Giáo trình Tiếng Anh chuyên ngành Y - Phần 1

Part I – Electronics – Unit 1
Điện Tử Y Sinh 4 – Học Viện Kỹ Thuật Quân Sự 1
Unit 1: Power supplies
Diodes have many uses in electronics. Perhaps their most obvious use is in
the area of power supplies. Power uses diodes as rectifies, to convert alternating
current to direct current. They also use diodes as regulators, to stabilize voltage.
This chapter covers the circuits which use diodes in these ways.
Power supplies also rely on other components that you have already
studied. Most electronic circuits use a great variety of devices. It is very important
to understand how they can perform together as a system.
As you can begin to understand how systems such as power supplies work
in electronics, you can begin to understand the principles of troubleshooting.
Troubleshooting is an analytical process by which you can determine the cause of
circuit problems. It is much easier to repair circuits when you know how and why
they work.
1. THE POWER SUPPLY
This chapter marks your beginning study of electronic circuits. The power
supply is the most fundamental circuit of any electronic system. The power supply
changes the available electric energy (usually ac) to the form required by the
various circuits cannot function. One of the first steps in troubleshooting any
electronic device is to check the supply voltages at various stages in the circuitry.
Power supplies can be very simple or very complicated depending on the
requirements of the system. A simple power supply may be required to furnish 12
V dc. A more complicated power supply may provide several voltages, some
positive and some negative with respect to the chassis ground. Some power supply
may have a wide tolerance regarding voltage.
The actual voltage output may vary ± 20 percent. Another power supply
may have to keep its output voltage within ± 0.01 percent. Obviously, the strict
tolerance complicates the design of the supply.
Figure 5-1 (4-1) shows a power supply as part of an electronic system.
Drawings of this type are called block diagrams. Block diagrams are very

useful for showing the various major circuits within electronic systems. The power
supply in Fig 5-1 (4-1) occupies one of the blocks within the system. It is the most
critical since it energizes the rest of the system. For example, if a problem
develops in the power supply, the fuse might “blow” (open).
In that case, none of the voltages could be supplied to the other circuits.
Another type of the problem might involve the loss of only one of the outputs of
the power supply. Suppose the +12V dc out drops to zero because of a component
failure in the power supply. Circuits A and circuits B would no longer work.
Part I – Electronics – Unit 1
Điện Tử Y Sinh 4 – Học Viện Kỹ Thuật Quân Sự 2
The second output of the power supply shown in Fig 5-1 (4-1) develops
both positive and negative dc voltages with respects to the common point (usually
the metal chassis). This out put could fail, too. It is also possible that only the
negative output could fail. In either case, circuit C would not work normally under
such conditions.
Troubleshooting electronic system can be made much easier with block
diagrams. If the symptoms indicate the failure of one of the block, the technician
can devote most attention to that part of the circuit. Since the power supply
energizes most or all the other blocks, it is one of the first things to check when
troubleshooting.
SELF TEST. Complete the missing word in each statement.
1. Power supply will usually change ac to
2. Power supply voltages are usually specified by using the chassis as
a reference.
3. Drawings such as Fig 1 are called diagrams.
4. On the block diagram, the circuit that energizes most or all the other blocks
is called the
2. RECTIFICATION
Most electronic circuits need dc. AC is supply by the power companies.
The purpose of the power supply is to change ac to dc rectification. Alternating

current flows in both directions, and direct current is only one direction. Since
diodes will conduct in only one direction, they are good rectifies.
Typical ac voltages supplied by the power companies to residential and
small commercial customers are 11V, 120V, 208V and 240V. Electronic circuits
often require much lower voltages. Transformers are used to step down the voltage
to the level needed. Fig 1-2 could be an electronic circuit, a battery being charged,
or some other device. In this chapter, the power supply circuits will be shown with
load resistors (RL).
The transformer in Fig 1-2 has a voltage ratio of 10:1. With 120V across
the primary, 12V ac is developed across the secondary. If it were not for the diode,
there would be 12V ac across the load resistor. The diode will allow the current to
flow through the load only in the direction of the arrow. Since current is flowing in
only one direction, it can be called direct current. When direct current flows
through a load, a dc voltage appears across the load.
Note the polarity across the load in Fig 1-2. Electrons always move from
negative to positive through a load. The positive end of the load is connected to
the cathode end of the rectifier. In any rectified circuit, the positive end of the load
will be that end which contacts the cathode of the rectifier. It can also be stated
Part I – Electronics – Unit 1
Điện Tử Y Sinh 4 – Học Viện Kỹ Thuật Quân Sự 3
that the negative end of the load will be in contact with the anode of the rectifier.
Fig 1-3 shows the negative side of a load resister connected to the anode end of the
resister.
It was stated that to forward bias a diode, the anode must be made positive
with respect to the cathode. It was also note that diode manufactures often mark
the cathode with a plus (+) sign. This may seem confusing. When the diode acts as
a rectifier, the function of the plus sign becomes clear. The plus sign is placed on
the cathode end to show the technician which end of the load will be positive.
Look at Fig 1-2 to see if this so.
Fig 1-4(a) shows the input waveform to the rectifier circuits of Figs 1-2 and

