required to undertake a new instrument of assessment designed to assess the particular
outcome(s) in which they were unsuccessful, for example, outcomes concerning the
ability to recall knowledge and understanding. In all cases evidence from the original
unit assessment, should be used for formative purposes prior to reassessment.
For all the outcomes the reassessment can be based on the original instruments of
assessment. There are two ways in which this can be accomplished:
1.
When candidates have not produced a satisfactory answer to a section or sections of
an assessment, e.g. parts a, b, etc. In these cases, the candidates should only be asked
to redo those section(s) in which they have not produced suitable responses.
Candidates should also be asked to complete this reassessment under the original
controlled conditions. This reassessment should take place as soon as practical after
the initial assessment, and staff should delay any discussion / analysis of the initial
assessment until reassessment has taken place. However, candidates should be
informed which sections of the assessments they will be required to redo and given
guidance on where they can find information to help them tackle the reassessment. In
cases where there are substantial numbers of candidates requiring reassessment, it
would be beneficial before reassessment to hold a revision lesson on the problem
area(s).
2.
When candidates have produced an answer which is substantially correct but which
contains minor errors, such as incorrectly reading an instrument (100mA instead of
10mA). In these situations, candidates could be reassessed by asking them to
demonstrate the correct answer orally. Again this should take place as soon after the
initial assessment and before any discussion / analysis of it.
The conditions under which assessment takes place.
Higher Still Arrangements and subject guides refer to assessment being carried out
under controlled conditions to ensure reliability and credibility for the purposes of
internal assessment, this means that assessment evidence should be compiled under
supervision to ensure that it is the candidates own work. Supervision may be carried
out by a teacher, invigilator, or other responsible person, e.g. a workplace provider.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2)
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Using internal assessment evidence to contribute to course estimates.
The unit assessments contained in this pack are designed exclusively for internal
assessment purposes and do not have the potential to generate evidence for external
assessment. To generate evidence for external assessment it is recommended that the
candidate undertakes an end of unit assessment that will comprise of two structured
questions that will test the candidates recall of knowledge and understanding.
Together with the results from other end of unit assessments from the Electronic and
Electrical Engineering Intermediate 2 course the candidate performance in the course
external assessment may be predicted. This evidence, if required, could support the
candidate through an appeal procedure.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2)
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SECTION 5
Resource requirements including course notes, book list, and audio/visual aid
list.
Course notes
Student notes covering all course work are provided in the student’s support material.
These should be either adopted by the centre or modified to suit the teaching approach
taken and the equipment available.
Book list
The following book will provide all the information required for the electrical and
electronics fundamentals course and is only one of many books that could be adopted
as a course book
Electronics for today and tomorrow
Tom Duncan
The following books provide useful background information.
Electronics: Circuits and Devices
Ralph J Smith, John Wiley and Sons, ISBN 0-471-08751-3
Electronic principles and Applications, John B Pratley, Arnold, ISBN 0-340-69275-8
Applied electronics, John C Morris, Arnold, ISBN 0-340-65284-5
Audio/visual aids
The electronics laboratory should have prominently displayed an electrical safety
notice. These are available from a variety of electrical and electronic wholesale
outlets and distributors and are relatively inexpensive.
Component manufacturers and distributors offer wall charts and posters showing
many aspects of electronics. These vary from resistor colour codes to product
processing details and application advertisements. They are generally free and
available on request. They are useful as visual aids on the walls of the electronics
teaching laboratory as they create atmosphere and over a period of time act as a
constant reminder to students.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2)
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SECTION 6
Electronics laboratory requirements including technical information sources,
components, material, facilities and equipment.
Technical information sources
It should be noted that developments in electronics, communications and computing
continue to offer tremendous opportunities for the dissemination and retrieval of
information. At the time of writing the Internet, CD-ROM and on-line component
distributors catalogues and web sites for manufacturers, suppliers and providers of
educational material are current examples. All of these and similar potential future
products originate from the electronics technology. Accordingly students should be
encouraged to use them and to explore and take advantage of such technological
products as they emerge. A culture of using the technology to its limits should be
encouraged.
There are numerous sources of technical information on electronics other than the
traditional library books. These sources, however are only helpful if they are both
accessible and relevant. The following has been refined through use and experience
but inevitably will be superseded by better methods as the technology advances and
they become available.
Component Distributors catalogues
MPS [ Maplin]
Web Site
http://www.maplin.co.uk
e-mail
<recipient>@maplin..co.uk
Telephone: Customer services
01702 554002
Telephone: Free technical helpline
01702 556001
Address
Maplin MPS, Freepost SMU 94, P.O. Box 777, Rayleigh Essex SS6 8LU
RS
Web Site
http://rswww.com
e-mail
http://rswww.com
Telephone: Customer services
01536 201201
Telephone: Free technical helpline
01536 402888
Address
RS Components Ltd. P.O.Box 99, Corby, Northants NN17 9RS
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 10
Teacher/Lecturer material
Farnell
Web Site
http://www.farnell.co.uk
e-mail
Enquiries@farnell.com
Telephone: Customer services
0113 263311
Telephone: Free technical helpline
0113 2799123
Address
Farnell Electronic Components, Canal Road, Leeds, LS12 2TU
Access should be provided for students to one or more of the above component
distributor’s catalogues in paper, CD-ROM or on-line form.
Selected data books, reference books and specialist texts should also be provided from
those offered by the above sources. There are so many good items on offer it is
impossible to recommend a definitive list, which is largely a matter of local
preference. Choices should be based on staff expertise, the teaching and learning
approaches used and the available budget. As many of the smaller specialised texts
are low cost it should be possible to provide several reference copies for use in the
electronics laboratory.
Many manufacturers of electronic components have web sites. These may be located
by a net search using the manufacturer’s name. Once into the web site it is often
possible to locate technical data, application information and in some situations
design tutorials.
Components
The Electronics Laboratory should offer access to component stocks as a standard
facility. This is for the benefit of both staff and students who will require access to
components for demonstrations, experimentation and for case study and project work.
The stock however, has to be managed and controlled if the quality of the facility is to
be sustained. The approach taken to this is a matter for the centre’s organisational
structure but experience suggests that one person needs to be clearly identified as
having responsibility for the stock, for issuing it and reordering.
Below is a typical basic selection of components
COMPONENT
resistors
DETAILS (RANGE)
Low cost metal film 0.25 W – standard preferred values from 1Ω to10
MΩ
High powered resistors 2.5 W silicon coated – standard available
values.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 11
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COMPONENT
DETAILS (RANGE)
Potentiometers
150mW carbon trimmers - standard preferred values from 100Ω to10
MΩ
capacitors
Metallised ceramic plate capacitors - standard preferred values from
1.8 pF to 120 pF
Resin dipped plate ceramic capacitors - standard preferred values
from 10 pF to 0.47µF
Radial polystyrene capacitors - standard preferred values from 100 pF
to 8200 pF
Radial lead electrolytic capacitors - standard preferred values from
1µF to 47000 µF
Axial lead electrolytic capacitors - standard preferred values from 1µF
to 47000 µF
diodes
1N4148, 1N4001, 1N5401, BZX85- range of voltages from 2.7V to
15V
Bridge rectifier
W005G
Light emitting diodes
3mm and 5mm red, orange and green
transistors
BC184L, BC214L, 2N3053, BFY50, TIP31A, TIP32A, TIP33A,
2N3055E, 2N2955, 2N3819, 2N3820
Op-amps
µA 741, LM324N, CA3140E
Logic chips
74 LS series TTL – selected functions as appropriate
4000 series CMOS - selected functions as appropriate
Other analogue integrated
circuits
NE555N timer, ICM7555 timer, L7805CP, L7812CP positive voltage
regulators, L7905CT, L7912CT negative voltage regulators
Other digital integrated
circuits
DACs and ADCs to suit laboratory applications
Fuses
As required for instruments
switches
Push button, toggle, slide and DIL miniature as required
transformers
As required
Lamps and bulbs
Low voltage and power selection to meet requirements
connectors
Terminal blocks, 4mm plugs and sockets in red, black, blue yellow
and green
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 12
Teacher/Lecturer material
Facilities
There are many possible layouts for an electronics teaching laboratory and each centre
will probably be developed to suit that centres particular requirements. However it is
important that whatever the layout the following features should be available. Each
student should have available a 2 metre run of surface and at least 4 -13 Amp sockets
outlets. This is necessary to allow adequate working surface for test equipment,
circuits, components and papers. A suitable device such as an RCD (Residual Current
Device) and a central safety switch with key lock should protect the socket outlets. A
specialist should undertake the design and installation of such facilities, as they
constitute important safety features.
Equipment
In the electronics laboratory each student should have access to the following
equipment. Ideally there should be one set of equipment per student.
1.
2.
3.
4.
5.
Multimeter
Dual power supply
Signal generator
Dual beam oscilloscope
Computer
The computer does not
necessarily form part of the set
It is also helpful to have a limited range of tools available such as
•
•
•
•
Long nosed pliers
Wire cutters
Wire strippers
Screw drivers
The types of tools and equipment on the market are constantly changing through a
process of continuous improvement. It is strongly recommended that before
purchasing any items for an electronics laboratory advice is sought from a current
user experienced in this area. Issues such as the cost of hand tools in relation to their
quality and life expectancy with inexperienced users who may damage or remove
them from the laboratory have to be given due consideration. Equipment may be
found which is both adequate for the teaching laboratory, student proof and
inexpensive requiring little maintenance.
There are many suppliers of test equipment and tools and those specialising in the
educational market are likely to offer products at an acceptable price. These suppliers
are also likely to have tools and equipment, which can survive the rigours of the
teaching laboratory.
Other centres, which have tried and tested equipment are often the best source of
information and should be consulted as part of the purchasing exercise.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 13
Teacher/Lecturer material
Software
The electronics teaching laboratory is greatly enriched by the presence of computers
with appropriate software. Since computers are one of the main products of
electronics technology they find extensive use in the application of the technology and
form part of the electronics environment. While the capital outlay for this may be
significant there has to be recognition of the part played by the computer with the
specialised software in the electronics environment.
Through access to suitable computing facilities and software students should be
exposed to this environment in the teaching laboratory. Commercial software for
word and data processing is widely available at reasonable cost and is generally
selected by centres to conform to their local policy. These may find application in the
creation of reports and the analysis of results.
