Applied ElectronicsApplied Electronics

Anwar Ali Sahito

Discussion timing after classDiscussion timing after class

• Follow strictly the discussion timingFollow strictly the discussion timing

• No discussion at the time when there is class of any other subjectof any other subject

D TiDay Time

WednesdayTh d

1.00 to 1..301 00 t 1 30Thursday

Saturday1.00 to 1.3012.00 to 1.00 

Teaching PlanTeaching PlanSr. # Topic Lectures

1. Introduction to Subject 01

2. Photo Transistors 012. Photo Transistors  01

3. Thermistors 02

4 Li h D d R i (L D R) 014. Light Dependent Resistors  (L.D.R) 01

5. Photocells 02

6. Vacuum Diode  &  Vacuum Triode 02

7. Cathode Ray Tube (CRT) 02y ( )

Teaching Plan (Cont.)Teaching Plan (Cont.)Sr. # Topic Lectures

8. Bandwidth, Distortion and Noise 02

9. Transistor Amplifier 019. Transistor Amplifier 01

10. Classification of Amplifiers (CE. CB, CC configuration) 02

11. Feedback Amplifiers 02

12. Introduction to Operational Amplifiers 01p p

13. Inverting and Non-Inverting OP- Amplifiers 02

14 Input impedance of Inverting and Non- 0214. Input impedance of Inverting and NonInverting OP- Amplifiers 02

Teaching Plan (Cont.)Teaching Plan (Cont.)Sr. # Topic Lectures

22. Synchronization 02

23 Multi-vibrators ( stable, mono-stable and 0223. Bi-stable) 02

24. Active Filters 02

25. Filter Classification 02

26. Losses and heat dissipation in solid state d i 0226. power devices 02

27. Junction temperature and Thermal resistance 01

28. Electrical model of heat-sink and design of Cooling system 01

Teaching Plan (Cont.)Teaching Plan (Cont.)Sr. # Topic Lectures

29. Electromagnetic Interference ( EMI ) 01

30. Conducted and Radiated Noise 0130. Conducted and Radiated Noise 01

31. Electromagnetic, Electrostatic and common impedance coupling 01

32. Methods of elimination and minimization of Interference and Noise 02

PresentationsPresentations

• Select a group of 5 to 6 students and submitSelect a group of 5 to 6 students and submit before 31st of July 2011.  

• No group can have more than 2 students from top 20 g p p• Group members and topic may be changed • Presentations will start after one month

• Submit in hard copy– Names & ID No. of all the group members with g pone group leader and one Assistant group leader

– Submit your three preferences of topics

Topics for the PresentationsTopics for the Presentations

• Photo transistorPhoto transistor

• Thermistor

i h d d i ( )• Light dependent resistor (LDR)

• Photo voltaic cell

• Vacuum diode  

• Vacuum triodeVacuum triode

• CRT 

• Transistor Amplifier configuration\Transistor Amplifier configuration\• Operational Amplifiers• Operations of operational Amplifiers• Operations of operational Amplifiers• Feedback AmplifiersP A lifi• Power Amplifiers

• Oscillators• Types of Oscillators• UJT Relaxation oscillator

• 555 timer IC555 timer IC • Multi vibrators• Active Filters• Active Filters• Noise and distortionEl t ti I t f• Electromagnetic Interference

• Voltage follower• Op‐Amp voltage adder and Substractor• Op‐Amp integrator and differentiator

PhototransistorPhototransistor

PhototransistorPhototransistor• The phototransistor is similar to a regular BJT except that

the base c rrent is prod ced and controlled b light insteadthe base current is produced and controlled by light instead of a voltage source.

• The phototransistor effectively converts variations in light t l t i l i lenergy to an electrical signal

• The collector-base pn junction is exposed to incident light through a lens opening in the transistor package.

• The phototransistor is a transistor in which base current is produced when light strikes the photosensitive semiconductor base region.

• When there is no incident light, there is only a small thermally generated collector-to-emitter leakage current i.e. I(CEO), this is called the dark current and is typically in the

f Arange of nA.

• When light strikes the collector-base pn junction, a b I i d d h i di lbase current, Iλ, is produced that is directly proportional to the light intensity.

• This action produces a collector current that• This action produces a collector current that increases with Iλ .

• Except for the way base current is generated theExcept for the way base current is generated, the phototransistor behaves as a conventional BJT.

• In many cases there is no electrical connection to ythe base

• The relationship between the collector current and h li h d b i h ithe light-generated base current in a phototransistor

is IC = βDC * Iλ .

Phototransistor symbolPhototransistor symbol

Phototransistor Chip structurePhototransistor Chip structure• Since the actual photo‐generation of base current occurs in the collector‐base region, the larger the physical area of this region, physical area of this region,the more base current is generated. 

• Thus, a typical phototransistor is designed to offer a large area to the incident light.

• A phototransistor can be either a two lead or a threeeither a two‐lead or a three‐lead device. 

• In the three‐lead fi ti th b l d iconfiguration, the base lead is 

brought out so that the device can be used as a 

ti l BJT ithconventional BJT with or without the additional light‐sensitivity feature. 

