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Form 5. Chapter 4: Electronics. Physics. Next >. The study of matter. 1. < Back. Next >. Physics: Chapter 4. Objectives: ( what you will learn ) 1)uses of Cathode Ray Oscilloscope 2)understanding semiconductor diodes 3)understanding transistors 4)analysing logic gates. 2. < Back. - PowerPoint PPT Presentation
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Chapter 4: Chapter 4: ElectronicsElectronics
Form 5Form 5
1
PhysicsNext >
The study of The study of mattermatter
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Objectives: Objectives: ((what you will learnwhat you will learn)) 1) uses of Cathode Ray Oscilloscope
2) understanding semiconductor diodes
3) understanding transistors
4) analysing logic gates
Physics: Chapter Physics: Chapter 44
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Maltese-cross TubeMaltese-cross TubeThermionic emission = emission of electrons from hot metal surface in vacuum
Cathode rays = electrons moving at high speeds after acceleration through high potential differenceA Maltese-cross tube is used to show the first two properties of cathode rays.
Properties:1. electrons moving at high
speeds in straight lines2. cause fluorescent
material to emit light 3. deflected by magnetic field4. deflected by electric field
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The direction of deflection of
cathode rays by magnetic field is
found with Fleming’s left-hand
rule.
“Maltese Cross” Crookes Tube
Invented in the 1880s by William Crookes during his
investigations into the nature of cathode rays.
It demonstrates that radiant matter is blocked by metal
objects.
Maltese-cross TubeMaltese-cross Tube
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Cathode Ray TubeCathode Ray Tube
It is widely used throughout industry and in laboratories to test and adjust electronic
equipment, and to follow rapid oscillations in electric voltages.
The oscilloscope is capable of following changes that occur within billionths of a
second.
Special converters attached to
oscilloscope can convert
mechanical vibrations, sound waves, and other
forms of oscillatory motion
into electrical impulses that can be observed on the face of CRT.
The Cathode Ray Tube (CRT)
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The Cathode Ray Tube (CRT)
Cathode Ray TubeCathode Ray Tube
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Cathode Ray Cathode Ray OscilloscopeOscilloscopeThe Cathode Ray Oscilloscope (C.R.O.) is divided into 3
parts:• Electron gun
• Deflection system
• Fluorescent screen
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Cathode Ray Cathode Ray OscilloscopeOscilloscope
Deflection system:• Y-plates: electric field deflects electrons vertically• X-plates: electric field deflects electrons horizontally
Fluorescent screen:• When fast electrons hit fluorescent screen, their kinetic
energy is converted into light – a spot of light is seen on the screen
• The walls of C.R.O. after anode is coated with graphite and grounded to keep out external electric fieldKinetic energy of electrons emerging from anode = eV
½ mv2 = eV
Electron gun:• The cathode emits electrons when heated• The grid controls the number of electrons reaching anodes
– control with brightness knob• The anode focus electrons into fine beam – control with
focus knob• The potential difference between anode and cathode
accelerates electrons to high velocity
2eVm
Velocity, v =
wheree = charge of electron, m = mass of electron
doctronics
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Cathode Ray Cathode Ray OscilloscopeOscilloscope
Uses of C.R.O.1. Measure potential difference• Switch off time-base• Connect voltage to be measured to Y-input• d.c. voltage: if x = deflection of light spot, voltage = xn
volts• a.c. voltage: 2 x (peak voltage, V0) = ln
x
l
Given:
Y-sensitivity = n V per division
r.m.s. voltage, Vrms = = volts
V0 1 ln
√2 √2 2
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Cathode Ray Cathode Ray OscilloscopeOscilloscope2. Measure short time interval
• Switch on time-base; one horizontal division = time interval, T
• Pulse A represents sound detected by microphone• Pulse B represents the echo• Say, time interval between A and B is 3 divisions = 3T• If d = distance of wall from microphone
3. Display waveform• Connect input voltage to Y-input• Switch on time-base • Adjust frequency to a steady trace formed on screen• The trace or waveform is the graph of voltage V
against time t
Speed of sound, v = =Distance travelled
2d
Time taken 3T
A B
3 divisions
wall
d
d
microphone
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Semiconductor Semiconductor diodesdiodesSemiconductors have resistance between that of
metals and insulators; e.g. carbons, germanium, silicon
Pure semiconductor: negative charge carriers = positive charge carriersor free electrons = holes
Doped semiconductor (with added impurity):n-type:free electrons > holes
(impurity of valency 5; arsenic or phosphorus)p-type:holes > free electrons
(impurity of valency 3, indium or gallium)
p n+ –
p-n junction
structure
+ –
symbol
+ –
actual diode
band
Semiconductor diode
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Semiconductor Semiconductor diodesdiodes
+
+
current
Forward bias
+
+
no current
Reverse bias
Ideal diode• Allows current through when connected in forward bias
• Stops current when connected in reverse bias (infinite resistance)
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Semiconductor Semiconductor diodesdiodesA diode is used as a
rectifier to convert a.c. to d.c.
