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9.1 CATHODE RAY OSCILLOSCOPE

What isthermionic

emission?

Thermionic emission is the process of emission ofelectrons from a heated metals surface.

How doesthermionicemission occur?

Metal consists of a large number of electronswhich are free to move.

At room temperature, the elect rons are free tomove but remain inside the metal.

The electrons cannot escape at the surfacebecause they are held back by the attractiveforces of the atomic nucleus.

If the metal is heated at a high temperature,some of the free electrons may gain sufficient

energy to escape from the metal.

What iscathoderays?

Thermionic emissions can be used to produce acontinuous flow of electrons in a cathode ray tube.

When the cathode is connected to the anode by anextra high tension (EHT) voltage supply, a narrow beamof fast electrons will move to the anode.

The beam of electrons moving from the cathode to theanode is called cathode rays.

Describethepropertiesofcathoderays

Cathode rays can be used in picture tube of atelevision, a cathode ray oscilloscope and the visualdisp lay on a radar screen.

The properties of cathode rays can be summarized:1. Negatively charged particles called electrons.2. Travel in straight lines and cast sharp shadows.3. Travel at very high speed and have kinetic energy.

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4. Can cause fluorescence. (A process where thekinetic energy of the electrons is converted intolight energy)

5. Deflected by electric and magnetic fields.

ToInvestigatethe propertiesof electronstreams in aMaltese crosscathode raytube.

Procedure Observation on the fluorescent screen

Connect only the 6.3 Vpower supply to thefilament

A dark shadow of the Maltese Cross isformed on the screen.

Connect the 6.3 V and EHTto the electrodes

A darker shadow of the Maltese Crossis seen on the screen. The shadow issurrounded by green light

Bring a pole of a bar magnetnear to the neck of the tube.

Two shadow are seen on the screen.The light shadow remains at the centreof screen while the dark one is shi fted.

Reverse the pole of the barmagnet

The light shadow remains at the centreof screen while the dark one is shiftedto the opposite direction.

Discussion1. When the 6.3 V power supply is switched on, the filament is

heated. The Maltese cross shadow is formed on the screen due

to the light from the filament.2. When the EHT power supply is switched on, a high voltage is

applied between the cathode and anode causing electrons toaccelerate at high speeds from cathode to anode. Theseelectrons are the cathode rays .

3. The cathode rays blocked by the Maltese Cross causing ashadow to form on the screen. The cathode rays travel instraight lines.

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4. The green screen formed around the shadow when the EHTpower supply is switched on shows that the kinetic energy ofthe electron is converted into light energy when the electrons hitthe fluorescent scree.

5. When a strong magnet is placed at the side of the Maltese Crosstube, the shadow formed is moved and distorted. This showsthat cathode rays are deflected by a magnetic field.

Investigate theproperties ofcathode raysin an electricfield

Observation:

(a) No vol tageconnected to thedeflecting plates

Top plate isconnected to EHT (+)and lower plate is

connected to EHT (-)

Top plate isconnected to EHT (-)and lower plate is

connected to EHT (+)No deflection The electron beamwill deflect upward

The electron beamwill deflect downward

Discussion1. The cathode ray is negatively charge.2. It will deflect in an electric field.

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Cathode RayOscilloscope

Uses a cathode ray tube that converts electronicand electrical signals to a visual display.

The graph produced consist of a horizontal axiswhich is normally a function of time, and a vertical

axis which is a function of the input voltage. The components in a cathode ray tube consists of a

vacuum glass tube with an electron gun, adeflection system for deflecting the electron beamand a fluorescent coated screen.

Describe thefunction ofthe cathoderay

oscilloscope,CRO

Electron gun The electron gun is used to produce a narrow beam

of electrons.

Filament Is heated when current flows through

it. It is used to heat up the cathode.Cathode Heated cathode emits electrons

through the process of thermionicemissions.

Control grid Control the number of electrons in theelectron beams.The more negative the grid, the fewerthe electrons are emitted from the

electron gun and the less thebrightness of the bright spot on thescreen.

Focusinganode

To focus the electrons into a beam andto attract electrons f rom the area of thecontrol grid.

Acceleratinganode

To accelerate the electron beamtowards the screen.

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DeflectionSystem

The deflection system allows the electron beam tobe deflected from its straight-line path when itleaves the electron gun.

Y-plates is to move the electron beam vertically up

and down the screen when an input vol tage isapplied across it.

No input voltageThe electron beamdoes not deflect andthe bright spot is atthe centre

+ve vol tage is

applied, the electronbeam deflectupward. The brightspot moves to thetop.

-ve voltage isapplied, the electronbeam deflectdownward. The

bright spot movesto the bottom

a.c voltage isapplied, the electronbeam deflects upand down. Thebright spot movesup and down toform a brightvertical trace on the

screen

The function of the X-plates is to sweep the electronbeam across the screen horizontally from left toright at a steady speed.

