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Form 5
Physics
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The study of matter
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Chapter 4:
Electronics
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Physics: Chapter 4
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Objectives:
(what you will learn)
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1)
uses of Cathode Ray Oscilloscope
2)
understanding semiconductor diodes
3)
understanding transistors
4)
analysing logic gates
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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
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Maltese-cross Tube
“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.
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The direction of
deflection of cathode
rays by magnetic field
is found with Fleming’s
left-hand rule.
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Cathode Ray Tube
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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The Cathode Ray Tube (CRT)
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.
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Cathode Ray Tube
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The Cathode Ray Tube (CRT)
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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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doctronics
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
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
Kinetic energy of electrons emerging from anode = eV
½ mv2 = eV
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Velocity, v =
2eV
m
where
e = charge of electron, m = mass of electron
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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
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r.m.s. voltage, Vrms =
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V0
√2
1
=
√2
ln
volts
2
x
l
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Given:
Y-sensitivity = n V per division
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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
Distance travelled
2d
Speed of sound, v =
=
Time taken
3T
wall
d
microphone
A
B
d
3 divisions
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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
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Semiconductor diodes
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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
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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)
Semiconductor diode
p-n junction
p
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n
+
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structure
–
band
–
+
symbol
–
+
actual diode
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Semiconductor diodes
Ideal diode
• Allows current through when connected in forward bias
• Stops current when connected in reverse bias (infinite resistance)
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+
current
+
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Forward bias
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+
no current
+
Reverse bias
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Semiconductor diodes
A diode is used as a rectifier
to convert a.c. to d.c.
VD
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a.c.
V
R
VR
Half-wave rectification
Current only flows through the diode during the positive half
cycle (as shown by +V).
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The voltage across the load, VR is direct voltage and the
current is d.c.
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Semiconductor diodes
A capacitor, C is
connected across load, R
to smoothen voltage, VR.
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VD
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a.c.
V
C
R
smoothing capacitor
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VR
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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.
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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.
B
E
B: base
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Some samples of the
actual transistors
C
B
n-p-n transistor
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C
E
p-n-p transistor
C: collector
E: emitter
Structure of an n-p-n transistor
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Transistors
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
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When Ib changed, it is amplified by the transistor,
producing larger change in Ic.
Ic
C
B
µA
Ib
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E
mA
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Transistors
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.
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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.
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Other devices may be used in place
of LDR for other functions.
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Logic Gates
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Logic gates = switching circuits used in computers
and electronic devices
A logic gate has one or more inputs but only one
output.
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Its action is summarized by an equation in Boolean
algebra, or with a truth table.
NOT logic gate
It is also called the inverting buffer.
Truth table
A
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X
Boolean equation
X=A
Input
Output
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Logic Gates
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AND and NAND logic gates
AND
NAND
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Input
A B
0 0
0 0
1 0
1 1
A
B
A
B
X=A•B
X=A•B
Output
AND NAND
0•0=0 0=1
0•1=0 0=1
1•0=0 0=1
1•1=1 1=0
Making a NAND gate out of
transistors and resistors
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Logic Gates
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OR and NOR logic gates
OR
NOR
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B
A
X=A+B
X=A+B
B
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Input
A B
0 0
0 1
1 0
1 1
Output
OR
NOR
0•0=0 0=1
0•1=1 1=0
1•0=1 1=0
1•1=1 1=0
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Summary
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What you have learned:
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22
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1.
Uses of Cathode Ray Oscilloscope
2.
Semiconductor diodes
3.
Transistors
4.
Logic gates
Thank You