TIẾNG ANH CHUYÊN NGÀNH CÔNG NGHỆ THÔNG TIN

UNIT 1
TRANSISTOR

1. Definition
In electronics, a transistor is a
semi-conductor device commonly used to
amplify or switch electronic signals. A
transistor is made of a solid piece of a
semiconductor material, with at least three
terminals for connection to an external
circuit. A voltage or current applied to one
pair of the transistor's terminals changes
Fig. 1 Some types of transistor

the current flowing through another pair of
terminals. Because the controlled (output) power can be much more than
the controlling (input) power, the transistor provides amplification of a
signal.
2. History
The first patent for the field-effect transistor principle was filed in Canada
by Austrian-Hungarian physicist, Julius Edgar Lilienfeld on 22 October
1925. But Lilienfeld did not publish any research articles about his devices.
In 1934 German physicist Dr. Oskar Heil patented another field-effect
transistor.
On 17 November 1947 John Bardeen and Walter Brattain, at AT&T
Bell Labs, observed that when electrical contacts were applied to a crystal
of germanium, the output power was larger than the input. William
Shockley saw the potential in this and worked over the next few months

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greatly expanding the knowledge of
semiconductors and could be described
as the father of the transistor, a legal
papers from the Bell Labs patent show
that William Shockley and Gerald
Pearson had built operational versions
from Lilienfeld's patents.

Fig 2. Transistor in Lilienfeld’s
experience

The first silicon transistor was produced by Texas Instruments in
1954. This was the work of Gordon Teal, an expert in growing crystals of
high purity, who had previously worked at Bell Labs. The first MOS
transistor actually built was by Kahng and Atalla at Bell Labs in 1960.
The transistor is considered by many to be the greatest invention of
the twentieth-century, and some consider it is one of the most important
technological breakthroughs in human history. It is the key active
component in practically all modern electronics and is the fundamental
building block of modern electronic devices like radio, telephone, computer
etc. Its importance in today's society rests on its ability to be mass produced
using a highly automated process (in fabrication) that achieves
astonishingly low per-transistor costs. Some transistors are packaged
individually but most are found in integrated circuits.
Although several companies each produce over a billion individually
packaged (known as discrete) transistors every year, the vast majority of
transistors produced are in integrated circuits (often shortened to IC,
microchips or simply chips) along with diodes, resistors, capacitors and
other electronic components to produce complete electronic circuits. A

logic gate consists of up to about twenty transistors whereas an advanced

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microprocessor, as of 2006, can use as many as 1.7 billion transistors
(MOSFETs). "About 60 million transistors were built this year [2002], for
[each] man, woman, and child on Earth."
The transistor's low cost, flexibility, and reliability have made it a
ubiquitous device. Transistorized mechatronic circuits have replaced
electromechanical devices in controlling appliances and machinery. It is
often easier and cheaper to use a standard microcontroller and write a
computer program to carry out a control function than to design an
equivalent mechanical control function.
3. Applications
The bipolar junction transistor, or BJT, was the most commonly used
transistor in the 1960s and 70s, after MOSFETs became widely available,
the BJT remained the transistor of choice for many analog circuits such as
simple amplifiers because of their greater linearity and ease of
manufacture. Desirable properties of MOSFETs, such as their utility in
low-power devices, usually in the CMOS configuration, allowed them to
capture nearly all market share for digital circuits; more recently
MOSFETs have captured most analog and power applications as well,
including modern clocked analog circuits, voltage regulators, amplifiers,
power transmitters, motor drivers, etc.
The essential usefulness of a transistor comes from its ability to use a
small signal applied between one pair of its terminals to control a much
larger signal at another pair of terminals. This property is called gain. A
transistor can control its output in proportion to the input signal, that is, can
act as an amplifier. Or, the transistor can be used to turn current on or off in

a circuit as an electrically controlled switch, where the amount of current is
determined by other circuit elements.