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FluorescentScreen

The fluorescent screen is coated on the insidesurface with some fluorescent material such asphosphor or zinc sulfide.

When electron beam strikes the screen, the material

becomes glows. This enables a bright spot toappear whenever an electron beam strikes thescreen.

The moving electrons have kinetic energy. Whenthis electrons strikes the screen, the fluorescentcoating on the screen converts the kinetic energy ofthe electrons into light energy.

Applicat ionof CRO

1. Measuring potential difference2. Measuring short intervals3. displaying wave forms

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Describe the work ing pr inciple of the cathode ray osci lloscope, CRO

Controlknob

Function

Powerswitch

Control the power supply

Focus Control the sharpness of the bright spot

Connected to the focusing anode

The sharpness of the bright spot is also affected by

the brightnessBrightness To control brightness or intensity of the bright spot

Connected to the control grid

Brightness level should be set as low as possible toobtain a clear and sharp trace

X-shift

To adjust the horizontal position of the bright spot onthe screen

Connected to the X-plates


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Y-shift

To adjust the vertical position of the bright spot orthe trace displayed

Connected to the Y-plates

Y gain

(volts / div)

To control the magnitude of the vertical deflection of

the bright spot or the trace displayed on the screenby adjusting amplitude

Connected to the Y-plates

Time-base(time/div)

To control the magnitude of the horizontal deflectionof the bright spot or the trace displayed on thescreen by adjusting the frequency

Connected to the X-plates

X-input A terminal to connect the voltage to the X-plates

Y-input A terminal to connect the voltage to the Y-plates

AC/DCswitch

To select the type of input received

When the switch is at DC posi tion, the a.c and d.cvoltages will be displayed

When the switch is at AC position, only the a.cvoltage will be displayed. Any signals of d.c voltagewill be blocked by a capacitor in the CRO

Earth To disconnect the input voltage at the Y-plates and toearth the input terminal

Display wave formsand measuring voltagefrom a DC sourceusing a CRO

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Measuring Potential Difference using the CRO

DC voltage = [displacement of the bright spot x [ the selected rangefrom the zero position] of the Y-gain control]

time-base off time-base on

(a) (b)

What is the value of the dcvoltage in figure (a) and (b) ifthe Y-gain contro l is 1 V/div?

Peak ac = [Height of vertical trace from x [ the selected rangevoltage the zero position] of the Y-gain control]

Y-gain = 2 V/divHeight of vertical tracefrom zero position =

Peak ac voltage =

Measure short t ime intervals

The time-base is set to 1 ms/div

It means I div = 0.001 s

The number of d iv is countedbetween two crests of a wave

The short time interval betweenpulses = Multiplying the numberof division by the time-base

Length between 2 signals = divTime base is set = 10 ms/divTime taken, t =

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Solve problems based on the CRO display

Example 1Diagram 1 shows a traceproduced by an ac power supplywhich is connected to Y-input ofan CRO setting at 20 V/div and 5ms/div.Calculate:(a) Period(b) Frequency(c) Peak voltage Diagram 1

Example 2Diagram 2 shows a traceproduced by an a.c powersupply connected to a CRO withthe time base is switched of.The Y-gain is set to 20 V/div.

Find the peak vol tage.

diagram 2

Example 3Diagram 3 shows a wave produced by anaudio generator displayed the screen of aCRO. The length between the two crests is 3cm.(a) If the time-base is set to 5 ms/div, find

the frequency.(b) What is the period of the wave?

(c) When the frequency of the wave isdouble, what is the length between thetwo crests?

Diagram 3

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4.2 SEMICONDUCTOR DIODES

Describe semiconductors in terms of resistance

MetalsGood conductors of electricitybecause they have free electronsthat can move easily betweenatomsThe resistance of metals isgenerally very low.

InsulatorsPoor conductors of electricitybecause they have too few freeelectrons to move about.The resistance of insulators isvery high.

Semiconductors

A material that has an elect rical conduct ivi ty that is between that of aconductor and an insulator.

The resistance of semiconductors is between that of conductors and

insulators. Semiconductors can be pure element such as sil icon or germanium.

At 0 Kelvin i t behaves as an insulator. When the temperatureincreases, the conductiv ity of the electricity will increase because itsresistance will be lowered.

Name two type of

charge carriers

Electricity conductivity in semiconductors occurs

because there is two type of charge carriers: Electron which is negatively charge

Hole which is positively charge

Characteristics of asilicon atom.

There are four electrons in the outermost shellof a silicon atom and they are shared betweenfour other neighbouring atoms to form fourcovalent bonds.

Each of the covalent bonds has a pair ofelectrons. Every atoms shares one electronwith each of its neighbours.

