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Diode &
Electronic Circuits
Semiconductors
 conductor
easily conducts electrical current
– valence electron can easily become free
electrons
insulator
– does not conduct electrical current
– valence electron are tightly bound to the
atoms.
semiconductor
– atoms are arranged in a crystal.
–


Semiconductors (cont.)


each atom shares its electrons with four neighboring atoms
the valence orbit can hold no more than eight electrons,
Electron and Hole Current
 heat
energy causes the atoms to vibrate
 dislodge an electron from the valence band to the
higher conduction band
 departure of the electron creates a vacancy in the
valence band called a hole
 hole will attract and capture any electron in the
immediate vicinity for recombination
Electronic Current
Hole Current
Doping a Semiconductor
An intrinsic (pure) semiconductor has very few free
electrons and holes.
 To increase the number of carrier (holes and
electrons), doping is used,
 n-Type Semiconductor

–
–

doped with a pentavalent impurity (donor
atom)
free electrons are called the majority
carries
p-Type Semiconductor
–
doped with a trivalent impurity (acceptor
atom)
Free electron
(n type)
Pentavalent dopant
Hole
(p type)
Trivalent dopant
PN Junction Diode
 half of a silicon crystal doped with p-type
impurity ,another half doped with n type impurity
 the border between p-type and n type is called the pn
junction
Anode
R
VS
p
=
n
Cathode
Forward
–
–
–
–
Bias
anode is more
positive than
cathode (>0.7v, the
knee voltage)
diode is conduct
with large current
ID
acts as a close
switch
neglect 0.7V in
ideal diode
Reverse Bias
– cathode is more
positive than anode
– diode passes a
negligible amount
of reverse current
– acts as an open
switch
– diode will
breakdown when
reverse voltage is
too large
Forward current in mA
200
175
150
125
100
75
50
25
knee
0
0
0.5
1.0
Forward bias in volts
1.5
breakdown
600
Reverse bias in Volts
400
200
0
20
40
60
80
100
120
140
Reverse
current
in mA
Ideal diode act as switch
Rectification and Smoothing
The above diagram show how to obtain a
regulated d.c. from the high voltage a.c. main
 This section will discussed the rectification and
smoothing

The half-wave
rectifier
Vin
Vin
Vout
Vout
Ideal: VP(in) = VP(out)
Halfwave Rectifier (cont.)
 During
the positive cycle of the a.c. source,
– the diode act as a close switch (neglect 0.7V)
– Vp(out) = Vp(in) = Vsec
 During the negative cycle of the a.c. source
– the diode act as an open switch
 Output Frequency : fout = fin
 D.C. value of Half-Wave Signal
Vdc 
V p ( out)

 0.318V p ( out)
Example 5.1
What are the peak load voltage and d.c. load voltage of fig. 5.8 ?
Fig 5.8 Example of half wave rectifier
 Solution

Given the turns ratio is 5 : 1. The r.m.s. secondary
voltage is one-fifth of the primary voltage
120
V2 
V  24V
5
•The peak secondary voltage is
V2 p
–
–
24V

 34V
0.707
The peak load voltage = peak secondary
voltage = 34V
The dc load voltage = 0.318 V2p =
10.8V
The bridge
rectifier
D1
Vin
D2
D3
D4
Vout
Vout
Full wave (Bridge) Rectifier
 During
the positive cycle,
– the purple diode D2 and D3 conduct,
– the red diode D1 and D4 do not conduct
 During the negative cycle
– the purple diode D2 and D3 do not conduct,
– the red diode D1 and D4 conduct
 Output Frequency
– Half wave : fout =2 fin
 D.C. value of Bridge Rectifier
Vdc 
2V p ( out)

 0.636Vm
Smoothing circuit
During the positive first quarter-cycle of the input,
the diode is forward-biased, allowing the capacitor
to charge to point a.
 When the input begins to decrease below its peak,
the diode becomes reverse-biased, the capacitor is
discharged from point a to point b

Point b is the point in which Vin is larger than the
Vc again. The capacitor resumes the charging
again until it reaches point c (= point a)
 Repeating the discharging from point a to point b
and the charging from point b to point c, we obtain
the following waveform.(it is much smooth than
the waveform without capacitor.)