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The two types of transistors have slight differences in how they are
used in a circuit. A bipolar transistor has terminals labeled base, collector,
and emitter. A small current at the base terminal (that is, flowing from the
base to the emitter) can control or switch a much larger current between the
collector and emitter terminals. For a field-effect transistor, the terminals
are labeled gate, source, and drain, and a voltage at the gate can control a
current between source and drain.
The fig. 3 represents a typical
bipolar transistor in a circuit. Charge
will

flow

between

emitter

and

collector terminals depending on the
current in the base. Since internally the
Fig 3. Typical circuit of transistor

base and emitter connections behave

like a semiconductor diode, a voltage drop develops between base and
emitter while the base current exists. The size of this voltage depends on
the material the transistor is made from, and is referred to as VBE
3.1. Transistor as a switch
Transistors are commonly used as electronic switches, for both high
power applications including switched-mode power supplies and low
power applications such as logic gates.
Using the simple transistor circuit it can be seen from the graph,
from point A to point B, as the base voltage rises the base and collector
current rise exponentially (the A-B segment should be curved), but the
collector voltage simultaneously drops because of the collector resistor.
Relevant equations:

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IBC
B

A

VB

Fig. 4 The transistor works as a switch

Following the Kirhoff laws, one can write expression:
VRC = IC × RC
VRC + VCE = VCC
If VCE could fall to 0 (perfect closed switch) then Ic could go no
higher than VCC / RC, even with higher base voltage and current. The

transistor is then said to be saturated. In actuality VCE drops to roughly VBE
÷ 2, rising with higher collector currents. Hence, values of input voltage
can be chosen such that the output is either completely off, or completely
on. The transistor is acting as a switch, and this type of operation is
common in digital circuits where only "on" and "off" values are relevant.
3.2. Transistor as an amplifier
The above common emitter amplifier is designed so that a small
change in voltage in (Vin) changes the small current through the base of the
transistor and the transistor's current amplification combined with the
properties of the circuit mean that small swings in Vin produce large
changes in Vout.
It is important that the operating parameters of the transistor are
chosen and the circuit designed such that as far as possible the transistor
operates within a linear portion of the graph, such as that shown between A
and B, otherwise the output signal will suffer distortion.

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Various configurations of single transistor amplifier are possible,
with some providing current gain, some voltage gain, and some both.
From mobile phones to televisions, vast numbers of products include
amplifiers for sound reproduction, radio transmission, and signal
processing. The first discrete transistor audio amplifiers barely supplied a
few hundred milliwatts, but power and audio fidelity gradually increased as
better transistors became available and amplifier architecture evolved.
Modern transistor audio amplifiers of up to a few hundred watts are
common and relatively inexpensive. Some musical instrument amplifier
manufacturers mix transistors and vacuum tubes in the same circuit, as
some believe tubes have a distinctive sound.

4. Advantages
The key advantages that have allowed transistors to replace their
vacuum tube predecessors in most applications are
- Small size and minimal weight, allowing the development of miniaturized
electronic devices.
- Highly automated manufacturing processes, resulting in low per-unit cost.
- Lower possible operating voltages, making transistors suitable for small,
battery-powered applications.
- No warm-up period for cathode heaters required after power application.
- Lower power dissipation and generally greater energy efficiency.
- Higher reliability and greater physical ruggedness.
- Extremely long life. Some transistorized devices have been in service for
more than 30 years.
- Complementary devices available, facilitating the design of complementary symmetry circuits, something not possible with vacuum tubes.

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- Insensitivity to mechanical shock and vibration, thus avoiding the
problem of microphonics in audio applications.
5. Disadvantages
- Silicon transistors do not operate at voltages higher than about 1,000 volts
(SiC devices can be operated as high as 3,000 volts). In contrast, electron
tubes have been developed that can be operated at tens of thousands volts.
- High power, high frequency operation, such as used in over-the-air
television broadcasting, is better achieved in electron tubes due to
improved electron mobility in a vacuum.
- On average, a higher degree of amplification linearity can be achieved in
electron tubes as compared to equivalent solid state devices, a characteristic
that may be important in high fidelity audio reproduction.