1-3. Two complete cycles are shown. In Fig 1-4(b) the waveform that appears
across the load resistor of Fig 1-2 is shown. The negative half of the cycle is
missing since the diode blocks it. This waveform is called half-wave, pulsating dc.
It represents only the positive half of the ac input to the rectifier.
In Fig 1-3 the diode has been reversed. This caused the positive half of the
cycle to be blocked Fig 1-4(c). The waveform is also half-wave, pulsating dc. Both
circuits, Figs 1-2 and 1-3 are classified as half-wave rectifiers.
The ground reference point will determine which way the waveform will be
shown for a rectifier circuit. For example, in Fig 1-3 the positive end of the local is
grounded. If an oscilloscope is connected across the load, the ground lead of the
oscilloscope will be positive and the probe top will be negative. Most
oscilloscopes show positive as ‘up’ and negative as ‘down’. The actual waveform
seen on the screen will then appear as that shown in Fig 1-4(c). Waveform can
appear up or down depending on circuit polarity, instrument polarity, and the
connection between instrument and circuit.
Half-wave rectifies are usually limited to low power applications. They take
useful output from the ac source for only half the input cycle. They are actually
not supplying any load current half the time. This limits the amount of electric
energy they can deliver over a given period of time. High power means delivering
large amounts of energy in given time. A half-wave rectifier is a poor choice in
high power applications.
TASK 1
Determine whether each statement is true or false.
1. Current that flows in both directions is called alternating current.
2. Diodes make good rectifiers because they will conduct in only one
direction.
3. A rectifier can be used in a power supply to step up voltage.
Part I – Electronics – Unit 1
Điện Tử Y Sinh 4 – Học Viện Kỹ Thuật Quân Sự 4
4. In a rectifier circuit, the positive end of the load will be connected to the

cathode of the rectifier.
5. A half-wave rectifier supplies load current only 50 percent of the time.
6. Half-wave rectifiers are usually used in high power applications.
TASK 2
Sentences in a text are held together by grammar links. Now mark the grammar
links in this paragraph by joining the words in italics with the words they refer to.
Another very important force in engineering is the one caused by elasticity. A
good example of
this is a spring. Springs exert more force the more they are
stretched.
This property provided a way of measuring force.
1. A repeated noun becomes a
pronoun.
Springs become they.
2. A word replaces an earlier
expression.
Force in engineering becomes one.
3. A word replaces a whole sentences or clause.
Spring exert more force the more they are stretched
becomes this property.
When the AC voltage is applied to the search coil, a magnetic field is
produced around it. If there is a metal object under ground, the field induces an
electric current in the object. The induced current in turn creates a magnetic field
around the object. This induces a voltage in the search coil. The induced voltage is
converted into an audible note by the circuitry in the box. This sound guides the
treasure hunter to the buried object.
Look back in the text and find the reference for the word in italics.
1. … , they are good rectifiers
2. … If
it were not for the diode.

3. … it can be called direct current.
4. … end
which contacts the cathode of the rectifier.
5. …
This may seem confusing.
6. … It represents only the positive half of …
7. …
This causes the positive half of the …
8. … They take useful output from…
9. … electric energy
they can deliver.
TASK 3
Part I – Electronics – Unit 1
Điện Tử Y Sinh 4 – Học Viện Kỹ Thuật Quân Sự 5
Put the word in bracket in the correct word form.
1. Alternating current flows in both (direct)
2. Electronic circuits often require much voltage. (low)
3. Transformers are used to step down the voltage to the level (need)
4. The positive end of the load to the cathode end of the rectifier.
(connection)
5. Wave-form can appear up or down on circuit polarity,
instrument polarity, and the between instrument and circuit.
(depend/connect)
6. A half-wave rectifier is a poor in high power applications.
(choose)
Part I – Electronics – Unit 2
Điện Tử Y Sinh 4 – Học Viện Kỹ Thuật Quân Sự 6
Unit 2: Electronics in the home
- Find out the meaning of these abbreviations: IC, CD, Hi-fi
- Read quickly through the text and write down the items mentioned.