Figure on the left shows the outer electrons ina silicon crystal which all are involved inperfect covalent bonds, leaving no freeelectrons to conduct electricity.

At very low temperature, pure silicon crystal isan insulator and has a high resistance tocurrent flow.

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As the temperature of pure si licon crystalincreases, the energy of the vibrating atoms inthe silicon crystal causes some electrons tobreak free.

For every electron that is broken free, there isa hole in the bonding structure between theatoms of the crystal. (atom X)

These holes are said to be carriers of posit ivecharge

One outer electron f rom the neighbouringatom (Y) will fi ll the hole and at the same timewill produce a hole at Y.

When the valence/outer electron moves to theleft, the hole move to the right

This is the physical orig in of the increase in theelectrical conductivity of semiconductors withtemperature

What is dopingprocess?

Doping is a process of adding a certain amount ofspecific impuri ties called dopants to a puresemiconductor to increase its electricityconductivity.

Describe n-typesemiconductor

phosphorus atom

n-type doping is to produce an abundance ofelectrons in the semiconductor

A s ilicon atom has four valence / outerelectrons which each electron is covalentlybonded with one of four adjacent silicon atoms

If atoms with five valence electrons(pentavalent atoms) are doped into the puresemiconductor, then each of the pentavalentatoms will have four covalent bonds and oneextra electrons.

It takes only a very small quantity of theimpurity to create enough free electrons toallow electric current to flow through silicon.

The free electrons are the majority carr iers andthe holes are the minority carriers

Since the pentavalent atom donates an extraelectron it is therefore called the donor atom.

Example: phosphorus, arsenic, or antimony


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Describe p-typesemiconductor

p-type doping is to create an abundance ofholes in the material.

If atoms of three valence electrons (trivalentatoms) are doped into the pure semiconductor ,

one electron is missing from one of the fourcovalent bonds. The deficiencies of valenceelectrons are called holes.

When current passes, a hole is filled by anelectron from a neighbouring atom. In this waythe hole moves from one atom to another.

The holes are the majority carriers and the freeelectrons are the minor ity carriers.

Since the trivalent atom accepts an electron, itis therefore called the acceptor atom.

Examples: boron, aluminium, gallium

Comparison between the n-type and p-type semiconductor

Aspect n-typeSemiconductor

p-typeSemiconductor

Puresemiconductor

Dopants material

Function of thedopants material

Valens electronsof the dopantmaterial

Majority chargecarriers

Minority chargecarriers

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Describesemiconductor diodes

The simplest semiconductor device is adiode.

A diode is made by joining a p-type andn-type semiconductors

A diode is a device that allows current toflow in one direction only but blocks it inthe opposite directions.

What is the p-njunct ion?

A p-n junct ion is formed when a n-typeand p-type semiconductors are joinedtogether.

The boundary between the p-type and n-type regions is called the junction.

At the p-n junct ion, elect rons f rom the n-side move to the p-side and recombinewith the holes.

Holes from the p-side similarly move intothe n-side, where they recombine withelectrons.

As a result of this f low, the n-side has anet positive charge, and the p-side has anet negative charge.

What is the depletionlayer?

The region around the junction is left withneither holes nor free electrons.

This neutral region which has no chargecarriers is called the depletion layer.

This layer which has no charge carrier is apoor conductor of electricity.

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Describe the function of diodes

What is forward bias?

The p-type of the diode is connected tothe posit ive terminal and the n-type isconnected to the negative terminal of a

battery. The diode conducts current because the

holes from the p-type material andelectrons f rom the n-type material areable to cross over the junction.

A l ight bulb wi ll l ight up.

What is reversed bias?

The n-type is connected to the posi tiveterminal and the p-type is connected tothe negative terminal of the battery.

The reversed polarity causes a verysmall current to flow as both electronsand holes are pulled away from the

junction.

When the potential dif ference due to thewiden depletion region equals thevoltage of the battery, the current willcease. Therefore the bulb does not lightup.

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Describethe use ofdiodes asrectifiers

A recti fier is an elect rical device that convertsalternating current to direct current.

Rectif ication is a process to convert an alternatingcurrent into a direct current by using a diode.

Two type of rectification:(a) Half-wave recti fication(b) Full-wave rectification

Half-waverectification

Norectification

A process where only half of every cycle of analternating current is made to flow in one directiononly.

Half-waverectificationby usingone diode

Inputac current

output varying dc

When a diode is connected in series with the

resistor, any current that passes through the resistormust also pass through the diode.

Since diode can only allow current to flow in onedirection, therefore the current will only flow in thefirst half-cycle when the diode in forward bias.

The current is blocked in the second half-cycle whenthe diode is in reverse bias.

Full-wave

rectification

A process where both halves of every cycle of an

alternating current is made to flow in the samedirection.

In the first half, the current flows from A to P to TU toR to B

In the second half, the current flows from B to S toTU to Q to A.