- Silicon transistors are much more sensitive than electron tubes to an
electromagnetic pulse, such as generated by an atmospheric nuclear
explosion.
Exercise 1: Answer the question following the text:
1. What is a transistor?
2. What are the transistors made of?
3. Why can transistors provide amplification of a signal?
4. Where are transistors used?
5. Which type of transistor was used in 1960s-1970s
6. What does MOSFET stand for?
7.Why is transistor used for amplifying signal?
8. What are the terminals of BJT ?
9. What are the terminals of FET?
10. What are the advantages of transistor compare to vacuum tube?

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11.When was the first MOS transistor built?
12. How many transistors are in the advanced microprocessor in 2006?
Exercise 2: Identify the statements are True or False:
1. A transistor is made of a solid piece of a semiconductor material, with at
least three terminals for connection to an external circuit
2. The transistor is the fundamental building block of modern electronic
devices like radio, telephone, computer and other electronic systems
3. The transistor is considered as one of the most important technological
breakthroughs in human history.
4. Transistorized mechatronic circuits couldn’t replace electromechanical
devices in controlling appliances and machinery
5. Transistors operating at high voltage not suitable for small, batterypowered applications.

6. Silicon transistors are much more sensitive than electron tubes to an
electromagnetic pulse
Exercise 3: Translate the text and summery in short paragraph.

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UNIT 2
SENSOR

1. Definition
A sensor is a device that measures a physical quantity and converts
it into a signal which can be read by an observer or by an instrument. For
example, a mercury thermometer converts the measured temperature into
expansion and contraction of a liquid which can be read on a calibrated
glass tube. A thermocouple converts temperature to an output voltage
which can be read by a voltmeter. For accuracy, all sensors need to be
calibrated against known standards.
Sensors are used in everyday
objects such as touch-sensitive elevator
buttons and lamps which dim or
brighten by touching the base. There are
also

innumerable

applications

for

sensors of which most people are never
aware.

Applications

include

cars,

machines, aerospace, medicine, manu-

Fig. 5 Humidity sensor

facturing and robotics.
A sensor's sensitivity indicates how much the sensor's output
changes when the measured quantity changes. For instance, if the mercury
in a thermometer moves 1 cm when the temperature changes by 1 °C, the
sensitivity is 1 cm/°C. Sensors that measure very small changes must have
very high sensitivities. Sensors also have an impact on what they measure;

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for instance, a room temperature thermometer inserted into a hot cup of
liquid cools the liquid while the liquid heats the thermometer.
Sensors need to be designed to have a
small effect on what is measured; making the
sensor smaller often improves this and may
introduce other advantages. Technological
progress allows more and more sensors to be

manufactured on a microscopic scale as
microsensors using MEMS (Micro Electro
Mechanical Systems) technology. In most

Fig. 6 Thermometer

cases, a microsensor reaches a significantly higher speed and sensitivity
compared with macroscopic approaches. A good sensor obeys the
following rules:


Is sensitive to the measured property



Is insensitive to any other property



Does not influence the measured property
Ideal sensors are designed to be linear. The output signal of such a

sensor is linearly proportional to the value of the measured property. The
sensitivity is then defined as the ratio between output signal and measured
property. For example, if a sensor measures temperature and has a voltage
output, the sensitivity is a constant with the unit [V/C]; this sensor is linear
because the ratio is constant at all points of measurement. If the sensor is
not ideal, several types of deviations can be observed:


The sensitivity may in practice differ from the value specified. This

is called a sensitivity error, but the sensor is still linear.


Since the range of the output signal is always limited, the output

signal will eventually reach a minimum or maximum when the measured

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property exceeds the limits. The full scale range defines the maximum
and minimum values of the measured property.


If the output signal is not zero when the measured property is zero,

the sensor has an offset or bias. This is defined as the output of the
sensor at zero input.


If the sensitivity is not constant over the range of the sensor, this is

called nonlinearity. Usually this is defined by the amount the output
differs from ideal behavior over the full range of the sensor, often noted
as a percentage of the full range.