In addition, however, circuit simulation and drawing software may also be used but is
less widely available and more difficult to locate in a form, which is cost effective for
the teaching, laboratory. To meet these criteria the software must be easy to use
without extensive training and be available at low cost with multiple copy site
licensing. The following packages might be considered.
Invent! CROCODILE CLIPS: Simple simulation of electronics and mechanics
Web Site
www.crododile-clips.com/education/v3.htm
e-mail
Sales@crocodile-clips.com
Telephone:
0131 226 1511
Fax
0131 226 1522
Address
Crocodile-clips, 11 Randolph Place, Edinburgh EH3 7TA
Electronics Workbench: circuit simulation and testing
Web Site
http://www.adeptscience.co.uk
e-mail
Info@adeptscience.co.uk
Telephone:
01462 480055
Fax
01462 480213
Address
Adept Science plc, 6 Business Centre West, Avenue One, Letchworth. Herts. SG6 2HB
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 14
Teacher/Lecturer material
Smart Draw: drawing of diagrams and plans with associated symbol libraries
Web Site
http://www.smartdraw.com
e-mail
Sales@ttp.co.uk
Telephone:
01889 564601
Fax
01889 563219
Address
The Thompson Partnership, Lion Buildings, Market Place, Uttoxeter, Staffs. ST14 8HZ
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 15
Teacher/Lecturer material
SECTION 7
Safety
The safety of teaching staff and students working in the electronics laboratory must be
the primary concern of everyone involved.
This has to precedence over all other activities and be sustained against all other
pressures
There are many aspects to safety as follows:
• Statutory requirements
• Centre procedures
• Centre structure
• Staff training and behaviour
• Laboratory features
• Student training and behaviour
It is beyond the scope of this document to provide details of all aspects of a centre’s
safety policy. Staff must, however, be content that all appropriate safety measures are
in place before embarking on work within the electronics laboratory.
Student training is a recurrent activity, which is likely to be the direct responsibility of
the lecturer/teacher. While this has to take place on a continuous basis as work in the
laboratory proceeds it is helpful to perform specific safety training at course
commencement. Such training might form part of the course induction as its relevance
extends across all course units. This is particularly important for electronic students,
as they should be encouraged to develop their own safety culture, which should
become a lifelong asset.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 16
Teacher/Lecturer material
SECTION 8
Acknowledgements
The support and assistance provided by colleagues at South Lanarkshire College who
have contributed material and helpful advice for this pack is gratefully acknowledged.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 17
Teacher/Lecturer material
STUDENT INFORMATION AND SUPPORT MATERIAL
SECTION
CONTENTS
Section 1
The learning outcomes
Section 2
Instruments of assessment
Section 3
Required achievement standard
Section 4
Student guide
Section 5
Safety
Section 6
Student notes
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2)
Student material
1
SECTION 1
The following information and support material will help you to work on the
Introduction to Semiconductor Applications unit. Information is provided about the
contents and the learning outcomes. There is a guide to working on the unit and
technical notes for you to study. Exercises and tutorial work are provided to help you
understand the technology and prepare you for the assessments, which are also
explained. As you will be working in the laboratory there is a safety reminder sheet
which you should study before you start.
The learning outcomes covered in the unit
Outcome 1
Interpret the operation of semiconductor diode circuits
Performance criteria
a. The identification of operational limits from manufacturer’s data sheets is correct.
b. The measurement and recording of diode circuit voltage levels are correct.
c. The explanation of the circuit operation is correct.
Evidence requirements
The student will be set tasks, which assess the ability to interpret the operation of two
different semiconductor diode circuits. The student will be required to take
measurements of diode circuit voltage levels and answer questions, which relate these
measurements to manufacturer’s data sheets. The measurements will be recorded in a
prespecified format. The student will be required to maintain a logbook, which
includes a brief explanation of the operation of the circuits.
Satisfactory achievement of the outcome will be based on the student attaining all of
the performance criteria and correctly answering the questions connected with PCs (b)
and (c).
Outcome 2
Outline the use of power control devices
Performance criteria
a. Power control devices are correctly identified from their symbols.
b. The actions of the power control devices are correctly described.
c. Applications for power control devices are correctly stated.
Evidence requirements
The student will be set a structured question to test understanding of the use of power
control devices.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2)
Student material
2
The student will be given a circuit diagram(s) containing a thyristor, diac and triac,
which he/she will be required to identify from their symbols. The student will be
required to describe the action of the device and a typical application for each.
Satisfactory achievement of the outcome is based on all parts of the question being
correctly answered.
Outcome 3
Interpret the operating conditions of a single-stage resistance loaded small signal
amplifier.
Performance criteria
a. The measurement and recording of circuit voltage levels are correct.
b. The measured voltage are correctly related to the biasing of the transistor
c. The gain of the circuit is correctly determined from the measurement of input and
output signals.
d. The circuit operation is described correctly.
Note on the range of the outcome
Amplifier: bipolar (common emitter) or FET (common source)
Evidence requirements
The student will be set a task, which assesses the ability to interpret the operating
conditions of a single-stage resistance, loaded small signal amplifier. The student will
be required to take measurements of circuit voltage levels and relate these
measurements to the biasing of the transistor and gain of the amplifier and describe
the overall circuit operation. The measurements will be recorded in a prespecified
format.
Satisfactory achievement of the outcome will be based on the student attaining all of
the performance criteria.
Outcome 4
Investigate operational amplifier circuits.
Performance criteria
a. Appropriate adjustments are made to the circuit to provide a voltage null at the
output.
b. The gain of inverting and non-inverting configurations is correctly calculated from
measurements taken and recorded.
c. The phase relationship between input and output signals in inverting and noninverting configurations is correctly recorded.
Evidence requirements
The student will complete practical exercises to demonstrate an ability to investigate
operational amplifier circuits and explain the requirement for offset null.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2)
Student material
3
The student will be given two preconstructed units in which adjustments and
measurements are to be made and recorded.
Exercises will be carried out in conjunction with a suitably constructed observation
checklist.
Satisfactory achievement of the outcome will be based on the student attaining all of
the performance criteria.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2)
Student material
4
SECTION 2
The assessment instruments for the outcomes
The Introduction to Semiconductor Applications unit will involve you in four main
topics with their assessment activities.
The first topic is semiconductor diodes and their applications and this will be
preceded by some explanation of semiconductor theory and P-N junction
characteristics. The applications will be assessed by two laboratory assignments on
semiconductor diode circuits combined with information that will have to be extracted
from relevant data sheets. The assignment should last a maximum of 60 minutes
The second topic covers electrical power control devices and their applications. The
assessment will be by structured questions. The test will last for a maximum of 45
minutes.
The third topic is small signal resistance loaded amplifiers. An explanation of their
operating conditions and biasing arrangements will be backed up with some practical
exercises. Both bipolar transistor (common emitter) and FET (common source)
circuits will be looked at. The assessment will be a laboratory assignment along with
some calculations on the results obtained.
Finally you will be asked to investigate the operation of inverting and non-inverting
operational amplifier configurations and to make adjustments to give a null output.
Again this will be preceded by an explanation of operational amplifiers and their
characteristics backed up with some tutorial exercises.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2)
Student material
5
SECTION 3
The required achievement standard for each assessment
Learning outcome1 semiconductor diode applications
You require to complete all parts of the exercises correctly to satisfactorily achieve
this outcome.
Learning outcome 2 power control devices
Again you must answer all the questions correctly to satisfactorily achieve this
outcome.
Learning outcome 3 small signal resistance loaded amplifiers
Satisfactory achievement in this outcome requires successful completion of the
practical exercises illustrated by providing correct answers to all associated questions.
Learning outcome 4 inverting and non-inverting operational amplifiers
Satisfactory achievement in this outcome requires successful completion of the
investigation and correct answers to the questions posed in the assessment.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2)
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SECTION 4
Students guide to working on the unit
The main purpose of this unit is to provide an introduction to basic analogue devices
and their application in electronic circuits
The first part of this unit covers semiconductor diodes their theory and applications;
you will examine the effect they have on electrical and electronic signals by a series
of practical exercises. You should try and understand diodes by studying these
practical examples.
The unit then requires you to look at the characteristics of more advanced
semiconductor devices, namely power control devices. You require an understanding
of their function and how they are used in Electrical and Electronic Engineering.
The third part of the unit looks at single stage resistance loaded amplifiers and again
this will involve a series of practical exercises to demonstrate how the correct
conditions for amplification can be set up for transistors. Again you should try and
understand diodes by studying these practical examples.
The final part of this unit looks at the operational amplifier in terms of its
characteristics and its two simplest applications. This will also be done using practical
exercises, which you should study in order to gain an insight into operational
amplifiers.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2)
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SECTION 5: SAFETY
Information sheets/references for safety and laboratory work
You should read these guidelines and discuss them with your tutor to clarify their
significance in your working environment.
• Enter the electronics laboratory only at agreed times
• Enter the electronics laboratory only when you are authorised
• You should only work on equipment when a supervisor is present
• Always avoid bulky, loose or trailing clothes, long loose hair, heavy metal
bracelets or watch straps
• Do not take food or drink into the electronics laboratory
• Avoid wet hand or clothes and clean up any liquid spillage
• Be as careful for the safety of others as yourself
• Think before you act, be tidy and systematic
• Keep passages and work areas free of obstructions
• Voltages above 120 V dc and 50 V rms. are always dangerous, take extra
precautions as voltages increase
• Never remove earth connections and make sure that all accessible conducting parts
of equipment or experiments are earthed. If in doubt check for earth continuity
• Multimeters and hand held probes should be of good fused design and are not
recommended for dangerous levels of voltage and power
• Understand the correct handling procedures for batteries, capacitors, inductors and
other energy storage devices. Always handle them carefully
• Fluorescent lights can cause rotating equipment to appear stationary. You should
be aware of this and take precautions as necessary
• Before equipment is made live all casings, covers or shrouds must be in place so
that no live parts can be touched with fingers
• Before equipment is made live all casings, covers or shrouds must be in place so
that no moving parts can be touched with fingers
• Before equipment is made live circuit connections and layouts should be checked
by your supervisor
• If you are working in a group, everyone in the group should give their assent
before the equipment is made live
• Never make changes to either circuits or mechanical layouts without isolating the
circuit by switching it off and removing connections to supplies
• Experimental equipment left unattended should be isolated from the supply unless
it has to be left on for some special reason, in which case a barrier and warning
notice are required
• Equipment found to be faulty in any way should be reported to your supervisor
immediately and not used until it is inspected and declared safe
• You should know what to do if there is an emergency in the electronics laboratory
• Use hand tools carefully and treat them with respect as they can be dangerous
when misused or faulty
• Do not remove equipment, tools or materials from the electronics laboratory
without authorisation from your supervisor
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2)
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SECTION 6
Introduction to Semiconductor Applications notes
Semiconductors
Semiconductors are materials whose ability to conduct electricity lies somewhere
between that of good conductors (like copper) and good insulators (like polythene).