The direction of the ac current passing through theresistor for each half cycle is the same ie T to U.

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Full-waverectificationby usingfour diodes

Inputac current

Describethe use of acapacitor tosmooth outoutput

current andoutputvoltage in arectifiercircuit

When the current passthrough the resistor andcapacitor, the capacitoris charged and storesenergy.

When there is nocurrent pass throughthe resistor andcapacitor, the capacitordischarge and theenergy from it is used toproduce voltage acrossthe resistor . As a resultit produces a smooth dc

output.

4.3 TRANSISTOR

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Describe atransistor interms of itsterminal

A transis tor has three leads connected to theemitter, base and collector.

The emitter emits or sends charge carriersthrough the thin base layer to be collected by the

collector.

There is two-type of transistor: npn transistor andpnp transistor.

In an npn transistor the emitter sends negativeelectrons to the collector.

In an pnp transistor, the p-type emitter sendspositive holes to the collector.

In both cases, the arrow on the emitter shows the


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direction of current flow.

The output current, of a transistor flows betweenthe emitter and the collector.

The current in the collector lead is called collector

current, IC. The base current, IBis used to control the

collector current through the transistor. The basecurrent can be used to switch the collectorcurrent on or off .

1. Base current is too small comparedto the collector current. The unit of

base current is A while the unit forthe collector current is mA.

( Ic>>>> IB)

Current = Collector current Amplication base current

2. Emitter current, IEis equal the sumof base currentand collector current

IE= IB+ IC I

e> I

c> I

B

Transistor asa currentamplifier

3. A small change in the base current, results in a big

change in the collector current, Ic>>>>Ib

Transistor asan automaticswitch

4. If there is no current flow in the base circuit, thenthere is also no current flow in the collector circuit.

IB= 0 thenIC= 0 transistor is switch offIBflows then IC flows transistor is switch on

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Describe how a transistor can beused as an automatic switch.

Potential divider circuit

Choose a sui table resistor R1and a variable resistor R2. The


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If the variable resistor in thetransis tor is replaced by a devicesuch as light dependent resistor

(LDR), a thermistor or amicrophone, the transistor canbe used as an automatic switchcontrol led by light, heat orsound.

voltage at base terminal can beadjusted to switch thetransistor on or off.

If the variable resistor = 0, base

voltage = 0 and the transistorremains off.

If the variable resistor isincreased, the base voltageincreases.

When the base vol tage reachescertain minimum value, thebase current switches thetransistor on.

The large collector current

flows through the transistorcausing the bulb to light up.

A l ight controlled switch

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1. The LDR has a very high resistance in darkness and a lowresistance in light. R is a fixed resistor .

2. The LDR and R form a potential divider in the circuit .

Circuit switches on the light atdaytime and switches off the bulb

at night automatically

Circuit switches on the bulb atnight and switches off the bulb at

day time automaticallyDraw a circuit diagram Draw a circuit diagram


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In daylight, the LDR has a verylow resistance as compared toR.

Therefore the base voltage is

high enough to switch thetransistor on and to light upthe bulb.

In darkness, the LDR has avery high resistance andtherefore the base voltage istoo low to switch the transistoron. The bulb light off

In daylight, the LDR has a verylow resistance as compared toR.

Therefore the base voltage is

too low to switch the transistoron

In darkness, the LDR has avery high resistance and thebase voltage is high to switchthe transistor on to light on thebulb.

9.4 LOGIC GATES

Logicgates asswitchingcircuits inelectronicsystems

Security lamps, alarm systems, and washingmachines can make some simple decisions.

The switching on and off operations are controlledby electronic switches made up of logic gates.

Logic gates work us ing tiny t ransistors as switches.They are manufactured as integrated circui t (IC), witheach chip holding several gates.

A logic gate is a c ircuit that has one or more inputsignals but only one output signal.

For each gate, the input or inputs are on the left ofthe symbol. The output is on the right

Each input and output can be either high (logic 1) orlow (logic 0).

A binary 0 represents 0 V, and a binary 1represents a non zero voltage.

Truthtable

A t ruth table lists all input possibili ties and thecorresponding output for each input.


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List, draw symbols and state the action of the logic gates

Gates Truth table Action

1. AND gate For the input to be

ON, both inputsmust be ON.Output in ON onlywhen both inputs

A and B are ON.

2. OR gate For the output tobe ON at least oneof the inputs mustbe ON.

Output Q is ONwhen input A or Bor both is ON

3. NOT gate The output is ONwhen the input isOFF, and viceversa

4. NAND gate It is equivalent toan AND gate withits output invertedby a NOT gate.Output Q is OFFwhen inputs A andB are both ON

5. NOR gate It is equivalent toan OR gate with i ts

output inverted bya NOT gate.Output Q is ONwhen both input Aand input B areOFF

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