If the deviation is caused by a rapid change of the measured property

over time, there is a dynamic error. Often, this behaviour is described
with a bode plot showing sensitivity error and phase shift as function of
the frequency of a periodic input signal.


If the output signal slowly changes independent of the measured

property, this is defined as drift.


Long term drift usually indicates a slow degradation of sensor

properties over a long period of time.


Noise is a random deviation of the signal that varies in time.



Hysteresis is an error caused by when the measured property reverses

direction, but there is some finite lag in time for the sensor to respond,
creating a different offset error in one direction than in the other.


If the sensor has a digital output, the output is essentially an

approximation of the measured property. The approximation error is also
called digitization error.



If the signal is monitored digitally, limitation of the sampling

frequency also can cause a dynamic error.

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

The sensor may to some extent be sensitive to properties other than

the property being measured. For example, most sensors are influenced
by the temperature of their environment.


All these deviations can be classified as systematic errors or random

errors. Systematic errors can sometimes be compensated for by means of
some kind of calibration strategy. Noise is a random error that can be
reduced by signal processing, such as filtering, usually at the expense of
the dynamic behavior of the sensor.
2. Resolution
The resolution of a sensor is the smallest change it can detect in the
quantity that it is measuring. Often in a digital display, the least significant
digit will fluctuate, indicating that changes of that magnitude are only just
resolved. The resolution is related to the precision with which the
measurement is made. For example, a scanning tunneling probe (a fine tip
near a surface collects an electron tunneling current) can resolve atoms and

molecules.
All living organisms contain biological sensors with functions
similar to those of the mechanical devices described. Most of these are
specialized cells that are sensitive to:


Light, motion, temperature, magnetic fields, gravity, humidity,

vibration, pressure, electrical fields, sound, and other physical aspects of
the external environment


Physical aspects of the internal environment, such as stretch, motion

of the organism, and position of appendages (proprioception)


Environmental molecules, including toxins, nutrients, and

pheromones


Estimation of biomolecules interaction and some kinetics parameters

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

Internal metabolic milieu, such as glucose level, oxygen level, or

osmolality


Internal signal molecules, such as hormones, neurotransmitters, and

cytokines


Differences between proteins of the organism itself and of the envi-

ronment or alien creatures
Artificial sensors that mimic biological sensors by using a biological
sensitive component are called biosensors.
Exercise 1: Answer the question following the text:
1. What is a sensor?
2. Where are sensors used?
3. What thing should be considered when making the sensors?
4. What properties the good sensors have
5. What is sensitivity error?
6. What does “linearity” means in term range of sensor?
7. What does “nonlinearity” means in term range of sensor?
8. What is a drift?
9. What causes the hysteresis in sensor?
10. What causes the digitization error?
11 What is the dynamic error?
12. What is the sensor resolution?
Exercise 2: Identify the statements are True or False:
1. Thermometer is one type of sensor
2. The application of sensor is not much

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3. Sensors that measure very small changes must have very high
insensitivities
4. Technological progress allows more and more sensors to be
manufactured on a microscopic scale.
5. If the sensitivity is not constant over the range of the sensor, this is called
nonlinearity
6. Biological sensors have functions similar to those of the mechanical
devices.
7. The resolution is related to the precision of the measurement.
8. Noise is a random error that can be reduced by signal processing
Exercise 3:
- Design a simple sensor, and then describe its working rule
- Summarizing the text of sensor in short paragraph (5-7 lines)

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UNIT 3
ACTUATOR

1. Definition
An actuator is a mechanical device for moving or controlling a
mechanism or system. An actuator typically is a mechanical device that
takes energy, usually created by air, electricity, or liquid, and converts that
into some kind of motion. The typical actuator types are called switches.
A biased switch is one

containing a spring that returns the
actuator to a certain position. The
"on-off" notation can be modified
by placing parentheses around all
positions other than the resting
position. For example, an (on)-off(on) switch can be switched on by