Examples of semiconductors are silicon, germanium, cadmium sulphide and gallium
arsenide.
Intrinsic semiconductors
Many properties of materials can be explained if we assume that the electrons
surrounding the nucleus of an atom are arranged in orbits or ‘shells’. The electrons in
the outermost shell are known as ‘valence’ electrons. These determine the chemical
combining power (valence) of the atom as well as many other physical properties.
These valence electrons form ‘bonds’ with the valence electrons of neighbouring
atoms to produce, in the case of most solids, a regular repeating three-dimensional
pattern of atoms called a crystal lattice.
An atom of a semiconductor material such as silicon and germanium has four valence
electrons as shown below in figure 1a. In a lattice, each one of these valence electrons
is shared with a nearby atom to form four ‘covalent’ bonds, as shown below in figure
1b.
Valence
electrons
Covalent bond
Fig 1b
Fig 1a
Simplified model of a
silicon atom
Nucleus and
inner electrons
Thus, every atom has a half share in eight valence electrons. It so happens that this
number of electrons gives a very stable arrangement. A strong crystal lattice results
and it is difficult for any electrons to escape from their atoms. Pure silicon and
germanium are therefore very good insulators (nearly perfect at absolute zero -273oC).
The atoms in the lattice structure vibrate and at normal temperatures the atoms vibrate
sufficiently for a few bonds to break and this sets some valence electrons free. When
this happens and the electron moves away, a deficit of negative charge exists where
the electron had previously been. This deficit of negative charge is called a ‘hole’ and
a hole has a positive charge of the same magnitude as the electrons negative charge.
The atom, now short of an electron, becomes a positive ion.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2)
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Free electron
Intrinsic semiconductor
hole
hole
Electron flow
(a)
Crystal lattice of silicon with
free electron and hole
Fig 2
(b)
When a battery is connected across a pure semiconductor, it attracts free electrons to
the positive terminal and provides a supply of free electrons at the negative terminal
(see figure 2). These free electrons drift through the semiconductor by ‘hopping’ from
one hole to another nearer the positive terminal each time. This makes it appear as if
the positive holes are moving towards the negative terminal.
The current in a pure semiconductor material is very small and can be thought of as a
stream of free electrons travelling in one direction and a stream of holes travelling in
the other. This is called intrinsic conduction because the charge carriers come from
inside the material.
If the temperature of a semiconductor rises, more bonds break and intrinsic
conduction increases because there are more free electrons and holes produces. The
resistance of a semiconductor decreases as temperature increases.
Extrinsic semiconductors
The use of semiconductors in diodes, transistors and integrated circuits, depends on
increasing the ability of pure semiconductors to conduct electricity. This is done by
adding a tiny, but controlled amount of impurities. This process is called doping and
the material obtained is called an extrinsic semiconductor because the impurity
supplies the charge carriers (which add to the intrinsic charge carriers). These
impurity atoms must be about the same physical size as the semiconductor atoms so
that they fit into the crystal lattice without causing distortion. There are two types of
extrinsic semiconductors.
n-type
This type is made by doping pure semiconductor with, for example, phosphorus. A
phosphorus atom has five valence electrons. The diagram shown in figure 3 shows
what happens when a phosphorus (or any other 5 – valence atom) is introduced into
the lattice of a silicon crystal.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 10
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Silicon atom
(nucleus and
inner electrons)
extra electron
Figure 3
Donor impurity atom
(phosphorus)
Crystal lattice of n-type silicon
Four of its valence electrons form covalent bonds with four neighbouring silicon
atoms but the fifth is spare and, being only loosely held, it can take part in conduction.
The impurity (phosphorus) atom is called a ‘donor’.
The ‘impure’ silicon is an n-type semiconductor because the majority charge carriers
are negative electrons. The overall charge in the crystal is still zero since every atom
present is electrically neutral. Impurity atoms are added in the amount that produces
the required conductivity.
A few positive holes are also present in n-type materials and they are the minority
carriers. The diagram of figure 4 shows conduction in an n-type material.
n-type semiconductor
Majority charge
carrier (electron)
Minority charge carrier (hole)
Electron flow
Figure 4
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 11
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p-type
In this type, the pure semiconductor is doped with, for example boron, which has
three valence electrons. The diagram, figure 5, shows what happens when one is
introduced into the silicon crystal.
Silicon atom
(nucleus and
inner electrons)
Figure 5
Donor impurity atom
(boron)
Hole (incomplete bond)
Crystal lattice of p-type silicon
Its three valence electrons each share an electron with three of the four silicon atoms
surrounding it. One bond is incomplete and the position of the missing electron, that is
the hole, behaves like a positive charge since it can attract an electron from a nearby
silicon atom. This causes another hole to be formed. For this reason the impurity
(boron) atom is called an acceptor.
The impure silicon is a p-type semiconductor since the majority of carriers causing
conduction are holes but note again that the semiconductor as a whole is electrically
neutral. In p-type material a few electrons are present as minority carriers. The
diagram of figure 6 shows how conduction occurs in such a material.
p-type semiconductor
Majority charge
carrier (hole)
Minority charge carrier (electron)
Electron flow
Figure 6
In both n- and p-types of semiconductor a temperature rise increases the proportion
of minority carriers and increases the conductivity of the semiconductor device.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 12
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The p-n junction
The operation of many semiconductor devices depends on the effects, which occur at
the boundary (junction), between p- and n- type materials found in the same
continuous crystal lattice.
Unbiased Junction
A p-n junction is shown in figure 7. As soon as the junction is produced, free
electrons near it in the n-type material are attracted across into the p-type material
where the electrons combine, and fill, the holes. As a result the n-type material
becomes positively charged (due to a loss of electrons) and the p-type material
becomes negatively charged (due to a gain of electrons – remember that both the nand p-types were previously electrically neutral). At the same time holes pass across
the junction from p- to n-type, capturing electrons there.
p-type material
p-type material
n-type material
Junction voltage
n-type material
Figure 7
Hole moving
across junction
Electron moving
across junction
Depletion or barrier layer
The exchange of charge soon stops because the negative charge now on the p-type
material opposes the further flow of electrons and the positive charge now on the ntype material opposes the further flow of holes. The region on either side of the
junction becomes fairly free of charge carriers (see figure 7), and this region is called
the depletion or barrier layer or region. It is less than 10 µm wide and is, in effect, an
insulator. A potential difference known as the junction voltage, acts across the
depletion layer from n- to p-type material, being about 0.6 V for silicon and 0.1 V for
germanium.
Reverse Biased Junction
If a battery is connected across the p-n junction with its positive terminal joined to the
n-type side and its negative terminal joined to the p-type side, it helps the junction
voltage. Electrons and holes are repelled farther from the junction and the depletion
layer widens as shown in figure 8a. only a few minority carriers cross the junction and
a tiny current, called ‘leakage’ current or ‘reverse’ current, flows. The resistance of
the junction is very high in reverse bias.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 13
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p-type
n-type
p-type
n-type
electrons
Holes
Electrons
Holes
Widened depletion layer
Figure 8b
Fig 8a
Forward Biased Junction
If a battery is connected so as to oppose the junction voltage, the depletion layer
narrows. When the battery voltage exceeds the junction voltage, appreciable current
flows because majority carriers are able to cross the junction. Electrons travel from nto p- side and holes travel in the opposite direction as shown in figure 8b. The
junction is then forward biased, that is the p-type side is connected to the positive
terminal of the battery and the n-type side is connected to the negative terminal of the
battery. A small leakage current again flows due to minority carriers. The resistance
of the junction is very low in forward bias.
Diodes, which are used only for one way conduction, are called signal diodes and
rectifier diodes and they are indicated by the symbol shown in figure 9a. Diodes used
for voltage regulation, called Zener, diodes, are indicated by the symbol in figure 9b,
while diodes used to give a light signal, are called light emitting diodes or LED’s use
the symbol shown in figure 9c.
Anode
Direction of
forward
current flow
(a)
(b)
(c)
Cathode
Figure 9 diode symbols
(a) Signal or rectifier diode. (b) Zener diode. (c) Light-emitting diode (LED).
The arrowhead part of the symbol indicates the connection to the part of the diode
called the anode. The flat bar indicates connection to the part called the cathode. For
normal (forward) conduction of current, the anode needs to be at a voltage, which is
more positive than the cathode.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 14
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Tutorial
Either insert the answer or delete the wrong answer
1)
Name two insulating materials
a)
b)
2)
Name the materials falling between the categories of insulators and
conductors.
3)
A simplified model of a silicon atom is shown in figure Q3. Name the parts
marked 1 and 2
2
Figure Q3
1
1
2
4)
A bond between atoms which results in them sharing electrons is called a
_________________________ bond
5)
The figure shown in figure Q5 is called a ________________ structure
Fig Q5
6)
Pure silicon and germanium are good INSULATORS / CONDUCTORS
especially at very low temperatures.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 15
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7)
A pure semiconductor is said to have a DOPED / PERFECT crystal lattice
structure
8)
The current in a pure semiconductor is very SMALL / LARGE when an
external battery is connected to it, and this conduction is called _________?
9)
Increasing the temperature of the semiconductor DECREASES /
INCREASES this conduction.
10)
Adding impurities to a pure semiconductor is known as _______________
This forms a material, which is known as a ___________________ semiconductor.
11)
There are Two types of these semiconductors known as ___________type,
and ___________ type.
12)
The junction shown in figure Q12 is FORWARD / REVERSE biased.