Electronics began at the start of the twentieth century with the invention of
the vacuum tube. The first devices for everyday use were radios, followed by
televisions, record players, and tape recorders. These devices were large and used
a lot of power.
The invention of the transistor in 1947 meant that much smaller, low
powered devices could be developed. A wide variety of electronic devices such as
hi-fi (high fidelity) units and portable radios become common in the home.
It was not until 1958 that microelectronics began with the development of
ICs (Integrated Circuits) on silicon chips. This leads to a great increase in the use
of electronics in everyday items. The introduction of the microprocessor allowed
electronics to be used for the control of many common processes.
Microprocessors are now used to control many household items such as
automatic washing-machines, dishwashers, central heating systems, sewing
machines and food processors. Electronic timers are found in digital alarm clocks,
water heaters, electric cookers, and microwave ovens. Telephones use electronics
to provide automatic dialing and answer phone facilities. New entertainment
devices have been developed such as video recorders and CD (Compact Disc)
players.
In the future, electronics are likely to become even more common in the
home multimedia entertainment systems and computer controlled robots are
developed.
TASK 1
Fill the gaps in this table with the help of the text.
DATE INVENTION APPLICATIONS IN THE
HOME
Early 20
th
century
Transistor
1958 Automatic washing machines

Future
Part I – Electronics – Unit 2
Điện Tử Y Sinh 4 – Học Viện Kỹ Thuật Quân Sự 7
TASK 2
Use the space below to make a list of ways in which you think electronics may be
used in the home in the future.





TASK 3. Adjective and noun formation
Use the suffixes in the boxes to write the adjective formed from the nouns and
nouns formed form the verbs.
-ic -es -al -ive -ant -ful
Noun Adjective
Centre
Electricity
Portability
Digit
Constant
Use
-ion -ment -er/ier -or -ty -ce
Verb Noun
Invent
Record
Part I – Electronics – Unit 2
Điện Tử Y Sinh 4 – Học Viện Kỹ Thuật Quân Sự 8
Amplify
Entertain

Use
Vary
Resist
Collect
Complete the sentence with verb and adjective or a noun form the box.
1. People change their mind for a of reasons.
2. Microelectronics began with the of ICs on silicon chips.
3. New entertainment devices have been developed such as video
and CD (Compact Disc) player.
4. New devices have been developed such as video recorder and
CD (Compact Disc) players.
5. Electronics timers are found in alarm clocks and water heater.
6. The EMF is 1.5V and the internal about 50
Ω.
7. Have characteristics that make them very in
electronic circuits.
8. Batteries are often used for equipment.
9. An current, voltage, or charge is one that is produced by
10. The carbon rod is in contact with the positive electrode is called the current

11. NiCad batteries have such as a small internal resistance that the charger must
produce a current output.
12. John is a regular of Holland’s health-care system.
Part I – Electronics – Unit 3
Điện Tử Y Sinh 4 – Học Viện Kỹ Thuật Quân Sự 9
Unit 3: Operational amplifiers
Integrated circuits technology has had quite an impact on modern electronic
design. Integrated circuits have given designers high performance for low cost. In
the linear field, perhaps the best example is the modern operational amplifier.
There are no perfect amplifiers through modern operational amplifiers

approach perfection. They have characteristics are as follows:
1.
Common-mode rejection. This gives them the ability to reduce hum and noise.
2.
High input impedance. They will not ‘load down’ a high-impedance signal
source.
3.
High gain. They have ‘gain to burn’ which can be reduced by using feedback.
4. Low output impedance. They are able to deliver a signal to a low-impedance
load.
No signal-stage amplifier circuit can rate high in all the above
characteristics. An operational amplifier is actually a combination of several
amplifier stages. Refer to Fig 3-1.
The first section of this combination circuit is a differential amplifier.
Differential amplifiers have common-mode rejection and high input impedance.
Some operational amplifiers may use field-effect transistors in this first section for
even better input impedance.
The second section of Fig 3-1 is another differential amplifier. This allows
the differential output of the first section to be used. Thus, common-mode
rejection, maximum signal swing, and additional gain are all realized.
The third section of Fig 3-1 is a common collector, or emitter-follower
stage. This configuration is known for its low output impedance. Notice that the
output is a single terminal. No differential output is possible. This is usually
referred to as a single-ended output. Most electronic applications require only a
single-ended output.
A single-ended output terminal can show only one phase with respect to
ground. This is why in Fig 3-1 one input is marked non-inverting and the other is
marked inverting. The non-inverting input will be in-phase with the output
terminal. The inverting input will be 180 out of phase with the output terminal.
Figure 3-2 shows the amplifier in a simplified way. Notice the triangle.