Fig. 7 Three pushbutton switches

moving the contact in either
direction away from the centre, but return to the central off position when
the contact is released. The momentary push-button switch is a type of
biased switch. The most common type is a "push-to-make" (or normallyopen or NO) switch, which makes contact when the button is pressed and
breaks when the button is released. Each key of a computer keyboard, for
example, is a normally-open "push-to-make" switch. A "push-to-break" (or
normally-closed or NC) switch, on the other hand, breaks contact when the
button is pressed and makes contact when it is released. An example of a

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push-to-break switch is a button used to release a door held open by an
electromagnet.
2. Some types of Actuator
-Knife switch: Knife switches are unique; the electrical contacts are
exposed, mounted on an insulating plastic or porcelain plate, unlike modern
switches in which the working parts are enclosed in an insulating plastic or
rubber housing to protect users from contact with hazardous voltages. The
"knife", a flat metal swinging arm, is moved by the user between two or
more gripping contacts of springy metal. The knife and contacts are

typically formed of copper, steel, or brass, depending on the application.
The primary advantage of a
knife switch is the extremely high
current capability inherent to the
design. The amount of surface area
on the "knife" that shorts the
Fig. 8 The symbol of witches
in diagram

contacts is also extremely high,
allowing a wide range of high vol-

tage or high amperage applications with no circuit degradation, choke, or
arcing during switch throw. Thicker components need only be
accompanied by wider contacts to conduct higher currents, which allow the
design to scale extremely well with size. Its disadvantage is that to operate
it, a user has to grasp the knife's insulated handle near the exposed contacts
and knife blade, causing a great risk of electric shock. Although knife
switches are inferior to traditional switches in applications where user
safety is paramount, they are still commonly employed in everyday highvoltage applications such as building transformers, large power relays, and
air-conditioning units.

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-An electronic switch: is an electrical component that can break an
electrical circuit, interrupting the current or diverting it from one conductor
to another. The most familiar form of switch is a manually operated
electromechanical device with one or more sets of electrical contacts. Each
set of contacts can be in one of two states: either 'closed' meaning the

contacts are touching and electricity can flow between them, or 'open',
meaning the contacts are separated and nonconducting. Since the advent of
digital logic in the 1950s, the term has spread to a variety of digital active
devices such as transistors and logic gates whose function is to change their
output state between two logic levels or connect different signal lines, and
even computers, network switches, whose function is to provide
connections between different ports in a computer network. The term
'switched' is also applied to telecommunications networks, and signifies a
network that is circuit switched, providing dedicated circuits for
communication between end nodes, such as the public switched telephone
network. The common feature of all these usages is they refer to devices
that control a binary state: they are either on or off, closed or open,
connected or not connected.
-Mercury tilt switch: The mercury switch consists of a drop of mercury
inside a glass bulb with 2 contacts. The two contacts pass through the glass,
and are connected by the mercury when the bulb is tilted to make the
mercury roll on to them. This type of switch performs much better than the
ball tilt switch, as the liquid metal connection is unaffected by dirt, debris
and oxidation, it wets the contacts ensuring a very low resistance bouncefree connection, and movement and vibration do not produce a poor
contact. These types can be used for precision works. It can also be used
where arcing is dangerous (such as in the presence of explosive vapour) as

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the entire unit is sealed. A simple semiconductor switch is a transistor.
Other types of switch include:
1. Centrifugal switch
2. DIP switch
3. Hall-effect switch

4. Inertial switch
5. Membrane switch
6. Toggle switch
7. Transfer switch
8. Time switch
9. Vandal resistant switch
10. Latching switch
3. Contacts
In the simplest case, a switch has two pieces of metal called contacts
that touch to make a circuit, and separate to break the circuit. The contact
material is chosen for its resistance to corrosion, because most metals form
insulating oxides that would prevent the switch from working. Contact
materials are also chosen on the basis of electrical conductivity, hardness
(resistance to abrasive wear), mechanical strength, low cost and low
toxicity Sometimes the contacts are plated with noble metals. They may be
designed to wipe against each other to clean off any contamination.
Nonmetallic conductors, such as conductive plastic, are sometimes used.
A pair of contacts is said to be 'closed' when there is no space
between them, allowing electricity to flow from one to the other. When the
contacts are separated by an insulating air gap, an air space, they are said to
be 'open', and no electricity can flow at typical voltages.
Some contacts are normally open (Abbreviated "n.o." or "no") until
closed by operation of the switch, while others are normally closed ("n.c.

