Concepts of Engineering
and Technology
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Basic Electricity and
Electronics
Module Two
Basic Electronics
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Transistor Basics

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A semiconductor device
Conductivity is controlled by current
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An example of a voltage controlled device is a
MOSFET
Made from a silicon crystal
Doped with impurities to allow
conductivity to be controlled
N and P type doping use different
conducting particles (electrons and holes)
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Silicon Crystal

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Four valence electrons
per atom
Bonds with four more
silicon atoms to
create a stable
molecule
Creates a solid threedimensional structure
Si
Si
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Si
Si
Si
The diamond unit cell is the
basic crystal building block.
An atom covalent
bonds to four
others, which in
turn bond to four
others, and so on.
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N-type Doping
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Adds arsenic, keeps the
stable crystal structure
Has an extra electron
Negative charge carrier
The extra electron
becomes free to conduct
current
More arsenic atoms
gives more conductivity
Si
Si
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As
Si
Si
P-type Doping
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Gallium has only three
electrons
Still keeps the stable
crystal structure
Creates a hole where an
electron is needed for a
bond
The hole is a positive
current carrying particle
Si
Si
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Ga
Si
Si
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The simplest semiconductor device is a
diode, which is half P material and half N
material
At the junction between the two regions,
electrons from N combine with holes
from P
This creates a charged depletion region
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N
P
N
P
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Reverse Bias
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Reverse bias creates a wider depletion region
No current flow (No Conduction)
Forward bias has opposite polarity, and when
greater than .7 V eliminates the depletion
region, allowing current to flow freely
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A Transistor

A transistor has 3 layers of semiconductor
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NPN or PNP
The regions are called the emitter, base, and
collector
N
P
Base
Emitter

N
Collector
The key to how it works is that the base is THIN
and LIGHTLY DOPED
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Turning a Transistor ON

It takes two connections to the power
supply
Base (P)
Emitter (N)
Collector (N)
Forward Bias
Reverse Bias
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
The emitter-base forward bias overcomes
the depletion region of the reverse biased
base-collector junction, creating an
electron flood that goes through the base
and spills over into the collector.
Forward Bias
Reverse Bias
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
Current flows from Emitter to Collector
(diffusion), negative to positive, while a
smaller current flows from Emitter to Base
(recombination).
Forward Bias
Reverse Bias
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
Here is the same circuit, shown using
schematic symbols:
RC
RB
VBB
IC
VCC
IB

IB = VBB - .7 / RB

IC = VCC (- VCE) / RC

IE = I C + IB
IB << IC
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
Here is the same circuit, shown slightly
different:
Power Supply
(VCC)
RC
Output
Signal In


This is a standard transistor switch
Used in computers and other electronic
devices
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The two states of a switch –
on and off
VCC
VO = VCC
Signal
In = 0
VCC
Transistor Off
This circuit is called
an inverter: When
the input is one the
output is zero, when
the input is zero the
output is one
VO = 0
Signal
In = 1
Transistor On
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Here is a better circuit:
VCC
T2
Signal In
Voltage Out
T1
T3
This circuit is called a Totem Pole,
and produces better 1’s and 0’s
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The Truth Table
A tool used to understand binary
logic
 Shows every possible input
 Shows the output for each input

What circuit
does this
represent?
Input
A
0
1
Output
X
1
0
An Inverter!
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Binary Logic
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A set of rules that applies to a digital
circuit
Logic defines the way the circuit will act
Given a set of inputs, the output will
produce a specific outcome
Always acts exactly the same way
We use a truth table to help us define how
we want the logic circuit to act
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
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We build the circuit to perform the
logic
Always does the same thing with the
same inputs
We must define each possible input
or input combination
We have to define exactly what
output we want for a particular input
Lets look at an example
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Two Bit Binary Adder
Adds two binary bits
 Binary can only have two values, 0
and 1

0
+0
0

0
+1
1
1
+0
1
1
+1
0
Carry 1
1 + 1 = 2, which is not valid binary
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Truth Table for Binary Addition
We have two inputs
 We generate two outputs