Often triangles are used in schematic diagram to represent amplifiers. Also notice
that the inverting input is marked with a minus (-) sign and the non-inverting input
is marked with a plus (+) sign. This is standard practice in linear diagrams.
Part I – Electronics – Unit 3
Điện Tử Y Sinh 4 – Học Viện Kỹ Thuật Quân Sự 10
Many circuits today are integrated. Everything is in one little package. A
technician can not see inside the package or make any internal measurements.
Therefore, it is seldom necessary to show the schematic details of the internal
circuit. Fig 3-3 is an example of the standard way of showing an operational
amplifier.
Modern operational amplifiers approach perfection. They have the desirable
characteristics listed earlier. Such amplifiers were originally used in analog
computes. They performed mathematical operations. This is where they got the
name “operational amplifiers”, or “op-amps”. Op-amplifiers are very popular,
mainly because they are available in compact, integrated form at very reasonable
costs.
TASK 1
Answer the following questions.
1. Refer to Fig 1 which section of the amplifier operated as an emitter follower to
produce low output impedance?
2. Refer Fig 1A signal is applied to the inverting input terminal. What is the phase
of the signal that appears at the output terminal as compared to the input signal?
3. Refer to Fig 1 is the output of the amplifier differential or single-ended?
TASK 2
Match a line in A with a line in B
1. An operational amplifier Have a. combination of several amplifier
stage.
2. Most electronic applications Will be b. common-mode rejection and a high
input impedance
3. A technician can not see

inside the package or make any
internal measurements.
Is c. a single-ended output.
4. Op-amps are very popular. Require d. in-phase with the output terminal.
5. The non-inverting input. Therefore e. it is seldom necessary to show the
schematic details of the internal
circuitry.
6. Differential amplifiers. Because f. they are available in compact
integrated form at very reasonable costs.
TASK 3
Here are some circuit symbols. Label them and describe their function.
Part I – Electronics – Unit 3
Điện Tử Y Sinh 4 – Học Viện Kỹ Thuật Quân Sự 11
Example:
What does a fuse do? It protects a circuit.
We can emphasize function by using this pattern:
The function of a fuse is to protect a circuit.
This list of function may help you
a. Adds capacitance to a circuit f. protect a circuit
b. Rectifies alternating currents g. Varies the current in a circuit
c. Adds resistance to a circuit h. Steps AC voltage up or down
d. Measures very small currents i. Receiver RF signals
e. Beaks a circuit j. Measures voltages
Part I – Electronics – Unit 3
Điện Tử Y Sinh 4 – Học Viện Kỹ Thuật Quân Sự 12
Language focus
Enumeration
Enumeration is useful when you want to list or state a number of factors:
Reasons, purposes, causes, results, symptoms, advantages, disadvantages,
recommendations, changes, etc.

In this part of the lesson we will learn how to link sentences using discourse
markers for Enumerating.
1. Adverbial connectives
- First, second, third, last, finally
- First, then, next, lastly
Example:
The cathode ray tube (CRT) operated as follows:
- First electrons are emitted from a heated cathode.
- Then these electrons are accelerated to give them a high velocity.
- Next they are formed into a beam which can be deflected vertically and
horizontally.
- Finally they are made to strike a screen on its inner surface with a
phosphor.
2. Determiners
The first One
The second Another + Noun
The third + Noun A third
The last
The final
3. Pronouns
The first
The second
The third
The last + is
One
Another
A third
Example:
Part I – Electronics – Unit 3
Điện Tử Y Sinh 4 – Học Viện Kỹ Thuật Quân Sự 13

Nuclear reaction can occur by two different processed:
The first is nuclear fission, the second is nuclear fusion.
Or the first process is fission, the second process is fusion
Exercise 1: Using the different markers to write sentences of Enumeration
1. According to one professor, there have been two changes in psychology in the
past 15 years.
- A movement from a behaviorist to a cognitive approach,
- A increased interaction between psychology and other sciences which study
people.
2. Common cold has several symptoms:
- Feel very unpleasant
- Your head aches
- You sneeze and cough
- Your nose is all stuffed up and it keeps running.
3. The order of a filter determines how sharply out-of-band signals are attenuated.
We shall see examples of filters.
- a first-order, high-pass filter,
- a low-pass filter
- to show how the two filters dealt with above can be made into a band pass filter.
Part I – Electronics – Unit 4
Điện Tử Y Sinh 4 – Học Viện Kỹ Thuật Quân Sự 14
Unit 4: Test and repair instruments
TASK 1
List as many instruments used for testing and repair in electronics as you
can. Compare your list with that of another group.
Read this text and check your answers. The following instruments are commonly
used for the test and repair of electronics circuits.
Multi-meter
This instrument can be used to measure a number of different electrical
quantities, such as voltage, current, and resistance, i.e. It is a combined voltmeter,