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or "nc") and opened by the switch action, where the abbreviations given are
commonly used on electronics diagrams for clarity of operation in
assembly, analysis or troubleshooting. They serve to synchronize meaning

with possible mistakes in wiring assembly, where wiring part of switch one
way and part another (usually opposite) way will pretty much guarantee
things won't work as designed.
Exercise 1: Answer the question following the text:
1. What is an actuator?
2. What is the normally close switch?
3. What is the normally open switch?
4. What is the function of “knife” portion in electric switch?
5. What is knife made of?
6. What are advantages, disadvantages of a knife switch?
7. What is electronic switch?
8. What is a mercury tilt switch?
9. What are the advantages of mercury tilt switch?
10. Where is mercury tilt switches used?
11. What are criterions of chosen material for making switch contact?
12. What is a contact?
Exercise 2: Identify the statements are True or False:
1. An on-off switch can be switched on by moving the contact in either
direction away from the centre, but return to the central off position
2. An NC switch makes contact when the button is pressed and breaks
when the button is released
3. The knife and contacts are typically made of copper, steel, or brass,
depending on the application.

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4. The knife switches are still employed in high-voltage applications such
as building transformers
5. The mercury tilt switch performs much better than the ball tilt switch

6. A simple semiconductor switch may be a transistor
Exercise 3: Translate the text and summery in short paragraph.

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UNIT 4
LOGIC GATE

1. Definition
A logic gate performs a logical operation on one or more logic inputs and
produces a single logic output. The logic normally performed is Boolean
logic and is most commonly found in digital circuits. Logic gates are
primarily implemented electronically using diodes or transistors, but can
also be constructed using electromagnetic relays, fluidics, optics,
molecules, or even mechanical elements.
In electronic logic, a logic level is represented by a voltage or
current, (which depends on the type of electronic logic in use). Each logic
gate requires power so that it can source and sink currents to achieve the
correct output voltage. In logic circuit diagrams the power is not shown,
but in a full electronic schematic, power connections are required.
A truth table is a table that describes the behavior of a logic gate. It
lists the value of the output for every possible combination of the inputs
and can be used to simplify the number of logic gates and level of nesting
in an electronic circuit. In general the truth table does not lead to an
efficient implementation like a minimization procedure, using Karnaugh
maps, the Quine–McCluskey algorithm or a heuristic algorithm is required
for reducing the circuit complexity.
2. Logic family
The simplest form of electronic logic is diode logic. This allows

AND and OR gates to be built, but not inverters, and so is an incomplete
form of logic. Further, without some kind of amplification it is not possible

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to have such basic logic operations cascaded as required for more complex
logic functions. To build a functionally complete logic system, relays,
valves (vacuum tubes), or transistors can be used. The simplest family of
logic gates using bipolar transistors is called resistor-transistor logic, or
RTL. Unlike diode logic gates, RTL gates can be cascaded indefinitely to
produce more complex logic functions. These gates were used in early
integrated circuits. For higher speed, the resistors used in RTL were
replaced by diodes, leading to diode-transistor logic, or DTL. It was then
discovered that one transistor could do the job of two diodes in the space of
one diode even better, by more quickly switching off the following stage,
so transistor-transistor logic, or TTL, was created. In virtually every type of
contemporary chip implementation of digital systems, the bipolar
transistors have been replaced by complementary field-effect transistors
(MOSFETs) to reduce size and power consumption still further, thereby
resulting in complementary Metal Oxide Semiconductor (CMOS) logic.
For small-scale logic, designers now use prefabricated logic gates
from families of devices such as the TTL 7400 series by Texas Instruments
and the CMOS 4000 series by RCA, and their more recent descendants.
Increasingly, these fixed-function logic gates are being replaced by
programmable logic devices, which allow designers to pack a large number
of mixed logic gates into a single integrated circuit. The fieldprogrammable nature of programmable logic devices such as FPGAs has
removed the 'hard' property of hardware; it is now possible to change the
logic design of a hardware system by reprogramming some of its
components, thus allowing the features or function of a hardware

implementation of a logic system to be changed.