P
N
A
Fig Q12
13)
The reading on the ammeter A will be LARGE / SMALL.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 16
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Exercise 1
Using a digital multimeter and the selection of diodes given to you, test the diodes in
the following ways:
• Set the multimeter to the lowest resistance setting (if the meter is not auto-ranging)
then measure and record the resistance with one lead connect to the anode and the
other connected to the cathode.
• Reverse the leads and repeat the tests.
• Repeat the tests with the other diodes with which you have been supplied.
• Set the multimeter to the diode test setting and repeat all of the tests carried out
above.
In the first set of tests a good diode should give a low resistance in one direction and a
high resistance (normally indicated as 2/) in the other direction. The second set of
test should give, for a good diode, a reading in the range of 0.55V to 0.75V in one
direction and an 2/ reading in the other.
Signal and rectifier diodes
Signal diodes are used for demodulation; clamping and gating are often point contact
diodes. These have very small junctions, which can pass only small amounts of
current, but they have the advantage that the capacitance between the anode and
cathode is very small, which is a desirable characteristic of any component used in
high frequency circuits.
Junction diodes having larger area junctions are better suited to rectifier circuits,
which operate at low frequencies but pass large amounts of current.
Exercise 2
Diode investigation
Object: to measure and plot the forward and reverse characteristics of a silicon diode.
Equipment: 0 – 10 V d.c. supply, prototype board, leads, 3 digital multimeters, 1kΩ
0.25 W resistor, 1N4001 diode
Method: connect the circuit as shown in figure 10 and measure the supply voltage, the
voltage across the diode and the diode current. Enter you results in table 1. Increase
the supply voltage until you have measured the current for each of the diode voltages
shown in table 1.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 17
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d.c. Power supply
Vd
-
+
1N4001
1KΩ
Vs
Figure10
Id
Vs
Vd
0
0.1
0.2
0.3
0.4
0.5
0.55
0.56
0.57
0.58
0.59
0.60
0.61
0.62
0.63
0.64
0.65
Id
Table 1
Now plot the diode voltage against the diode current. (ensure that you make
maximum use of the graph paper).
Id
mA
Vd
Reconnect the circuit as shown in figure 11 by reversing the diode connections. Take
the measurement as before and complete table 2
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 18
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0.66
d.c. Power supply
Vd
-
+
1N4001
1kΩ
Vs
Figure11
Id
Vs
Vd
Id
0
1
2
3
4
5
6
Table 2
Now plot the diode voltage against the diode current on a second sheet on graph
paper. (ensure that you make maximum use of the graph paper).
-Vd
-Id
µA
Characteristics of diodes
A resistor is specified by its values of resistance and its power rating, so asking for a
47 kΩ 0.25 W resistor ensures that the correct item is obtained. Diodes are less easy
to specify because they do not obey Ohm’s law, and so have no consistently valid
resistance value. To specify diodes completely, their characteristics have to be given
i.e. graphs of current plotted against voltage in both directions. (refer to your results
for exercise 1). Diodes can be specified by their voltage and current ratings but this
does not give the full picture. The two main electrical properties of the diode are its
average forward current If (the amount of current in can continue to pass) and its
maximum reverse voltage Vrrm (the maximum repetitive reverse voltage it can
withstand before it breaks down).
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 19
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Typical characteristics for a silicon diode are shown in fig 12 . The characteristics
show voltage plotted on the X-axis, with forward (anode Positive) plotted on the right
hand side and reverse voltage (anode negative) plotted on the left-hand side. Note that
the scale used in plotting reverse voltage is very different from the scale used in
plotting forward voltage because the values differ.
Current in the forward direction is plotted on the Y-axis above the centre line, with
reverse current plotted below the centre line, again on different scales because of the
great difference in values. This method of plotting is always used for diode
characteristics, so that the scale markings must always be examined carefully.
tF
(mA)
100
80
60
40
20
-150
-100
-50
V(VOLTS)
F
0.2
V(VOLTS)
R
-2
0.4
0.6
Figure 12
-4
-6
tR
µ
(A)
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Zener diode
As we have already noted the amount of doping affect the quantity of charge carriers
available within a semiconductor, but this process can be taken a stage further. With
precise control of the doping the characteristics can be as shown in figure 13 .
IF
VR
VB
VF
0.2
0.4
0.6
5.1V
IR
In this case a very precise reverse breakdown characteristic has been produced and the
diode can recover from this breakdown every time. This type of diode is known as a
Zener diode and is always used in the reverse direction. The symbol for a Zener diode
is shown in figure 14
Figure 14
Zener reverse bias conditions
The reverse current that flows in a p-n junction is due to minority carriers crossing the
depletion region. However if the reverse voltage is continuously increased, the point
of reverse breakdown is eventually reached and the current increases very suddenly.
Zener diodes have a low reverse breakdown and they can recover from this reverse
breakdown as long as their other ratings such as power are not exceeded. Strictly
speaking, Zener breakdown applies to breakdowns at less than 5 volts and above 5
volts the breakdown is due to the avalanche effect. As mentioned earlier the Zener is
always connected in the reverse direction the cathode being connected to the positive
of the supply and the anode to the negative. Once the diode is reversed biased
breakdown will occur as soon as the applied voltage reaches the value specific to that
diode. After breakdown occurs, the voltage across the diode remains virtually
constant at that value for a large variation of current through it.
The Zener diode is used as a voltage reference source, for protection of analogue
meter movements and for waveform shaping.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 21
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The power rating of a zener diode shall not be exceeded to prevent damage e.g. a
BZX79 series diode with a rating of 5.1 volts can dissipate 500mW( see
manufacturer’s catalogues) and its maximum current is given by:
Imax =
Power
= 0.5 W = 0.1 A = 100mA
Voltage 5.1V
If the diode was used to supply a reference voltage (i.e. negligible current) of 5.1 V
from a 9 V battery (whose output reduces with use) the maximum voltage to dropped
across the series resistor is 9 –5.1 = 3.9 V
The value of the series resistor required to protect the zener diode is therefore:
V
Imax
=
3.9
0.1
= 39Ω
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 22
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DIODE APPLICATIONS
Rectification
One of the most common applications for a diode is rectification. This uses the
diode’s ability to conduct in only one direction, to convert an alternating current (a.c.)
into a uni-directional current (d.c.)
Half-Wave rectification
When only one diode is used in a circuit as shown in figure 15 then current will flow
in the load only when point A in the circuit is more positive than point B
A
ac
supply
I
Load
Figure 15
B
For positive half cycles, A is positive relative to B, the diode conducts and a current
flows in the load resistor R.. The waveform at R is the same as that at the transformer
secondary, less the small volt drop across the diode, (the barrier potential) as shown in
figure 16.
For negative half cycles, A is negative relative to B, the diode does not conduct and
therefore no current flows in the load resistor R . Due to the fact that the voltage
across the resistor is zero for every half cycle the average dc level is very low for this
type of circuit (see figure 16)
V
H ALF W AV E
t
Figure 16
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Full-Wave rectifier (Bi-Phase)
The circuit diagram for the Bi-phase rectifier is shown in figure 17.
For positive half cycles, A is positive relative to B, and B is positive relative to C.
therefore D1 is forward biased, and D2 is reverse biased. D1 conducts and current
flows in the load.
D1
A
∼
B
IL
C
D2
V
FUL L W AV E
t
For negative half cycles, C is positive with respect to B and B is positive with respect
to A. This means that D2 is forward biased and D1 is now reverse biased. D2 now
conducts and current flows in the load in the same direction as before. The output
waveform is also shown in figure 17. As can be seen in the output waveform the
average d.c. output voltage is higher than the half-wave rectifier output.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 24
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The bridge rectifier
The bridge rectifier circuit arrangement is the most commonly used arrangement for
single-phase rectifiers. The circuit connections can be shown in a number of ways, but
the two most common diagrams are shown in figure 18.
D1
D2
D3
ac
ac
Load
D2
D4
D4
D3
D1
load
Figure 18
In the full-wave rectifier two diodes are conducting and two diodes are blocking at
any one time. The current path for the positive half cycle is from the ac supply,
through D1 down through the load then through D4 and back to the ac supply. The
current path for the negative half cycle is from the ac supply, through D2 down
through the load then through D3 and back to the ac supply. These current paths are
shown in figure 19.
+
D3
D1
D3
D1
ac
ac
-
D4
D2
load
D4
D2
Figure 19
The output waveform is the same as that shown in figure17
As with the Bi-Phase circuit the average dc. output voltage is 0.64 x the peak ac.input
voltage. This is much better than the half wave circuit where the value is only 0.32 x
the peak ac input voltage. The major problem with this type of dc voltage is that it
returns to zero twice every cycle and this would not be very useful in electronic
circuits.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 25
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load
In order to improve this waveform a reservoir capacitor is added across the output of
the rectifier and this produces the output voltage as shown in figure 20. As the output
voltage is increasing the capacitor charges up and as the output voltage starts to return
to zero the capacitor discharges into the load and hence helps to maintain the output
voltage at a higher level. The effect of the capacitor can be observed by carrying out
practical exercise 3. This, as can be seen during the practical exercise still leaves a
small ripple on the output voltage which can be reduced by connecting a simple filter
before the load, as shown in figure 21
V
AVERAGE
DC LEVEL
t
0
Figure 20
R
C
Vout
Figure 21
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 26
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In all rectifier circuits, every diode will be reversed biased for half of the ac cycle.
The amount of this reverse bias depends on the type of circuit used, and is greatest for
a half wave circuit feeding into a reservoir capacitor.
Table 3 shows the operating results which may be expected from rectifiers of the three
types already described given an ac input of E volts peak and smoothing by reservoir
capacitor.
Note that in all rectifier circuits, reversing the connections of the diodes reverses the
polarity of the output voltage.
CIRCUIT
DC. OUTPUT
(10KΩ)
REVERSE
VOLTAGE ON
DIODES
DC. OUTPUT
(FULL LOAD)
Half-wave
E – 0.7V
2E
0.32E
Bi-phase
E – 0.7V
E
0.64E
Bridge
E – 1.4V
E
0.64E
Table 3
Summary
All rectifier diodes must be chosen for the correct forward current and the correct
reverse voltage when not conducting.
The addition of a reservoir capacitor brings the output voltage up to almost peak ac
value.