ammeter and ohmmeter. Multi-meters can have analogue or digital displays and
can be switched to different measuring ranges.
Logic probe
This instrument is used for measuring voltage levels and pulses in digital
logic circuits. When the probe is placed on the pin of a logic IC, small colored
LEDs light up to indicate if a pulse is detected or whether the pin is at high or a
low logic level.
Oscilloscope
This instrument is used to measure fast - moving signals. It shows how a
signal varies with time or relative to another signal. It used a cathode ray tube to
display the waveform of the measured signal on a screen.
Function generator
This instrument contains a triangular wave oscillator which can be switched
to produce triangular, square, or sine waves over a range of frequencies. It used ti
test and adjust a variety of electronic equipment such as audio amplifiers. The
function generator provides a known signal which can be injected into a circuit.
Often it is used with an oscilloscope so that a visual display of the waveform can
be seen.
TASK 2
Which of the instruments would you use to do the following?
1. To check a fuse.
2. To determine the frequency response of an audio amplifier.
3. To test for the presence of a control signal on the output pin of computer
chip.
4. To determine the value of the current through a transformer.
Part I – Electronics – Unit 4
Điện Tử Y Sinh 4 – Học Viện Kỹ Thuật Quân Sự 15
5. To measure the frequency of an oscillator.
TASK 3. Compound nouns.
Study these examples of compound nouns.

A signal generator = equipment for generating signals.
A cassette player – equipment for playing cassettes.
1. Playing CDs?
2. Receiving batteries (signals)?
3. Charging batteries?
4. Amplifying aerial (signals)?
5. Filtering (out) noise?
6. Synthesizing speech?
7. Cleaning cassette heads?
8. Amplifying (the) power (of a signal)?
9. Sensing vibration?
10. Scanning (the human) body (for disease)?
TASK 4
Read the text and put the verbs in bracket in to the correct verb form.
For fault – finding you must have at least a multi-meter, either analogue or digital.
An oscilloscope (not/be) absolutely essential but you will
(find) yourself very restricted without one. It’s like trying to repair (try/repair) a car
while (wear) a blindfold.
For audio equipment, a signal source (need). Clearly, a function
generator is useful but simpler and cheaper alternative work well in most cases. You
only need a (fix) frequency source, say 400 or 1000 Hz sine or square
wave. For cassette recorders a tape with a constant 4000 Hz wave
(record) on both channels is adequate for most fault-finding. However, for
(check) playback levels and frequency response and aligning the tape head, proper
test tapes, which are expensive (require).
For serious work, a collection of test leads and audio connectors is essential. Most
modern audio equipment uses phono sockets so it’s worthwhile
(invest) in cables which terminate in phono plugs. For other types of sockets,
adaptors are available.
Part I – Electronics – Unit 5

Điện Tử Y Sinh 4 – Học Viện Kỹ Thuật Quân Sự 16
Unit 5: Junction transistors
Transistors are solid-state devices similar in some ways to the diodes you
have studied. Transistors are more complex and can be used in many more ways.
They are very important and can be found in almost all modern electronic
equipment.
The most important feature of transistors is their ability to amplify signals.
Amplification can make a weak signal strong enough to be useful in some
electronic applications. For example, an audio amplifier can be used to supply a
strong signal to a loudspeaker.
1. Amplification
Amplification is one of the most basic ideas in electronics. Amplifiers make
sounds louder and signal levels greater and, in general, provide a function called
gain. Fig 5-1 shows the general function of an amplifier. Note that the amplifier
must be provided with two inputs: a DC power supply and the input signal. The
signal is the electrical quantity that is too small in its present form to be usable.
With again, it can become usable. As shown in Fig 5-1, the output signal is greater
because of the gain of the amplifier. Gain can be measured in several ways. If an
oscilloscope is used to measure the amplifier input voltage and output voltage (not
the power supply), then the voltage gain can be observed. A certain amplifier may
provide an output voltage that is 10 times greater than the input voltage. The
voltage gain of the amplifier is 10. If an ammeter is used to measure amplifier
input and output currents, the current gain can be obtained. With a 0.1 – An input
signal. An amplifier might produce a 0.5 - An output signal for a current gain of 5.
If the voltage gain and the current gain are known, the power gain can be
established. An amplifier that produces a voltage gain of 10 and a current gain of 5
will give the following power gain:
P = V.I or P
gain
= V