Input Input Output Carry
Σ (sum) Co
A
B
How do
we do
this?
0
0
1
1
0
1
0
1
0
1
1
0
0
0
0
1
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First, we
have to
understand
binary!
Binary Numbers
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Binary only has two values, 0 and 1
The decimal number system has ten
values, 0 through 9
How do we count higher than 9 in decimal?
We add decimal places
How do we count higher than one in
binary?
We add binary bits
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
Decimal places are each multiples of ten
100 = 1 (ones)
101 = 10 (tens)
102 = 100 (hundreds)
103 = 1000 (thousands)

Binary bits are each multiples of two
20 = 1 (ones)
21 = 2 (twos)
22 = 4 (fours)
23 = 8 (eights)
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Reading Binary Numbers

A decimal number like 9437 reads:
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The binary number 1011 has values:
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One eight, no fours, one two, one one.
1011 has a decimal value of 11 (eleven)
How do read the binary 1111?
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Nine thousand, four hundred, thirty, seven.
Decimal Fifteen
How do you count higher than fifteen?

Add more binary bits (decimal places)
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Binary Bit Values

To count up to 1000 (decimal) you need
ten binary bits
Bit
number
Decimal value
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9
8
7
6
5
4
3
2
1
0
5
1
2
2
5
6
1
2
8
6
4
3
2
1
6
8
4
2
1
To count higher, you need more binary
bits
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Binary to Decimal

The binary number:
1001011001
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Has a decimal value:
512 + 64 + 16 + 8 + 1 = 601
1 0 0 1 0 1 1 0 0 1

9
8
7
6
5
4
3
2
1
0
5
1
2
2
5
6
1
2
8
6
4
3
2
1
6
8
4
2
1
This process involves addition
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Decimal to Binary
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Decimal to binary is a little harder
The process involves subtraction
For example, consider the decimal number:
361
Go back to the binary count:
9
8
7
6
5
4
3
2
1
0
5
1
2
2
5
6
1
2
8
6
4
3
2
1
6
8
4
2
1
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0 1 0 1 1 0 1 0 0 1
9
8
7
6
5
4
3
2
1
0
5
1
2
2
5
6
1
2
8
6
4
3
2
1
6
8
4
2
1
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36110 equals 01011010012
8210 equals 00010100102
102310 equals 11111111112
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Binary Count
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The easiest way to
show all possible input
values is a binary count
Keep adding one to the
previous value
Everyone should be
able to count in binary
D = 1, C = 2, B = 4,
A=8
A B C D Deci Hex
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
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0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
0
1
2
3
4
5
6
7
8
9
A
B
C
D
E
F
A B C D
What happens if
you have to keep
counting?
0
0
0
1
0
0
0
1
0
0
0
1
1
1
1
1
Deci
Hex
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
0
1
2
3
4
5
6
7
8
9
A
B
C
D
E
F
0
0
1
1
0
0
1
1
0
0
1
1
0
1
1
1
16
17
31
255
10
11
1F
FF
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Hexadecimal
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Computers and electronic devices
communicate in binary
All those 1’s and 0’s are confusing
Is there an easy way to tell what the
binary values are?
YES – it’s called Hexadecimal
Hexadecimal is a base 16 number system
Hexadecimal exactly represents 4 binary
bits
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Using Hex
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To create hex, start with bit 0 and group
the binary number into groups of 4
For example, our decimal 601 would be:

10010110012 or 25916

70110 = 10101111012 = 2BDh
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Using Hex


To create hex, start with bit 0 and group
the binary number into groups of 4
For example, our decimal 601 would be:

10 0101 10012 or 25916

70110 = 10 1011 11012 = 2BDh
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
There are several ways to indicate
the hex number system:
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Use the subscript 16
Use a lower case h right after the
number
Use a dollar sign, e.g. $2BD
Hex is a shorthand way of
representing binary
The hex number always exactly
represents 4 binary bits
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Back to Addition


Add 2 4-bit binary numbers:
111
0101
5
+ 0011
+3
1
1000
8
1 + 1 = 2, which is written 10 in binary
This 1 is a carry
into the next bit
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2 Bit Binary Adder

Lets go back to the truth table for a
binary adder:
Now that we
understand
binary, we can
figure out how
to do this!
Input Input Output Carry
Σ
A
B
Co
(sum)
0
0
0
0
0
1
1
0
1
0
1
0
1
1
0
1
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We Use Transistors

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All logic uses transistors
We use voltage to represent binary
TTL (transistor – transistor logic) is
common:
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+ 5 V = binary 1
0 V = binary 0
These voltages will turn on or off
transistors
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