The ripple is at supply frequency when using a half wave rectifier and is at twice
supply frequency when using a full wave rectifier.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 27
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Exercise 3
Equipment
1 signal generator
1 dual beam oscilloscope
1 digital multimeter
1 1N4001 diode
1 470µF/ 25V electrolytic capacitor
1 rectifier pack (as available)
0.5 W resistors 470R, 1K, 4K7
Prototype board and wires
Object
To investigate the operation, and sketch the waveforms of a half-wave and full-wave
rectifier.
Method
Part1
1. Connect the circuit as shown in the diagram of figure 1, set the output of the
signal generator to 4 Vpk- pk. Measure the voltage across the load resistor and
note it on table 1, then with the aid of the oscilloscope, draw a sketch of the input
and output waveforms on the graph paper provided.
2. Connect the capacitor across the load and repeat step 1
3. Disconnect the capacitor, the reverse the diode and repeat the measurements of
step 1.
A
1N4001
4V pk-pk
1kHz
B
1KΩ
Figure 1
Part 2
WARNING: DO NOT ATTEMPT TO DISPLAY / MEASURE THE
INPUT AND OUTPUT WAVEFORMS OF A FULL WAVE
RECTIFIER AT THE SAME TIME, ON YOUR OSCILLOSCOPE.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 28
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1. Connect the circuit as shown in figure 2
2. Measure and record in table 2 the voltage across the load resistor and draw the
waveform, on the graph paper.
3. Connect the capacitor across the load resistor and repeat step 2
4. Change the load resistor to i) 1K0, ii) 470R and repeat steps 2 and 3 for each
resistor.
∼
ac
+ve
Rectifier
pack
4.7KΩ
∼
Figure2
Table 1
SUPPLY
LOAD VOLTAGE
LOAD VOLTAGE
VOLTAGE AC
(WITHOUT
CAPACITOR)
(WITH
CAPACITOR)
RES.
SUPPLY
LOAD VOLTAGE
LOAD VOLTAGE
VOLTAGE
AC
(WITHOUT
CAPACITOR)
(WITH
CAPACITOR)
COMMENTS
COMMENTS
4K7
1K0
470R
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Conclusions (with reference to the voltages and waveforms obtained write your own
description of differences between half-wave and full-wave rectification, and also on
the effectiveness of capacitor smoothing under differing load conditions)
Zener stabiliser
The circuit of a simple Zener stabiliser is shown in fig 22. The input to the circuit is
an unstabilised d.c. voltage. Resistor RS is connected in series with the Zener diode
and it provides the necessary protection against excess current flow at the breakdown
point. RL is the normal load connected across the diode. The Zener is reversed biased
and, under normal conditions, will operate in the reverse conduction region of its
characteristics.
Is
Rs
IL
Vin
Vz = 20V
RL
Figure 22
Iz
The output voltage across the load will remain constant for i) changes in load current
and ii) changes in supply voltage.
If the current in the load increases the current in the Zener diode will fall (assuming
that it is not already at its minimum value) this allows the current in the series resistor
RS to remain constant. Therefore the voltage drop across RS remains constant and
hence the output voltage remains constant.
If the supply voltage increases, the current in the Zener diode increases causing the
voltage drop across the series resistor to increase. The output voltage therefore
remains constant.
Care must be taken to ensure that the current through the Zener diode does not
increase beyond the maximum value or permanent damage to the Zener diode will
occur.
Example
If the Zener diode circuit of fig 22, stabilises with a minimum current of 1mA.
Calculate the maximum and minimum supply voltages between which stabilisation is
obtained. The load resistance RL is 1kΩ, RS is 300Ω and the Zener diode is a BZX79.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 30
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1/
IL = VZ / RL = 20 / 1000 = 20mA
IZ = Imax = 100mA (from data sheets)
IS = IL + IZ = 120 mA
Vsupply(max) = IS * RS + VZ = (120mA * 300) + 20 = 56 V
2/
IL = VZ / RL = 20 / 1000 = 20mA
IZ = 1mA (minimum value for stabilisation)
IS = IL + IZ = 21 mA
Vsupply(min) = IS * RS + VZ = (21mA * 300) + 20 = 26.3 V
If the supply voltage of example 1 is 50 V, determine the minimum value of RL(i.e.
the maximum current the circuit can supply) if stabilisation is to remain effective.
IS = (Vin – VZ) / RS = (50 – 20) / 300 = 100mA
IZ = 1mA (minimum value for stabilisation)
IL = IS - IZ = 99mA
RL = VZ / IL = 20 / 99mA = 202Ω
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 31
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Exercise 3
Q1. Calculate a suitable value of series resistor for a Zener stabiliser circuit whose
input voltage varies from 15 V to 35 V for a stabilised output of 12 V. the load
resistor is 680Ω and the Zener characteristics give a minimum current of 2mA and a
maximum of 200mA.
Q2. Calculate the value for a suitable series resistor for a zener stabiliser circuit whose
load current varies from 20mA to 120mA with an output voltage of 5.1 V, the input
voltage is 9 V and the minimum Zener current must be 1mA and the maximum
permitted is 150mA.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 32
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Diode Clipping
It is sometimes necessary to limit or clip a waveform to a particular predetermined
level. Examples of clipping are shown in figures 23 and 24
Clipper
circuit
output
Clipper
circuit
input
output
input
Figure 24
Figure 23
In figure 23 the clipper is used to remove high voltage spikes from the input
waveform.
In figure 24 the clipper is used to produce a square wave from the sine wave input.
R
0.6V
Figure 25
During the positive half cycle of the input waveform the diode is reverse biased
(non-conducting) therefore the output takes the same shape as the input.
During the negative half cycle the diode is forward biased (conducting) and the
voltage dropped across the diode is 0.6 V (the forward voltage drop across a silicon
diode). As the diode is connected across the output then 0.6 V appears at the output
If the input voltage is large enough then this 0.6 V can be neglected and the
therefore the circuit shown in figure 25 can be used to completely clip the negative
half cycle.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 33
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+2V
2V
Figure 26
To produce clipping at other levels, an emf may be added in series. In the circuit
shown in figure 26 the series emf used causes the diode to be forward biased until
the input voltage is greater than 2 V when it will become reverse biased and the
output will then follow the same shape as the input. During the negative half cycle
the output will remain at +2 V.
Output
D1
D2
+2V
Input
4V
2V
-4V
Figure 27
For the circuit shown in figure 27 diode D1 is reversed biased for the positive half
cycle. The cathode of D2 is at +2V and only become forward biased when its anode
potential rises above +2V, at that point the output stays at +2V until the anode
voltage drops below + 2V (i.e. The positive peak is clipped to +2V). during the
negative half cycle of the input, diode D2 is reverse biased. The anode of D1 is at –
4V and is reverse biased. D1 only become forward biased when its cathode
potential drops below –4V , at that the output stays at –4V until its cathode rises
above –4V (i.e. The negative half cycle is clipped at –4V).
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 34
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Zener clipping
Zener diodes can also be used in clipping circuits as shown in figure 28. During the
positive half cycle of the input diode ZD1 conducts only when its cathode reaches
breakdown voltage (+6Volts in this case) where the output remains until the input
voltage drops below this value (i.e. The input positive peak is clipped at +6V). During
the negative half cycle the diode ZD1 is forward biased, assuming this is an ideal
diode Vout = 0V clipping all the negative half cycle.
Rs
+6V
Input
ZD1
Output
Figure 28
Diode Clamping
A clamper circuit or dc restorer is used to provide an ac waveform with a dc level. A
clamper does not change the shape of the input signal, it just provides it with a dc
component. As an example a video signal contains a dc level which corresponds to
the average brightness of the screen. This level is sometimes lost due to ac coupling
and has to be restored the circuit shown in figure 29 shows a clamping circuit.
Y
+5V
X
C1
Vin
+10V
R1
D1
Vout
0V
-5V
Figure 29
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 35
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Note: The impedance of a capacitor is dependent on the highest frequency component
of the input waveform.
XC = 1 / 2πfC
If the component frequency is high the impedance will be low.
The frequency components of the supply waveform can be determined by the slope of
the waveform, the steeper the slope the higher the frequency. Therefore as the leading
and trailing edges of the square wave input in figure 29 have a very high frequency
components the impedance of the capacitor is very low.
Circuit operation
In the circuit of figure 29, point Y goes from +5V to –5V during the trailing edge of
the input waveform and the potential at point X becomes the same as that at point Y
due to the low impedance of the capacitor. This means that point X is negative and
diode D1 is forward biased (short circuit across R1) so Vout becomes 0 Volts and the
capacitor C1 is quickly charged through D1. During the transition to the leading edge
point X goes from –5V to +5V, the diode is reverse biased. Therefore the voltage drop
across R1 is 10V with point X being positive. C1 will charge through R1 which will
cause Vout to fall, but if the RC time constant is large compared to the period of the
wave form the output will be that shown in figure 29 (remains constant until the
trailing edge appears).
The same circuit can be used to restore the dc level to an ac sine wave.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 36
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Exercise
Equipment
1 dual dc power supply
1 signal generator
1 oscilloscope
1 10µF / 63 V capacitor
1 BZX 85 5V1 diode
2 1N4148 diode
1 100 KΩ 0.5W resistor
1 180 Ω 0.5W resistor
Object
To investigate diode clipping and clamping
Method
Connect the circuit as shown in figure 30. Set the output of the signal generator to 4V
pk – pk 1KHz and measure and record the output using the oscilloscope.
100kΩ
Vin
4V
1kHz
Vout
1N4148
Figure 30
After completing the previous part of this exercise switch of your equipment and add
the dc power supply to the circuit as shown in figure 31. Set the dc supply to 2V and
measure and record the output as before.
100kΩ
1N4148
Vin
4V
1kHz
Vout
2V
Figure 31
Adjust the dc supply to 1V and measure and record the output.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 37
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Switch off your equipment and replace the 1N4148 diode with the zener diode and
remove the dc power supply from the circuit. This should give you the circuit as
shown in figure 32
180Ω
BZX 85
5V1
Vin
8V
1kHz
Vout
Figure 32
Connect the signal generator and set the frequency to 1kHz. Switch on and adjust the
input to 8V then measure and record the input and output waveforms with the aid of
you oscilloscope.
Part 2 diode clamping
Connect the circuit as shown in figure 33 and adjust the input to 10 V peak to peak
10kHz square wave. Measure and record the input and output waveforms with the aid
of the oscilloscope.