gain
.I
gain
= 10.5 = 50
Only amplifiers provide a power gain. Other devices might give a voltage
or a current gain. A step-up transformer provides voltage gain. Why can it not be
considered an amplifier? The answer is that it does not provide any power gain. If
the transformer steps up the voltage 10 times, then it steps down the current 10
times. The power gain, ignoring loss in the transformer, will be
P
gain
= V
gain
.I
gain
= 10.0,1 = 1
A step-down transformer provides a current gain. It cannot be considered
an amplifier. The current gain is offset by a voltage loss, and thus, there is no
power gain.
Part I – Electronics – Unit 5
Điện Tử Y Sinh 4 – Học Viện Kỹ Thuật Quân Sự 17
Even though power gain seems to be the important idea, some amplifiers
are classified as voltage amplifiers. In some circuits, only the voltage gain is
mentioned. This is especially true in amplifiers designed to handle very small
electric signals. You will run across many voltage amplifiers or small signal
amplifiers in electronic devices, you should remember that they provide power
gain, too.
Amplifiers designed to handle large signals are usually called power
amplifiers. In the electronic system of Fig 5-2 the speaker requires several watts
for good volume. The signal from the pickup arm is a fraction of a milliwatt

(mW). A total power gain of thousands is needed. However, only the final large-
signal amplifier is called a power amplifier.
SELF TEST
Determine whether each statement is true or false.
1. An amplifier needs an input signal and a power supply to develop output
signal.
2. An amplifier has a voltage gain of 50. If the input signal is 2 mV the output
signal should be 50 mV.
3. The input signal to an amplifier is 1 mA. The output signal is 10 mA. The
amplifier has a current gain of 10 W.
4. A step transformer is a true voltage amplifier.
5. All amplifiers have power gain.
TASK 1
Each of these verbs has a related noun ending in –er or –or which refers to an
instrument or component. Complete the column of nouns. You have met these
nouns in this and earlier units.
Ex: Record
→ recorder.
VERBS NOUNS
Indicate
Transmit
Transform
Measure
Charge
Part I – Electronics – Unit 5
Điện Tử Y Sinh 4 – Học Viện Kỹ Thuật Quân Sự 18
Rectify
Amplify
Collect
Detect

Tune
Process
Oscillate
2. Transistors
Transistors make good amplifiers. That can provide the power gain that is
needed. There are several important types of transistors. This chapter will be
mainly concerned with the bipolar junction transistors.
Bipolar junction transistors are similar to junction diodes, but one more
junction is included. Fig 5-3 shows one way to make a transistor. A P-type
semiconductors region is located between two N-type semiconductors regions.
The polarity of thee regions is controlled by the valence of the materials used in
the doping process.
The transistor regions shown in Fig 5-3 are named emitter, base, and
collector. The emitter is very rich in current carriers. Its job is to send its carrier in
to base region and then on to the collector. The collector collects the carriers. The
emitter emits the carriers. The base acts as the control region. As will be seen later,
the base can allow few for many carriers to flow from the emitter to the collector.
The transistor of Fig 5-3 is bipolar because both holes and elections will
take part in the current flow through the device. The N-type regions contain free
electrons which are negative carriers. The P-type region contains free holes which
are positive carriers. Two (bi) polarities carriers are present. Note that there are
also two PN junctions in the transistor. It is a bipolar junction transistor.
The transistor shown in Fig 5-3 would be classified as an NPN transistor.
Another way to make a bipolar junction is to make the emitter and collector of P-
type material and the base of N-type material. This type would be classified as a
PNP transistor. Fig 5-4 shows both possibilities and the schematic symbols for
each. You should memorize the symbols. Remember that the emitter lead is
always the one with the arrow. Also remember that if the arrow is NOT pointing
in, the transistor is an NPN type.
Part I – Electronics – Unit 5

Điện Tử Y Sinh 4 – Học Viện Kỹ Thuật Quân Sự 19
The two transistor junctions must be biased properly. This is why you can
not replace an NPN transistor with a PNP transistor. The polarities would be
wrong. Transistor bias is shown in Fig 5-5. The collector-base junction must be
reverse-biased for proper operation. In an NPN transistor, the collector will have
to be positive with respect to the base. In a PNP transistor, the collector will have
to be negative with respect to the base. PNP and NPN transistors are not
interchangeable.
The base-emitter junction must be forward biased, as shown in Fig 5-5.
This makes the resistance of the emitter junction very low as compared with the
resistance of the collector-base junction. A forward-biased semiconductor junction
has low resistance. Fig 5-6 compared the two junction resistance.
The large difference in junction resistance makes the transistor capable of
power gain. Assume that a current if flowing through the two resistance showns in
Fig 5-6. Power can be calculated by
R
I
P


2
The power gain from Rbe to Rcb could be established by calculating the
power in each and dividing:
BE
CB
gain
RI
RI
P




2
2
If the current through R
CB
were equal to the current through R
BE
, I
2
would
be cancelled out and the power gain would be
BE
CB
gain
R
R
P

