chapter 4 electronic.pptx

8/14/2019 Chapter 4 Electronic.pptx

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CHAPTER4 ELECTRONICPresented By Thomas Cheah


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4.1 CATHODE-RAYOSCILLOSCOPE(CRO)

Thermionic emission

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

How the Thermionic emission occur?

Metal consists of a large number of electrons which arefree to move.

At room temperature, the electrons are free to move butremain inside the metal.

The electrons cannot escape at the surface becausethey are held back by the attractive forces of the atomicnucleus.

If the metal is heated at a high temperature, some of thefree electrons may gain sufficient energy to escape fromthe metal.


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Factors affecting the rate of thermionic emission

Surface area of metal

Temperature of metal

Types of metal

Nature of metal surface


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Electron Gun

An assembly of cathode and anode in a vacuum tube.

It is used to produce beams of fast-moving electrons

known as cathode rays.


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The properties of cathode rays

Movement is in a straight line because it is light and has

high velocity.

Has momentum and energy; fluorescent effect when

connects with fluorescent items.

Negatively charged deflected towards positive plates.

Can be deflected by magnetic fields.

When collides with metal targets, kinetic energy 99%

light and X-rays A deflection tube or the Maltese cross tube is used to

study the properties of cathode rays.


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To Investigate the properties of electron streams in

a Maltese cross cathode ray tube.


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Electron gun:

The cathode emits electrons when heated

The grid controls the number of electrons reaching

anodescontrol with brightness knob

The anode focus electrons into fine beamcontrol 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 lighta 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


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Cathode ray oscilloscopes are used to:

Measure potential difference

Measure short time intervals

Display wave forms


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Measure potential difference


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o Measure potential difference

o Peak to Peak potential difference

o Peak potential difference

o Root mean square potential difference


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Measure short time intervals


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EXERCISE

Figure shows a trace on an oscilloscope for an a.c

source.If the Y-gain is set to 1.5 Vcm-1 and the time-

base is 2 ms cm-1.

(a) Calculate the peak voltage,Vp of the a.c source.

(b) Calculate the frequency, f of the a.c source


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EXERCISE

(c) Sketch the trace displayed on the screen if the

settings are changed to 1 Vcm-1 and 1ms cm-1.


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EXERCISE

An ultrasound signal is transmitted vertically down to the sea bed.

Transmitted and reflected signals are input into an oscilloscope with a

time base setting of 150 ms cm-1. The diagram shows the trace of the two

signals on the screen of the oscilloscope. The speed of sound in water is

1200 ms-1. What is the depth of the sea?


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4.2 SEMICONDUCTORDIODE

Metals

Good conductors of electricity because they have free

electrons that can move easily between atoms

The resistance of metals is generally very low.

Insulators

Poor conductors of electricity because they have too

few free electrons to move about.

The resistance of insulators is very high.


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Semiconductors

A material that has an electrical conductivity that is

between that of a conductor and an insulator.

The resistance of semiconductors is between that of

conductors and insulators. Semiconductors can be pure element such as silicon or

germanium.

At 0 Kelvin it behaves as an insulator. When the

temperature increases, the conductivity of the electricity

will increase because its resistance will be lowered.


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Materials usually used in the electronics industry as

semiconductors are silicone and germanium.

Doping process is the addition of a small quantity of

foreign objects into a semiconductor to increase its

conductivity. The atom size of the foreign object has

to be about the same size as the atom size of the

semiconductor.


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Diode

A semiconductor diode is also known as a p-n junction.

A diode allows current to flow in one direction only.

A diode consists of a combination of an n-type and a p-type

semiconductor.

At the junction of these two semiconductors, the electrons from the n-

type semiconductor will float over to fill up the holes in the p-typesemiconductor.

This will cause opposite charges to exist within the semiconductors; and

this will create a a layer of about 1 mthickness which prevents further

floatation of the electrons.

This layer is known as the depletion layer. The potential difference across the depletion layer is known asjunction

voltage.

The junction voltage of a diode must be overcome before current can

flow.

Junction voltages for silicone and germanium are approximately 0.6 Vand 0.1 V respectively.


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Type of Connections

Forward-biased

Reverse-biased

Forward-biased

The p-type of the diode is connected to the positive terminal and

the n-type is connected to the negative terminal of a battery.

The diode conducts current because the holes from the p-type

material and electrons from the n-type material are able to crossover the junction.

A light bulb will light up.


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Forward-biased

+

+


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Reverse-biased

+

+

no current


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Diodes as Rectifiers

Rectification is the process of converting a.c. to d.c.

This is done with a diode as diodes allow current to flow

only in one direction.

3 type of rectifications Half-wave rectification

Full-wave rectification

Smoothing


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Half-wave rectification

When a diode is connected in series with the resistor, any

current that passes through the resistor must also pass

through the diode.

Since diode can only allow current to flow in one direction,therefore the current will only flow in the first half-cycle

when the diode in forward bias.

The current is blocked in the second half-cycle when the

diode is in reverse bias.


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Half-wave rectification

R


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Full-wave rectification

A process where both halves of every cycle of an alternating

current is made to flow in the same direction.

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

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

The direction of the ac current passing through the resistor for

each half cycle is the same ie T to U.


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4.3 TRANSISTORS

Transistors are electronic devices that act as a

transfer resistor to control the current and potential

difference within an electronic circuit.

Transistors are a combination of two types of

semiconductors, i.e. type p and type n.

Transistors have three electrodes:

Base (B)

Collector (C)

Emitter (E)


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4.3 TRANSISTORS


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4.3 TRANSISTORS

Transistor as a current amplifier


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4.3 TRANSISTORS

Component Function

Microphone Converts sound signals to

electrical signals

Capacitor Prevents d.c. from flowing

into the transistor andloudspeaker

Transistor Amplifies input signal

Loudspeaker Converts electrical signals tosound


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4.3 TRANSISTORS

Light controlled switch

Light-dependent resistor (LDR) changes resistance

depending on presence of light

Very high resistance in the dark

Low resistance in bright light


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4.3 TRANSISTORS

Heat controlled switch

Thermistor is a heat-dependent resistor

Resistance increases when it is cold

Resistance drops when it is hot


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4.4 LOGICGATE

Logic Gate

electronic switches that have one or more input and

only one output

Truth table

a table which lists all possible situations for input andoutput through logic gates

the number of possible combinations N for n input

variables:


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chapter 4 electronic.pptx

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