Y
X
10µF/63 V
Vin
100kΩ
1N4148
Vout
Figure 33
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 38
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POWER CONTROL DEVICES
Thyristors
A thyristor is a semiconductor device, which acts, in a similar fashion to a rectifier
diode with one important difference. When the thyristor is forward biased, unlike a
normal diode, it will not conduct until a trigger voltage is applied to the gate. The
symbol for a thyristor is shown in figure 33
Anode
Anode
Gate
Alternative symbol
Gate
Cathode
Figure 33
Cathode
Operation
The thyristor is triggered into a conducting state by applying a positive voltage (or
pulse) to the gate, while the anode is positive with respect to the cathode (the thyristor
is forward biased) and the current flowing exceeds the latching current. Once the
thyristor is in its conducting state, the gate losses all control over the device. In order
to bring the thyristor back into its non-conducting state, the anode current has to be
reduced below the holding current. This can be achieved by reducing the voltage
across the anode and cathode to zero volts or by reverse biasing the thyristor (as will
happen if the supply is ac). The gate current required to trigger the thyristor is much
smaller than the main current that will flow through the device.
Latching current
The latching current is the minimum current at which the gate voltage can be removed
while the thyristor remains in its conducting state. If the gate voltage is removed
before this value of current is flowing, the thyristor will revert back to the nonconducting state.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 39
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Holding current
The holding current is the minimum current which must be flowing through the
thyristor in order to hold the device in conduction after it has been switched on. If the
current is reduced below this value the device will switch off.
Operating modes
The thyristor is said to have three modes of operation, forward blocking, forward
conduction and reverse blocking. These are all shown in figure 35 .
IF
FO RW AR D
CO N DUCTIO N
RE G IO N (O N )
FO R. I G = 0
Varying Values
Of IG
VR
V BR ( R )
VF
RE VE RSE
BLO C KIN G
FO RW AR D
BLO C KIN G
(O FF)
Figure 35
Applications
The thyristor has no moving parts and can operate at extremely high speeds without
arcing. Typical applications of the thyristor include controlling the power in light
dimmer circuits, motor speed control circuits and alarm circuits. These power control
applications can be achieved by varying the point at which the thyristor is switched on
during the positive half cycle, which is known as phase control, or by firing the
thyristor for several cycles then having several cycles with no triggering. This second
method is known as burst triggering and is suitable for applications where this will not
be noticed, such as an electric furnace. It cannot be used for lamp dimming as the
lamp would be on full brightness for several second and then off for several seconds,
whereas this would have no adverse effect on the heating of the electric furnace.
Since the thyristor is a half wave device it can only control power in the positive or
negative half cycles of an ac waveform. To overcome this problem the triac was
developed.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 40
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Triac
The triac is a single devices containing essentially two thyristors back to back. When
triggered, the triac can therefore conduct during both the positive and negative half
cycles of the ac supply. The power available to the load can therefore be controlled
from zero up to full wave. The symbol for the triac is shown in figure 36.
MT1
Gate
Figure 36
MT2
Operation
The triac operates in a similar fashion to the thyristor and has to be triggered before it
will conduct. In the case of the triac however, terminal MT1 can either be positive or
negative with respect to MT2. It is also possible to use positive or negative gate pulses
to trigger the triac into conduction. The triac has four modes of operation and these
are forward blocking, forward conduction, reverse blocking and reverse conduction.
Applications
Typical applications of the triac include ac power control of electric furnaces, lamp
dimmer circuits and motor speed control.
Diac
The diac is a two terminal device containing a two directional zener diode. It is
mainly used as a triggering device for the thyristor and triac. The symbol for a diac
and its physical appearance are shown in figure 37.
Physical
appearance
Symbol
Glass
encapsulation
Non-polarised
Figure 37
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 41
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The diac conducts or turns on when some predetermined voltage level is reached, say
30 volts and therefore it can be used to trigger the gate of a thyristor or triac each time
the input wave form reaches this predetermined value. Since the diac contains back to
back zener diodes it conducts on both positive and negative half cycles.
A typical lamp dimmer circuit using a triac and a diac is shown in figure 38
Lamp
R
Triac
VR
a.c. mains
supply
Figure 38
C
Diac
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 42
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VS
LO A D
T IM E
Th
a.c.
S U P P LY
VS
IG
IG
G AT E
T R IG G E R
C IR C U IT
IL
T IM E
IL
T IM E
Figure 39
VS
LO A D
T IM E
Th
a.c.
S U P P LY
VS
IG
IG
G AT E
T R IG G E R
C IR C U IT
IL
T IM E
IL
T IM E
VS
LO A D
T IM E
IG
Triac
a.c.
SU PPLY
VS
IT
IL
T IM E
G AT E
T RIG G ER
C IR C U IT
IL
T IM E
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 43
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Exercise
mA
Equipment
1 C106 thyristor
1 1kΩ 0.5W resistor
1 6V 60mA Lamp
1 dc power supply
1 multimeter
1 switch
1kΩ
+
6V dc
-
Switch
Figure 40
Object
Lamp
To investigate the operation of a thyristor
Method
Connect the circuit shown in fig 40 and switch on the power supply to 6 volts with the
switch in the open position and note that the lamp is off. Put the switch into the on
position and the lamp should light. Now return the switch to the off position and the
lamp should remain lit.
Measure and record the current through the thyristor. Reduce the supply voltage
slowly, noting the current just before the lamp goes off. This value is known as the
holding current.
Switch off and reverse the connections from the power supply as shown in figure 41.
Switch on to 6V and close the switch, note what happens to the lamp.
mA
6V dc
+
1KΩ
Switch
Figure 41
lamp
The lamp did not light when the switch was closed as the thyristor is in reverse
blocking mode.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 44
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Tutorial
1.
Draw the BS symbol for a Thyristor, a Triac and a Diac
2.
Give two examples of where you would normally us a Thyristor
3.
Explain the function of a Thyristor (the use of input and output waveforms
may help to illustrate the operation of the device).
4.
Give two examples of where you would normally us a Triac.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 45
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5.
Explain the function of a Triac (the use of input and output waveforms may
help to illustrate the operation of the device).
6.
Give an example of where you would normally use a Diac
7.
Explain the function of a Diac
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 46
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TRANSISTOR AMPLIFIERS
Before looking at transistor amplifiers and their operation it is necessary to first look
at the types of transistor available and how they operate.
Two sorts of transistor
Despite the widespread use of integrated circuits, the discrete transistor is still in great
demand by the circuit designer. The use of a transistor as an amplifier and as a switch
is explained in this section. Since the transistor is capable of amplifying a signal, it is
said to be an active component. There are two types of transistor in use:
(a) the Bipolar or junction transistor (BJT)
(b) the unipolar or field-effect transistor (FET)
Usually when people talk about the transistor they mean a discrete bipolar transistor
since this type is more widely used than the unipolar type. Bipolar means that the
transistors operation depends upon two charge carriers at the one time, holes and
electrons. Unipolar means that the transistor’s operation depends on the flow of
electrons or holes but not both.
What do bipolar transistor look like ?
There are thousands of bipolar transistors. Figure 42 shows what a few commonly
used types look like.
e
C
b
b
TAG
c
e
c
b
b
e
TAG
TO 18 O U TLIN E
(M ETA L CA N )
c
b
e
c
e c b
b
b
c (CASE)
b c e
E-LIN E O UT LIN E
(P LA STIC S )
e
TO 92a O U TLINE
(P LA STIC S )
TO 3 O UT LIN E
(M ETA L CA SE )
c
e
TAG
e
Figure 42
c
b
bce
e
TO 220 O U TLINE
(P LA STIC S W ITH
M ETA L M O U N TIN G
SU R FAC E )
TO 39 O U TLIN E
(M ETA L CA N )
TO 5 O UT LIN E
(M ETA L CA N )
TAG
The case, which protects the actual transistor, is known as the encapsulation. The two
encapsulations in wide spread use are metal and plastic. The shape or outline of the
transistor is chosen by the manufacturer and depends on what it is to be used for. Each
transistor is given an unique code, which allows identification. American
manufacturers use a code, which begins ‘2N’ (for diodes they use ‘1N’) the number
which follows the prefix is the registered number of that device. European
manufacturers use a code, which indicates the type of semiconductor material used,
silicon or germanium, and the intended use of the transistor. For example, the BC107
transistor, the first letter tells what the semiconductor material is; the letter ‘A’ means
germanium and ‘B’ means silicon. The second letter indicates the main use for the
transistor; the letter ‘C’ indicates that it can be used as an audio frequency amplifier.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 47
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The letter ‘S’ would mean its main use is for switching and the letter ‘F’ indicates
radio frequency amplification.
The bipolar transistor
As stated earlier the transistor is called bipolar because both electrons and holes flow
during its operation.
The semiconductor diode, covered in outcome 1, is a two terminal device having one
pn junction in its structure see figure 43.
P
N
figure 43
The transistor is made up of three ‘doped’ semiconductor layers, two relatively wide
regions, collector and emitter, with an extremely thin layer, base, between them. The
transistor is a three terminal device.
The arrangement can be NPN or PNP and this, along with the current paths is shown
in figure 44
IC
IC
C
C
N
IB
P
IB
P
N
B
N
B
P
E
E
I
IE = IC + IB
I
IC
IE = IC + IB
C
IB
IC
C
IB
B
B
E
Figure44
E
IE
IE
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 48
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In the example given in figure 44 the arrows indicate the direction of conventional
current flow, but in the following descriptions it is electron flow which will be
discussed.
What a transistor does
Action
There are two current paths through a transistor. One is the base emitter path and the
other is the collector-emitter (via base) path. The value of the transistor is that the
circuits can be linked so that the current in one path can control the current in the
other.
Electrons
Holes
IC
C
+0.6V
VBE
N
IB
P
B
Base emitter
path
N
Collector
Emitter path
+6V
0V
E
I
Figure45
0V
If a potential difference of +6V is connected across an NPN transistor so that the
collector is positive with respect to the emitter (leaving the base unconnected), the
base collector junction is reversed biased since the positive of the supply goes to the
n-type collector.