Transistor
Transistor characteristics
 A bipolar transistor is an electronic device made of
materials such as silicon and germanium.
 It consists of three layers, either P-N-P or N-P-N.
 Transistor can be used as amplifier or switch.
 In this section, only n-p-n transistor will be
discussed.
 The
three terminals of the transistor are
collector, base and emitter.
 Under normal operating condition, the
junction of collector and base should be
reverse biased and the junction of base and
emitter junction should be forward biased.
 In
the following diagram, the base of the transistor
is connected to the common point, i.e., the
junction of the two power supplies, this method is
known as common base.
 The methods of common emitter and common
collector are also used in circuits.
 In the common emitter configuration, the input is
applied to the base and the output is taken from
the collector.
Symbol of NPN
transistor
Arrangement of an NPN
transistor
Transistor switch and amplifier
 Under
steady state, IB is kept constant. The current
gain of the transistor is dc = IC / IB.
 In the linear region, dc will not vary too much
with IB.
 The current flowing into the base and into the
collector will flow out of the transistor from the
emitter, therefore, IE = IC + IB or IE = (1+ dc)IB.
 When
the transistor is operating as an common
emitter amplifier, the input is applied to the base
and output is taken from the collector.
 It should be biased to have VCE approximately
equal to VCC/2 at the steady state. Then the
amplifier will have widest dynamic range.
 When an input signal applied, IB varies and IC
will vary accordingly.
 If the input is too large, IB will swing out of the
linear region and the output will be distorted.
 The
relationship between the input and
output voltages can be shown in the
following formula:
Vout  GVin
 Where
Vin and Vout are input and output
voltage respectively, G is the voltage gain
of the amplifier.
Common emitter transistor amplifier
Amplifiers are classified in four types, they are
classes A, AB, B and C.
 They are distinguished by the conduction angle of
collector current.
 For class A amplifier, the whole input waveform
will appear at the output. Its conduction angle is
360 . The transistor is only operating in linear
region.
 For class B amplifier, only half of the input
waveform will appear at the output. Its conduction
angle is 180.

 The
conduction angle of class AB amplifier is
between that of class A and class B, less than 360
but larger than 180.
 For class C, less than half of the waveform will
appear at the output. Its conduction angle is less
than 180.
Classes of amplifier
Type of amplifier
Conduction angle
Class A
360
Class AB
Between 360 and 180
Class B
180
Class C
Less than 180
 Among
the four types, only class A amplifier does
not have high distortion output. However, it has a
draw back of low efficiency. Therefore, it is
usually used on small signal amplifier.
 Class AB can also work on analogue signals but at
least two transistors will be used to build an
amplifier to achieve low output distortion. It has
higher efficiency than class A amplifier.
 Because of high output distortion, class B and C
amplifiers are seldomly seen operating on
analogue signals. But their high efficiency make
them favorable on radio frequency high power
applications.
When the transistor is operating in the cutoff state,
IB is very small or equal to zero. IC is very small
too and VCE  VCC.
 When the transistor is operating in the saturation
state, IB and IC are both high, then VCE  0.
 A transistor amplifier will not have undistorted
output when it is operating in cutoff or saturation
state.
 However, transistor behaves as a switch in this
two states.

 In
the cutoff state, the transistor is said to be
turned off, and the output voltage is high.
 In the saturation state, the transistor is said to be
turned on, and the output voltage is low.
 Since the logic ICs can only deliver very small
output current, the transistor switch can be used to
boost the output current to drive the high power
devices.
 When
the input is at high voltage level, its output
is at low level and vice versa. This switch is also
an inverter.
 The applications include switch mode power
supply, controlling electromechanical devices (eg.
solennoid or motor) and high power electrical
devices (eg. light bulb or heater).
Transistor switch
Example 1
An input of 10mV is applied to an amplifier with
voltage gain 200. What is the output voltage?
Output voltage Vout= GVin = 200 x 10mV = 2V
Example 2
In the circuit diagram shown in fig. 5.17, VCC =15V,
RC=1k, dc =100. What is the most appropriate
VCE? Calculate IC, IB, IE and RB under steady state.
If the same transistor is used as a switch as on fig.
5.18 and the load resistor equals to 200,
calculate RB.
Example 2
VCE = VCC / 2 = 7.5V
IC = (VCC - VCE )/ RC = (15V-7.5V)/1000 = 7.5mA
IB = IC/dc = 7.5mA/100 = 0.075mA or 75A
IE = IC + IB = 7.5mA + 0.075mA =7.575mA
Since VBE =0.7V, therefore, RB = (VCC - VBE )/ IB
=(15V-0.7V)/0.075mA=191k
Example 2
When the transistor is used as a switch, RB of lower
value should be chosen.
When the switch is turned on, IC = VCC / RC =
15V/200 = 75mA
IB should be higher than IC / dc =
75mA/100=0.75mA
RB should be lower than (VCC - VBE )/ IB =(15V0.7V)/0.75mA=19k
To make sure that the transistor can turn on, RB
should be chosen to be 1/10 of the above
calculated value, that is 1.9k
Logic Function
NOT Function
YA
A

