1. Form 5
Physics
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The study of matter
Chapter 4: Electronics 1

2. 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
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3. Maltese-cross Tube
Thermionic emission = emission of electrons from hot metal surface in vacuum Cathode rays = electrons moving at high speeds after acceleration through high potential difference
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A Maltese-cross tube is used to show the first two properties of cathode rays.
Properties: 1. electrons moving at high speeds in straight lines 2. cause fluorescent material to emit light 3. deflected by magnetic field 4. deflected by electric field 3

4. Maltese-cross Tube 4 “Maltese Cross” Crookes Tube
Invented in the 1880s by William Crookes during his investigations into the nature of cathode rays.
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It demonstrates that radiant matter is blocked by metal objects.
The direction of deflection of cathode rays by magnetic field is found with Fleming’s left-hand rule. 4

5. The Cathode Ray Tube (CRT)
The oscilloscope is capable of following changes that occur within billionths of a second.
It is widely used throughout industry and in laboratories to test and adjust electronic equipment, and to follow rapid oscillations in electric voltages.
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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. 5
The Cathode Ray Tube (CRT)

6. The Cathode Ray Tube (CRT)
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The Cathode Ray Tube (CRT)

7. Cathode Ray Oscilloscope
The Cathode Ray Oscilloscope (C.R.O.) is divided into 3 parts:
Electron gun
Deflection system
Fluorescent screen
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8. Cathode Ray Oscilloscope
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
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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 field
doctronics
Kinetic energy of electrons emerging from anode = eV
½ mv2 = eV 8
2eV m
Velocity, v =
where
e = charge of electron, m = mass of electron

9. Cathode Ray Oscilloscope
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
r.m.s. voltage, Vrms = = volts
V ln
√ √2 2
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Given:
Y-sensitivity = n V per division 9

10. Cathode Ray Oscilloscope
2. 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
Speed of sound, v = =
Distance travelled 2d
Time taken 3T
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wall d
microphone A B
3 divisions
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 10

11. Semiconductor diodes
Semiconductors have resistance between that of metals and insulators; e.g. carbons, germanium, silicon
Pure semiconductor:
negative charge carriers = positive charge carriers
or 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)
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Semiconductor diode p n + –
p-n junction
structure + –
actual diode
band + –
symbol 11

12. Semiconductor diodes 12 Ideal diode
Allows current through when connected in forward bias
Stops current when connected in reverse bias (infinite resistance)
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current
Forward bias +
no current
Reverse bias 12

13. Half-wave rectification
Semiconductor diodes
A diode is used as a rectifier
to convert a.c. to d.c. VD VR R
a.c.V
Half-wave rectification
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Current only flows through the diode during the positive half cycle (as shown by +V). 13
The voltage across the load, VR is direct voltage and the current is d.c.

14. Semiconductor diodes
A capacitor, C is connected across load, R to smoothen voltage, VR.
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a.c.V
smoothing capacitor C 14

15. Semiconductor diodes
2 diodes are used in a simple full-wave rectification.
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4 diodes are used in a bridge full-wave rectification. 15

16. 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.
n-p-n transistor B C E
p-n-p transistor B C E
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Some samples of the actual transistors
B: base C: collector E: emitter
Structure of an n-p-n transistor 16

17. Transistors 17 Transistor 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.
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Next > Ib B C E mA µA Ic 17

18. Transistors 18 Transistor as a switch
The 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.
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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. 18

19. Logic Gates
Logic 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.
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NOT logic gate
It is also called the inverting buffer. A X
Input
Output
Boolean equation
X = A
Truth table 19

20. Making a NAND gate out of transistors and resistors
Logic Gates
AND and NAND logic gates A B
X = A • B
AND A B
X = A • B
NAND
Making a NAND gate out of transistors and resistors
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1 = 0
1 • 1 = 1 1
0 = 1
1 • 0 = 0
0 • 1 = 0
0 • 0 = 0
NAND
AND B A
Output
Input 20

21. Logic Gates 21 1 = 0 1 • 1 = 1 1 1 • 0 = 1 0 • 1 = 1 0 = 1 0 • 0 = 0
OR and NOR logic gates A OR
X = A + B B A
NOR
X = A + B
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1 = 0
1 • 1 = 1 1
1 • 0 = 1
0 • 1 = 1
0 = 1
0 • 0 = 0
NOR OR B A
Output
Input 21

22. Summary 22 What you have learned: Thank You
Uses of Cathode Ray Oscilloscope
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2. Semiconductor diodes
3. Transistors
4. Logic gates 22
Thank You