If the base – emitter junction is now forward biased by applying a voltage (VBE) of
about + 0.6V, electrons flow from the n-type emitter across the junction (as they
would in a junction diode) into the p-type base. The loss of these electrons is made up
by electrons entering the emitter from the external supply, to form the emitter current
IE. (see figure 45)
In the base, only a very small proportion of the electrons (about 1%) from the emitter
combine with holes because the base is very thin (less than 10 microns) and is also
lightly doped. Most of the electrons pass through the base region under the strong
attraction of the positive voltage on the collector. They cross the base collector
junction and become the collector current IC.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 49
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The small amounts of electron – hole recombination’s occurring in the base gives it a
momentary negative charge, which is immediately compensated by a flow of positive
charged holes from the power supply to the base. This flow of holes from the external
supply creates a small base current IB and enables the transistor to maintain the much
larger collector current.
If we regard IB as the input current to the transistor and IC as the output current from
it, then the transistor acts.
i. As a switch in which IB turns on and controls IC (i.e. IC = 0 if IB = 0 and IB = 0
until VBE approaches +0.6V)
ii. As a current amplifier since IC is greater than IB.
Further points
Note the for an NPN transistor the collector and base must be positive with respect to
the emitter; for a PNP type they must be negative.
The input (base emitter) and output (collector emitter) circuits have a common
connection at the emitter. The transistor is then said to be in common - emitter mode.
Two other less usual modes are common - collector and common – base.
dc. current gain
Typically IC is 10 to 1000 times greater than IB depending on the type of transistor.
The dc. current gain hfe is an important property of the transistor and is defined by:
hfe =
IC
IB
For example if IC = 5mA and IB = 0.05mA, then hfe = 5/ 0.05 = 100. Although hfe is
approximately constant for one transistor over a limited range of IC values, it varies
between transistors of the same type due to manufacturing tolerances.
Also note that the current entering the transistor equals that leaving the transistor.
IE = IC + IB
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 50
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Transistor as a switch
Transistors have many advantages over other electrically operated switches such as
relays. They are small, cheap, reliable, have no moving parts, their life is almost
indefinite (in well-designed circuits) and they can switch millions of times a second.
VCC +6V
IC
RC
C
RB
IB
1K
2N3019
B
10 K
Vin
VCE = Vout
Figure46
E
VBE
IE
0V
The basic common emitter switching circuit is shown in figure 46. It contains a
protective resistor RB in the base circuit and a ‘load ‘ resistor RL in the collector
circuit. RL and the transistor form a potential divider across the power supply voltage
VCC
Vin is the input signal (usually dc) applied to the base emitter circuit. It is the value of
Vin, which causes the switching of the circuit output Vout(that is the VCE – taken off
across the collector and emitter). The output switches between a ‘high’ value (near
VCC) and a ‘low’ value (near 0V). the action of this circuit can be explained as
follows.
If Vin is gradually increased from 0V to 6V and measurements are take of Vin, Vout, IC
and IB, the two graphs can be produced as shown in figure 47
Vo/V
A6
(TRANSISTOR PARTLY ON)
B
6
5
5
4
(a)
0.6V < Vi < 1.4V AN D h F E = I C /I B
I C /mA
I B = 0 = I C (TRANSISTOR OFF)
Vi > 1.4V AND
I C Vcc/R L
(TRANSISTOR
FULLY ON)
4
h FE
= I C /I B
(TRANSISTOR PARTLY ON)
3
Vi > 1.4V AND
I C Vcc/R L
(TRANSISTOR
FULLY ON)
2
1
(b)
C
3
2
Figure 47
1
D
0
1
0.6
2
3
4
5
6
Vi/V
0
1
2
3
4
5
6
I B /µ A
1.4
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 51
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From figure 47 the following can be deduced.
i. When VIN is less than + 0.6V, VOUT = VCC = 6V (point AB on graph a) and also
IB = IC = 0 (point 0 on graph b). In this region the transistor is not switched on
and so it has a very high resistance (infinite in an ideal transistor). Since the
resistor RL and the transistor act as a voltage divider, and all the voltage will be
across the transistor. Since there is an infinite resistance there must also be zero
current.
ii. When Vin is greater than 0.6V but less than 1.4 V, VOUT falls rapidly as Vin
increases (BC on graph a), and IC increases linearly as IB increases (OP on graph
b). In this region the transistor is partly on and its resistance decreases rapidly.
As the resistance decreases so the voltage dropped across the transistor decreases
and the voltage across the resistor RL increases. Since the resistance of the
transistor decreases, the currents IC and IB increase.
iii. When Vin is greater than 1.4V VOUT = 0v (CD on graph a) the resistance of the
transistor has reached its minimum and so the voltage across the transistor, VOUT
levels out. This means that the current IC reaches its maximum and will not be
increased by further increases in IB (PQ on graph b). In this region the transistor
is fully on and is said to be saturated because increasing IB does not increase IC.
When the transistor is being used as a switch its action can be summarised by the two
ideal simple circuits shown in figure 48
+6V
+6V
ICRL = 0V
RL
ICRL = 6V
RL
SW
SW
Vout = 6V
0V
0V Figure 48
S open = transistor off
Vout = 0V
S closed = transistor on
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 52
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Field effect transistor (FET)
In a bipolar junction transistor, the small input (base ) current controls the larger
output (collector current; it is a current controlled device. In the FET, the input
voltage controls the output current. The input current is usually negligible (less than
1pA = 1 Pico ampere = 1 x 10 –12 A). This is very useful as it gives the device a very
high input impedance which allows it to match easily with high output impedance
devices.
The FET is a unipolar device where only one type of charge carrier is involved, either
electrons or holes, depending on wither the device is a n-channel or a p-channel. The
FET consists of a bar or channel of n- or p-type semiconductor material with metal
contacts at the ends. These contacts are called the drain (D) and source (S). a third
contact, the gate (G) is connected to a small p- or n-type region, between the drain and
source, which forms a p-n junction. A simplified diagram of an N channel and P
channel FET is shown in figure 49
Gate
Gate
Source
Source
Drain
Drain
p
n
n
p
p
n
n
p
Gate
Gate
Depletion
region
Depletion
region
VGS
VGS
VDS
Figure 49
VDS
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 53
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The action of an FET
The channel acts as a conductor which is narrower in the middle because of the
depletion layer at the p-n junctions, caused by the different gate material at either side
of the channel. Since the depletion layer is short of charge carriers it behaves like an
insulator. When the drain is made positive with respect to the source, electrons flow
from source to drain. The gate is usually negative and reverse biases the p-n junction
widening the depletion region (figure 50) this narrows the cross sectional area
available for current flow therefore the current is reduced.
Narrower
channel
Current reduced
Figure 50
Channel
closed
Current stopped
For a given drain-source voltage VDS, ID is controlled by the gate-source voltage VGS
(more correctly – by the electric field produced by VGS) and ID decreases as VGS goes
more negative.
As the FET is a unipolar device it is less susceptible to temperature effects, as there
are no minority charge carriers and it also has a lower noise level since the charge
carriers do not have to cross a P-N junction. Some advantages in applications of
FET’s are as follows:
Advantages
• High input impedance
• Simple biasing techniques
• Relatively insensitive to temperature
• Low noise level
Applications
• Constant current resistors
• Choppers
• Modulators
• Voltage controlled switches
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 54
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The transistor amplifier (biasing)
In order to make best use of a transistor as an amplifier the circuit should be set up to
ensure that the transistor, when handling the input signal, is always operating in the
linear portion of its characteristic. This is region from O-P on graph b figure 47, this is
when the transistor does not switch off (point O) and does not reach saturation (point
P). This set up is known as biasing the transistor. Three types of bias circuits are
shown in figure 51. The simplest uses a single resistor connected between the supply
voltage and the base of the transistor figure 51 (a). This type of bias is seldom used
because the correct value for the resistor depends on the hfe of the transistor being
used, and would not work properly if the same resistor was used with another
transistor of the same type.
The circuit shown in figure51 (b) is a considerable improvement on the first because
the bias resistor is fed from the variable voltage at the collector. This small change
makes the circuit to some extent self adjusting. If the collector current rises, due to
some change, the voltage at the collector will drop (ohms law) this will reduce the
base current which in turn reduces the collector current. The opposite happens if the
collector current falls the collector voltage rises and the transistor is turned on more
allowing more collector current to flow.
R1
R2
R2
R1
RL
R1
Figure51
R2
RE
a
b
c
The circuit shown in figure 51 (c) is the most commonly used circuit for small signal
amplifiers. The two resistors in the base circuit provide a voltage divider that give the
correct bias voltage and the resistor in the emitter circuit provides control of the base
emitter current. This type of circuit means that there is very little problem in changing
from one transistor to another.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 55
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An amplifier constructed using a FET has a much simpler bias arrangement and the
circuit shown in figure 52 shows the bias for a FET in depletion mode. For correct
bias, the voltage of the gate should be negative with respect to the source voltage, or
to put it another way, the source voltage must be positive with respect to the gate
voltage. In this circuit the positive voltage is obtained from the voltage drop across R3
in series with the source. When there is no input signal the gate voltage is kept at 0
volts by resistor R1, which needs to have a very large value since no current flows in
the gate circuit.
+6V
R2
1K
OUT
IN
Figure 52
R1
1M
R3
470R
0V
Summary (biasing)
The purpose of biasing the transistor is to set its output current to a value, which
permits the best use to be made of its transfer characteristics, to give the maximum
output without distortion. For a linear amplifier having a resistive load, the most
useful bias setting is to have a collector voltage close to half the supply voltage (this is
known as class A). The biasing method chosen must be stable, lest the bias setting be
upset by small changes in component values. The output voltage will be a minimum
when the transistor is conducting heavily and it will be a maximum when the
transistor is nearly off.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 56
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Small signal amplification
In order to use the amplifier circuit for ac signals capacitors have to be added to the
input and output lines. This will prevent the dc bias voltages being effected by any
external voltages, and also prevent the bias voltage from effecting the external
components, such as the signal source. These coupling capacitors are shown as C1 and
C2 in figure 53. A third capacitor CE is required to by-pass the emitter resistor for ac
signals this allows the small input voltage to drive a small current through the base
emitter circuit without having to flow through the emitter resistor. When the emitter
resistor was introduced to stabilise the biasing, it did so by providing negative
feedback to the dc circuit, but this negative feedback is not required for the ac signal
and therefore the capacitor can remove it for ac only. Using capacitors of different
values will allow the amount of negative feedback to be controlled thereby giving
some control over the gain of the amplifier.