Actually, the currents are not equal in transistor, but they are very close.
Thus, we are making only a small error. A typical resistance value for R
CB
might
be 10,000
Ω. It is high since the collector base junction is reverse-biased. A typical
value for R
BE
might be 100Ω. It is low because the base emitter junction is
forward-biased. The power gain for this typical would be:
100

100
000,10

BE
CB
gain
R
R
P
Perhaps the biggest puzzle is why the current the reverse-biased junction as
high as the current through the forward-biased junction.
Diode theory tells us to expect almost no current through a reverse-biased
junction. This is true in a diode but not true in the collector-base junction of a
transistor.
Fig 5-7 shows why the collector-base current is high. The collector-base
voltage V
CB
produces a reverse bias across the collector-base junction. The base-
Part I – Electronics – Unit 5
Điện Tử Y Sinh 4 – Học Viện Kỹ Thuật Quân Sự 20
emitter voltage V
BE
produces forward bias across the base-emitter junction. If the
transistor were simply two diode junctions, the results would be:
- I
B
and I
E
would be high.
- I

C
would be zero.
The base region of the transistor is very narrow (about 0.0025 cm, or 0.001
in). The base region is lightly doped. It has only a few holes. It is not likely that an
electron coming from the emitter will find a hole in the base with which to
combine.
With so few electron-hole combinations in the base region, the base current
is very low.
The collector is an N-type region but is charged positively by V
CB
. Since
the base is such a narrow region, the positive field of the collector is quite strong
and the great majority of the electrons coming from the emitter attracted and
collected by the collector.
Thus,
- I
E
and I
C
high
- I
B
is low.
The emitter current of Fig 5-7 is the highest current in the circuit. The
collector current is just a bit less. Typically, about 99 percent of the emitter
carriers go on to the collector. About 1 percent of the emitter carriers combines
with carriers in the base and become base current. The current equation for Fig 5-7
is
I
E

= I
C
+ I
B
By using typical percentages it can be stated as
100% = 99% + 1%
The base current is quite small but very important. Suppose, for example
that the base lead of the transistor in Fig 5-7 is opened. With the lead open, there
can be no base current. The two voltages V
CB
and V
BE
would add in series to make
the collector positive with respect to the emitter. You might guess that current
would continue to flow from the emitter to the collector, but it does not.
With no base current, there will be no emitter current and no collector
current. The base-emitter junction must be forward-biased for the emitter to emit.
Opening the base lead removes this forward bias. If the emitter in not emitting,
there is nothing for the collector to collect. Even though the base current is very
low, it must be present for the transistor to conduct from emitter to collector.
Part I – Electronics – Unit 5
Điện Tử Y Sinh 4 – Học Viện Kỹ Thuật Quân Sự 21
The fact that a flow base current controls much higher currents in the
emitter and collector is very important. This shows how the transistor is capable of
good current gain.
TASK 1
These verbs in the box are often used in electronics.
Conduct, emit, rectify, sample, dissipate, process, record, suppress
Fill the gaps in these sentences with an appropriate verb from the list above. Make
sure the verb is in the correct form.

1. Computers data.
2. You can sound on the tape.
3. A bridge circuits is used to alternating current produce direct
current.
4. All metals, and some non-metals such as carbon, electricity.
5. To prevent radio interference, you must . any sources
interference such as car ignition systems.
6. Power transistors ……………… heat. Therefore they must be mount on a
heat sink.
7. The electron gun is a CRT ………………… a stream of electrons.
8. When recording a CD, sound is ………………… 44,100 times every
second.
Part I – Electronics – Unit 6
Điện Tử Y Sinh 4 – Học Viện Kỹ Thuật Quân Sự 22
Unit 6: Conductors, insulators and semiconductors
If we connect a battery across a body there is a movement of free electrons
towards the positive end. This movement of electrons is an electric current. All
materials can be classified into three groups according to how readily they permit
an electric current to flow. These are: conductor, insulator and semiconductor.
In the first category, metals are substances which provide an easy path for
an electric current. All metals are conductor. However, some metals do not
conduct well. Manganese, for example, is a poor conductor. Copper is a good
conductor, therefore it is widely used for cables. A non-metal which conducts well
is carbon. Salt water is an example for a liquid conductor.
A material which does not easily release electrons is called an insulator.
Rubber, nylon, porcelain and air are all insulators. There are no perfect insulators.
All insulators will allow some flow of electrons, however this can usually be
ignored because the flow they permit is so small.
Semiconductors are midway between conductor and insulator. Under
certain conditions they allow a current to flow easily but under others they behave