It has one input and one
output

The output Y is equal to
the complement of
A(input).
INPUT
OUTPUT

It is denoted by a prime
(‘)after the alphabet A.
A
0
YA
1

The logical operator is
( )and the operand is A.
1
0
 It
is important to point out that each input
can only take one of the two logic values, 1
or 0
 To work out the output for each
combinations of input, we have to use
Boolean Algebra
 Logic Gate is the equivalent electronic
device that performs a Boolean Algebraic
function.
AND Function
Y is HIGH only when
both A and B are HIGH.
 Y = A•B (sometimes the
dot • is omitted) stands
for a Boolean operation,
AND multipication.
 AND multiplication is
exactly the same as
ordinary multiplication.

A
Y=A˙B
B
INPUTS
A
B
0
0
0
1
1
0
1
1
OUTPUT
Y = A˙B
0
0
0
1
OR Gate
Y is HIGH unless both
input A, B are LOW.
 Y = A+B stands for a
Boolean operation,


A
Y=A+B
B
IN PUTS
OUTPUT
OR addition.
A
B
Y = A+B
OR addition is similar
to ordinary addition
except that 1+1 = 1
0
0
0
0
1
1
1
0
1
1
1
1
Multiple Level of Gates

B
A
C
D
The output (1st level) gate is a 3 input AND gate
–
–
2 inputs are input A,D
remaining input is fed from the output of an
OR gate
the OR gate is called 2rd level gate
 output Y is a product of three terms

Y  A  (B  C)  D
A
B
X  A  B B C
C
 The
–
output gate is a 2 input OR gate
each input from output of AND gate
 The
two AND gates are called 2nd level gate
 Output X is a sum of two terms
Truth Table for X  A  B  B  C
A
0
0
0
0
1
1
1
1
B
0
0
1
1
0
0
1
1
C
0
1
0
1
0
1
0
1
B
1
1
0
0
1
1
0
0
AB
0
0
0
0
0
0
1
1
B C X  A  B B C
0
0
1
1
0
0
0
0
0
0
1
1
0
1
0
1
Truth Table for Y  A  (B  C)  D
A
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
B
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
C
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
D
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
B+C
0
0
1
1
1
1
1
1
0
0
1
1
1
1
1
1
Y  A  ( B  C)  D
0
0
0
0
0
0
0
0
0
0
0
1
0
1
0
1
NAND Function
Y is LOW only if A,
B, are HIGH
 NAND is generated
by inverting the
output of an AND
function

A
Y  AB
B
IN PUTS
OUTPUT
A
B
Y  AB
0
0
1
0
1
1
1
0
1
1
1
0
NOR Function
A
Y is LOW unless
both A and B are
LOW
 NOR is generated
by inverting the
output of an OR
function.
Y AB

B
IN PUTS
OUTPUT
A
B
Y AB
0
0
1
0
1
0
1
0
0
1
1
0
TTL and CMOS
 Logic
gates are available in integrated
circuit (IC) form. transistor-transistor
logic (TTL) or complementary metaloxide semiconductor (CMOS)
 TTL and CMOS chips are designated by an
industry-standard numbering system. Low
Power Schottky TTL, is designated
74LSXX, where XX is a different number
for each logic function
 Relatively
higher switching speed of TTL
low power consumption of CMOS
 Logic gates come in packages containing
several gates.
 Common groupings available in DIP
packages are six 1-input, four 2-input gates,
three 3-input gates, or two 4-input gates.
Table 7 Some Common Logic Gate Packages
Gate
74LS00
74LS02
74LS04
74LS11
4011B
4001B
4069UB
4073B
74HC00A
74HC02A
74HC04A
74HC11
Family
TTL
TTL
TTL
TTL
CMOS
CMOS
CMOS
CMOS
High-Speed CMOS
High-Speed CMOS
High-Speed CMOS
High-Speed CMOS
Function
Quad 2-input NAND gate
Quad 2-input NOR gate
Hex inverter
Triple 3-input AND gate
Quad 2-input NAND gate
Quad 2-input NOR gate
Hex inverter
Triple 3-input AND gate
Quad 2-input NAND gate
Quad 2-input NOR gate
Hex inverter
Triple 3-input AND gate