+Vc
RC
R
C2
1
C1
Figure 53
Tr1
2N2222
OUTPUT
INPUT
R2
RE
CE
0V
Since as stated earlier the amplifier output goes up when the input goes down, and the
output goes down when the input goes up, the amplifier is an inverting amplifier.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 57
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Practical exercise (common emitter)
Equipment
1 dc power supply
1 signal generator
1 oscilloscope
1 multimeter
1 prototype board
Components as per the circuit diagram
Object
To investigate the operation of a small signal common emitter amplifier.
Method
Connect the circuit as shown in figure 54. Set the power supply at 12V and take notes
and readings of each circuit as you progress.
+12V
R1
15K
RL
1K8
22µF / 25V
22µF / 25V
TR1
BC107
C2
C1
Input
0V
(part 1)
R2
3K3
RE
470R
Output
Figure 54
0V
Using a suitable voltmeter, measure the following dc voltages (no ac signal present).
Voltage across R2 = …………………………………………………….
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 58
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Voltage across RE = ……………………………………………………..
Base- Emitter voltage = ……………………………………………………
Collector voltage = ……………………………………………………….
Voltage across RL = ……………………………………………………..
b)
Using the signal generator, apply a sinewave of 20mV peak to peak, at the frequency
of 1 kHz into the input of the amplifier. Using the oscilloscope, observe and sketch
the following waveforms.
+V
t
Base signal
-V
+V
Collector
signal
t
-V
+V
Emitter signal
t
-V
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 59
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c) Try switching the scope between ac and dc coupling. What do you find?
Answer
…………………………………………………………………………………………
…………………………………………………………………………………………
…………………………………………………………………………………………
d) Calculate the amplifier voltage gain.
Input signal = 20mV peak to peak
Output signal = ………………………………………..
Voltage gain = output signal / input signal =
comments
…………………………………………………………………………………………
…………………………………………………………………………………………
Part 2
+12V
R1
15K
RL
1K8
22µF / 25V
22µF / 25V
TR1
BC107
C2
C1
Input
20mV
pk - pk
(part 2)
R2
3K3
RE
470R
C3
10µF
25V
Output
0V
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 60
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2a) Using a suitable voltmeter, measure the following dc voltages ( no ac signal
present).
Voltage across R2 = …………………………………………………….
Voltage across RE = ……………………………………………………..
Base- Emitter voltage = ……………………………………………………
Collector voltage = ……………………………………………………….
Voltage across RL = ……………………………………………………..
Does the addition of C3 affect the dc voltages? ……………………………
b) Again apply a sinewave of 20mV peak to peak, at the frequency of 1 kHz into the
input of the amplifier. Using the oscilloscope, observe and sketch the following
waveforms.
+V
Emitter signal
t
-V
+V
Base signal
t
-V
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 61
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+V
Collector
signal
t
-V
b) Does the addition of C3 alter the waveforms? And if so how?
…………………………………………………………………………………………
…………………………………………………………………………………………
…………………………………………………………………………………………
…………………………………………………………………………………………
c)
Calculate the amplifier voltage gain.
Input signal = 20mV peak to peak
Output signal = ………………………………………..
Voltage gain = output signal / input signal =
comments
…………………………………………………………………………………………
…………………………………………………………………………………………
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 62
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Practical exercise (common source)
Equipment
1 dc power supply
1 signal generator
1 oscilloscope
1 multimeter
1 prototype board
components as per the circuit diagram
Object
To investigate the operation of a small signal common source amplifier.
Method
Connect the circuit as shown in figure 56. Set the power supply at 20V and take notes
and reading of each circuit as you progress.
VDD
20V
RD
4.7k
C2
4.7µF
C1
4.7µF
Figure 56
Vin
500mV
1kHz
RG
560k
RS
1.2k
CS
10µF
Using a suitable voltmeter, measure and record the following dc voltages (no ac signal
present)
Voltage across RG
= ………………………………………………….
Voltage across RS
= ………………………………………………….
Gate – Source voltage = ……………………………………………………
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 63
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Drain voltage
= ……………………………………………………………
Voltage across RD
= ………………………………………………….
b)
Using the signal generator, apply a sinewave of 500mV peak to peak, at the frequency
of 1 kHz into the input of the amplifier. Using the oscilloscope, observe and sketch
the following waveforms.
+V
Gate signal
t
-V
+V
t
Source signal
-V
+V
t
Drain signal
-V
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 64
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c) Calculate the amplifier voltage gain.
Input signal = 500mV peak to peak
Output signal = ………………………………………..
Voltage gain = output signal / input signal =
comments
…………………………………………………………………………………………
…………………………………………………………………………………………
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 65
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THE OPERATIONAL AMPLIFIER
Introduction
Operational amplifiers (op-amps) are amplifier circuits in which the circuit
components (transistors, resistors, etc.) are integrated on to a tiny silicon chip and
packaged in IC form. They are extremely versatile devices and are used in a wide
range of applications including amplification, signal processing and waveform
generation. Many different types of op-amps are currently available, however the
most commonly used is the 741. The symbol for the op-amp together with the pin out
data for the 741 is shown in figure 57.
+ VS
NC
+VS
O/P
8
7
6
1
2
Offset
Null1
Inv
I/P
Offset
Null2
5
Inverting
input
_
+
Non-inverting
input
Output
5
1
Figure57
4
3
Non-inv - VS
I/P
-VS
There are two input terminals, one called the inverting input (marked - ) and the other
called the non-inverting input (marked + ). The output (assuming zero offset) will be
the difference in the signal between both inputs multiplied by the gain.
Ideally when both inputs are the same the output should be zero. In practice some
offset always occurs (caused by a slight difference in internal components). With both
inputs held at zero volts, the resulting output can be easily trimmed to zero by an
offset nulling technique such as that shown in figure 58
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 66
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Offset null control
2
_
6
741
3
+
Figure58
Output
5
1
10K
+ VS
The 10K potentiometer should be adjusted so that the output voltage is zero.
There are two power supply terminals marked + Vs and –Vs (sometimes marked
+Vcc and –Vcc). Operation is normally from a dual balanced power supply at ±12V
or ±15V , although the circuit can be adjusted to operate from a single power supply.
Properties of Op-amps
The main properties of Op-amps are:
1. A very high ‘open loop’ gain (about 100,000 for the 741) which decreases as the
signal frequency increases.
2. A very high input impedance (typically 106 to 1012Ω), meaning very little current
is drawn from the signal source.
3. A very low output impedance (commonly 100Ω), meaning the efficient transfer of
its output voltage to any load greater than a few kilohms.
Inverting amplifier circuit
R1
+Vs
R2
2
_
741
3
Input
R3
Figure 59
7
6
+
4
-Vs
Output
0V
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 67
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The circuit given in figure 59 is that of an inverting amplifier using a balanced power
supply. The dc bias conditions are set by connecting the + input to the mid voltage
point (ie 0V) via resistor R3 (since the current through R3 is very small, the voltage
across R3 is also very small and the non-inverting input + is at virtually 0V.
The input signal is applied via R2 to the inverting ( - ) input of the op-amp. The
output will therefore be in antiphase with the input (ie 180o for an ac signal).
Resistor R1 provides negative feedback for the amplifier and it is this feedback which
determines the overall gain of the amplifier circuit.
Amplifier gain = - ( R1/ R2)
For example if R1 = 220K, R2 = 22K and R3 = 22K the the gain of the amplifier will
be –10
Ie. -220 / 22 = -10
Note coupling capacitors can be added to the input and output terminals if required
when amplifying ac signals.
Non-inverting amplifier circuit
R1
+Vs
2
_
7
741
3
Input
R2
R3
Figure 60
6
+
4
-Vs
Output
0V
The diagram of figure 60 shows the circuit for a non- inverting amplifier again using a
balanced power supply.
Non inverting amplifiers also make use of negative feedback to stabilise the circuit
conditions in the same way as inverting amplifiers, but the input signal is now applied
to the ( + ) terminal of the op-amp. The output signal will be in phase with the input
signal.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 68
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Amplifier gain = (R1 + R2) / R2 = (R1 / R2 ) + 1
Using the same values as previously the amplifier gain will be = 11
Gain = (220 / 22) + 1 = 11
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 69
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Practical exercise (op-amps)
Equipment
1 dual power supply
1 signal generator
1dual beam oscilloscope
1 prototype board
1 741 op-amp
2 10K 0.25W resistors (R2 & R3)
1 100K 0.25W resistor (R1)
Object
To investigate inverting and non-inverting amplifiers
Method
Using the equipment supplied connect up the circuit shown in figure 61
R1
+Vs
R2
741
3
Input
Figure 61
7
2 _
6
+
R3
4
-Vs
Output
0V
a) Calculate the gain of the amplifier using R1/ R2.
Gain = R1 / R2 = ………………………..
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 70
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b) Using the signal generator and oscilloscope, apply an input signal of 500mV, peak
to peak sine wave at a frequency of 1 kHz to the amplifier. Observe and sketch the
following waveforms.
+V
Input
signal
t
-V
+V
Output
signal
t
-V
c) Calculate the gain of the amplifier
Gain = output signal / input signal =
Compare this to the theoretical gain calculated earlier and comment on any difference.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 71
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Part 2
`Non-inverting amplifier
Connect the circuit as shown in figure 62
R1 = 100K
R2 = 10K
R3 = 10K
+Vs
2
_
7
741
3
Input
R2
R3
Figure 60
6
+
4
-Vs
Output
0V
a) Calculate the gain of the amplifier using (R1 / R2 ) + 1
gain = (R1 / R2 ) + 1 =
b) Using the signal generator and oscilloscope, apply an input signal of 500mV, peak
to peak sine wave at a frequency of 1 kHz to the amplifier. Observe and sketch the
following waveforms.
+V
Input
signal
t
-V
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 72
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+V
Output
signal
t
-V
c) Calculate the gain of the amplifier
Gain = output signal / input signal =
Compare this to the theoretical gain calculated earlier and comment on any difference.
Part 3
Repeat the previous experiment with frequencies of 10kHz, 100kHz and 500kHz and
note the effect this has on the gain of the circuit.
Electronic and Electrical Fundamentals: Introduction to Semiconductor Applications (Int 2) 73
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