as insulators. Germanium and silicon are semiconductors. Mixtures of certain
metallic oxides also act as semiconductors. These are known as thermistors. The
resistance of thermistor falls rapidly as their temperature rises. They are, therefore
used in temperature – sensing devices.
TASK 1
Read the text and decide if these statements are true or false in relation to the
information in the text.
1. Electrons flow from positive to negative.
2. Copper provides an easy path for an electric current.
3. All metals are good conductors.
4. All good conductors are metals.
5. Air is not a perfect insulator.
6. Rubber readily releases electrons.
7. The resistance of a thermistor is higher at low temperatures than at high
temperatures.
TASK 2. Contextual reference.
Rewrite the following sentences, replacing the words in italics with expressions
from the passage that have similar meanings to:
1. The flow of free electrons is an electric current.
Part I – Electronics – Unit 6
Điện Tử Y Sinh 4 – Học Viện Kỹ Thuật Quân Sự 23
2. Materials in the first group are called conductors.
3. Materials providing a path for an electric current are conductors.
4. All insulators permit some flow of electrons.
5. Germanium sometimes acts as an insulator and sometimes as a conductor.
TASK 3
Look back in the text and find the reference for the words in italics. What do the
pronouns in these sentences refer to?
1. All materials can be classified into three groups according to how readily they
permit an electric current to flow.

a. Three groups b. All materials c. Free electrons
2. Under certain conditions
they allow a current to flow easily but under others
they behave as insulators.
a. Conductors b. Semiconductors c. Insulators.
3.
These are known as thermistors.
a. Metallic oxides b. Semiconductors c. mixtures of certain
metallic oxides.
4.
They are therefore used in temperature-sensing devices.
a. Thermistors b. Semiconductors c. Metallic oxides.
Part I – Electronics – Unit 6
Điện Tử Y Sinh 4 – Học Viện Kỹ Thuật Quân Sự 24
Language focus
Adjective clauses
An adjective clause is a dependent clause that modifies a noun.
It describes, identifies, or gives further information about a noun (An adjective
clause is also called a relative clause).
1. Using subject pronouns: who, which, that.
Who is used for people.
Which is used for things.
That is used for both people and things.
Note: An object pronoun is often omitted from an adjective clause (A subject
pronoun, however, may not be omitted)
2. Punctuation of adjective clause:
a. Don’t use commas if the adjective clause is necessary to identify the noun it
modifies.
b. Use commas if the adjective clause simply gives additional information and it is
not necessary to identify the noun it modifies.

Exercise 1: Combine the sentences, using the second structure as an adjective
clause.
Example:
- The error sensor output is fed to the controller.
- The controller sends a signal to the power supply of the motor.
→ The error sensor output if fed to the controller which sends a signal to the
power supply of the motor.
1.
- The rotor contains an armature.
- Armature is a set of wire loops wound on a steel core. (which)
2.
-
This device provides a 50Ω resistance discharge path for the defibrillator
paddles.
- Discharge path stimulates the torso resistance. (That)
3.
- The instrument will then perform a calibration and self-test procedure.
- Self-test procedure consists of checking the position of the feed table. (which)
4.
- The sequence number is displayed in the centre of the top line of the display.
- The sequence number will be assigned to the next test. (that)
5.
- Test results are listed in the package insert.
- Test results should be obtained. (that)
Part I – Electronics – Unit 6
Điện Tử Y Sinh 4 – Học Viện Kỹ Thuật Quân Sự 25
Exercise 2: Add commas where necessary.
1. The display changes to the Test Screen which shows the sequence number of
the strip being tested.
2. Do not use any material that will scratch the feed table insert.

3. The moving coil which gives the instrument its name is composed of fine
copper wire wound on a thin rectangular aluminum former.
4. The oxygen content of the air flowing into the bellows is controlled by a
percentage control valve which regulates the resistance to room air and oxygen
appropriately.
5. The direction of the air in the pneumatic system is determined by the main
solenoid which is switched appropriately by the system electronics.
6. The third terminal of the pot is connected to the zero voltage supply rail which
is earthed.
7. The envelope contains nitrogen which helps to prevent corrosion of the contact.
Exercise 3: Short relative clauses 1.
We can join these sentences by using a relative clause.
- The lines are arranged in two groups.
- The lines carry the supply.
→ The lines which carry the supply are arranged in two groups.
Relative clauses with certain active verbs can be shorten by omitting the relative
words and changing the verb to its – ing form. These verbs include:
Carry form
Contain hold
Consist of measure
We can shorten the relative clause like this:
The lines carrying the supply are arranged in two groups.
The lines are suspended from insulators.
The insulators are made of porcelain.
The lines are suspended from insulators which are made of porcelain.
Relative clause like this with passive verbs can be shortened by omitting the
relative word and the verb to be:
The lines are suspended from insulators made of porcelain.
Now link each group of sentences into one sentence. Use short relative clause
where possible. (Number 2 can not be shortened)