VD
VRR
a.c.V
Half-wave rectification
Current only flows through the diode during the positive half cycle (as shown by +V).
The voltage across the load, VR is direct voltage and the current is d.c.
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Semiconductor Semiconductor diodesdiodes
A capacitor, C is connected across load, R to smoothen voltage, VR.
VD
VRR
a.c.V
smoothing capacitor
C
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Semiconductor Semiconductor diodesdiodes
4 diodes are used in a bridge full-wave rectification.
2 diodes are used in a simple full-wave rectification.
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TransistorsTransistors
n-p-n transistor
B
C
E
p-n-p transistor
B
C
E
B: base C: collector E: emitter
Structure of an n-p-n transistor
Some samples of the actual transistors
Transistor is an electronic device containing at least 3 layers of semiconductor and electrical contacts, used in a circuit as amplifier, detector, or switch.
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TransistorsTransistorsTransistor as a current amplifier
The base current Ib controls the collector current Ic
Ic is many times larger than Ib.
When Ib = 0, Ic = 0
When Ib changed, it is amplified by the transistor, producing larger change in Ic.
Ib
BC
E
mA
µA
Ic
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TransistorsTransistorsTransistor as a switchThe transistor can be used as a switch to switch on a lamp, L.
The light-dependent resistor (LDR) has resistance of 2 kΩ in bright light and 20 kΩ in the dark.
During the day, resistance R1 is much less than resistance R2. So the potential difference across LDR is much smaller than across R2.
The base current Ib is small, the collector current Ic is small, and the relay is not activated. The lamp L is off.
The reverse happens when in the dark. R1 increases to maximum, potential difference across LDR increases, and Ib increases.
The transistor amplifies the increase resulting in large Ic, thus activating relay and lamp L is switched on.
Other devices may be used in place of LDR for other functions.
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Logic GatesLogic GatesLogic gates = switching circuits used in computers and electronic devices
A logic gate has one or more inputs but only one output.
Its action is summarized by an equation in Boolean algebra, or with a truth table.
NOT logic gateIt is also called the inverting buffer.
A X
Input Output
Boolean equation
X = A
Truth table
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Logic GatesLogic GatesAND and NAND logic gates
A
BX = A • BAND
NANDA
BX = A • B
1 = 01 • 1 = 1110 = 11 • 0 = 0010 = 10 • 1 = 0000 = 10 • 0 = 000
NANDANDBAOutputInput
Making a NAND gate out of transistors and
resistors
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Logic GatesLogic GatesOR and NOR logic gates
A
BX = A + B
NOR
A
BX = A + B
OR
1 = 01 • 1 = 1111 = 01 • 0 = 1011 = 00 • 1 = 1100 = 10 • 0 = 000NORORBA
OutputInput
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SummarySummary
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What you have learned:What you have learned:
1. Uses of Cathode Ray Oscilloscope
Thank YouThank You
2.2. Semiconductor diodes
3.3. Transistors
4.4. Logic gates