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Electronic logic gates differ significantly from their relay-and-switch
equivalents. They are much faster, consume much less power, and are
much smaller (all by a factor of a million or more in most cases). Also,
there is a fundamental structural difference. The switch circuit creates a
continuous metallic path for current to flow (in either direction) between its
input and its output. The semiconductor logic gate, on the other hand, acts
as a high-gain voltage amplifier, which sinks a tiny current at its input and
produces a low-impedance voltage at its output. It is not possible for
current to flow between the output and the input of a semiconductor logic
gate.
Another important advantage of standardized integrated circuit logic
families, such as the 7400 and 4000 families, is that they can be cascaded.
This means that the output of one gate can be wired to the inputs of one or
several other gates, and so on. Systems with varying degrees of complexity
can be built without great concern of the designer for the internal workings
of the gates. The output of one gate can only drive a finite number of inputs
to other gates, a number called the 'fanout limit'. Also, there is always a
delay, called the 'propagation delay', from a change in input of a gate to the
corresponding change in its output. When gates are cascaded, the total
propagation delay is approximately the sum of the individual delays, an
effect which can become a problem in high-speed circuits. Additional delay
can be caused when a large number of inputs are connected to an output,
due to the distributed capacitance of all the inputs and wiring and the finite
amount of current that each output can provide.
3. Types of Logic gates

NAND and NOR logic gates are the two pillars of logic, in that all
other types of Boolean logic gates (i.e., AND, OR, NOT, XOR, NOR) can

23


be created from a suitable network of just
NAND or just NOR gate(s). They can be
built from relays or transistors, or any
other technology that can create an
inverter and a two-input AND or OR
gate. Hence the NAND and NOR gates
are called the universal gates. For an
input of 2 variables, there are 16 possible
Boolean algebraic functions. These 16
functions are enumerated below, together
with their outputs for each combination

Fig. 9 Logic state of two inputs

of inputs variables.
In the 1980s, schematics were the predominant method to design
both circuit boards and custom ICs known as gate arrays. Today, custom
ICs and the field-programmable gate array are typically designed with
Hardware Description Languages (HDL) such as Verilog or VHDL. The
need for complex logic symbols has diminished and distinctive shape
symbols are still the predominate style.
Two more gates are the exclusive-OR or XOR function and its
inverse, exclusive-NOR or XNOR. The two input Exclusive-OR is true
only when the two input values are different, false if they are equal,

regardless of the value.
If there are more than two inputs, the gate generates a true at its
output if the number of trues at its input is odd. In practice, these gates are
built from combinations of simpler logic gates. By use of De Morgan's
theorem, an AND gate can be turned into an OR gate by inverting the sense

24


of the logic at its inputs and outputs. This
leads to a separate set of symbols with
inverted inputs and the opposite core
symbol.
These symbols can make circuit
diagrams for circuits using active low
signals much clearer and help to show
accidental connection of an active high
output to an active low input or vice-versa.

Fig 10. One chip with four NANDs

Exercise 1: Answer the question following the text:
1. What does logic gate perform?
2. What relation between Boolean logic and logic gate?
3. What is the truth table?
4. How are the outputs created in logic gate?
5. Which algorithms are use to reduce degree of logic gate complexity?
6. Which components are used to build complete logic systems?
7. What do the RTL, DTL, TTL stand for?
8. What differences between RTL, DTL and TTL?

9. What is CMOS logic?
10. Why will fix function logic gate be replaced by PLD?
11. What are important features of FPGA?
12. What are the significant difference between logic gate and relay-and
switch equivalents?
13. What major advantages are in 7400 and 4000 families?
14. Why propagation delays happen in logic system?

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