• In the two‐lead configuration, the base is not electrically available, and the device can ,be used only with light as the input. In many applications, the phototransistor is used in pthe two‐lead version.

Phototransistor Biasing CircuitPhototransistor Biasing Circuit

Phototransistor collector characteristicsPhototransistor collector characteristics

Each individual curve on the graph corresponds to a certain value of lightcertain value of light intensity (in this case, the units are          mW/cm2 ) and thatmW/cm2 ) and that the collector current increases with light intensity.

Phototransistors are not sensitive to all light but only to light within a certain range of wavelengths. They are most sensitive to particular wavelengths asThey are most sensitive to particular wavelengths, as shown by the peak of the spectral response curve in Figure

PhotodarlingtonPhotodarlington 

• The photodarlington consists of a phototransistor connected in a darlington garrangement with a conventional BJT, as shown in Figure. g

• Because of the higher current gain, this device has a much higherhas a much higher collector current and exhibits a greater light sensitivity than does asensitivity than does a regular phototransistor.

Applications of PhototransistorApplications of Phototransistor

• Phototransistors are used in a wide variety of yapplications. 

• A light‐operated relay circuit is shown in Figure. • The phototransistor Q1 drives the BJT Q2. When there is sufficient incident light on Q1, transistor Q2 is driven into saturation, and collector currentQ2 is driven into saturation, and collector current through the relay coil energizes the relay. 

• The diode across the relay coil prevents by its li i i i l l i flimiting action, a large voltage transient from occurring at the collector of Q2 when the transistor turns off.

• Figure shows a circuit in which a relay is de‐which a relay is deenergized by incident light on the phototransistorphototransistor. 

• When there is insufficient light, g ,transistor Q2 is biased on, keeping the relay energizedenergized. 

• When there is sufficient light, phototransistor Q1g p 1turns on; this pulls the base of Q2 low, thus turning Q2 off and de‐turning Q2 off and deenergizing the relay.

• These relay circuits can be used in a variety of applications such as automatic door activatorsapplications such as automatic door activators, process counters and various alarm systems. 

• Another simple application is illustrated in Figure.Another simple application is illustrated in Figure. • The phototransistor is normally on, holding the gate of the SCR low. 

• When the light is interrupted, the phototransistor turns off. 

• The high going transition on the collector triggers• The high‐going transition on the collector triggers the SCR and sets off the alarm mechanism. 

• The momentary contact switch SW1 provides for y 1 presetting the alarm. 

• Smoke detection and intrusion detection are ibl f thi i itpossible uses for this circuit.  

Thermistor

ThermistorThermistor

• Thermistor, a word formed by combining , y gthermal with resistor, 

• refers to a device whose electrical resistance, or ability t d t l t i it i t ll d b t tto conduct electricity, is controlled by temperature.

• A thermistor is a temperature‐sensing element composed of semiconductor material which exhibits acomposed of semiconductor material which exhibits a large change in resistance proportional to a small change in temperature. 

• Thermistors are widely used as inrush current limiters, temperature sensors, self‐resetting over‐current protectors, and self‐regulating heating elements.protectors, and self regulating heating elements.

• Assuming as a first‐order approximation thatAssuming, as a first order approximation, that the relationship between resistance and temperature is linear then:temperature is linear, then:

ΔR = k ΔT  

Wh ΔR h i i• Where  ΔR = change in resistance

• ΔT = change in temperature

• k = first‐order temperature coefficient of resistance

Types of ThermistorsTypes of Thermistors

• Thermistors can be classified into two types, depending yp , p gon the sign of k. 

• If k is positive, the resistance increases with increasing t t d th d i i ll d ititemperature, and the device is called a positive temperature coefficient (PTC) thermistor, or posistor. 

• If k is negative, the resistance decreases withIf k is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor. 

• Resistors that are not thermistors are designed to have a k as close to zero as possible (smallest possible k), so that their resistance remains nearly constant over athat their resistance remains nearly constant over a wide temperature range

• Instead of the temperatureInstead of the temperature coefficient k, sometimes the temperature coefficient of presistance α (alpha) or αT is used. 

• It is defined asIt is defined as

• For example for the common• For example, for the common PT100 sensor, α = 0.00385 or 0 385 %/°C0.385 %/ C. 

Resistance = Voltage ÷ Current

• At constant temperature the line is straight, so the thermistor resistance is constant

• If the temperature increases the resistance decreases

• If the temperature decreases the resistance increases

Thermistor

High temperature

Current

High temperature

Low temperature

Potential Difference

NTC ThermistorNTC Thermistor

• Many NTC thermistors are made from a pressedMany NTC thermistors are made from a pressed disc or cast chip of a semiconductor such as a sintered metal oxide. 

• They work because raising the temperature of a semiconductor increases the number ofsemiconductor increases the number of electrons able to move about and carry charge ‐ it promotes them into the conduction band. p

• The more charge carriers that are available, the more current a material can conduct. 

• I electric current (amperes)• I = electric current (amperes)n = density of charge carriers (count/m³)A = cross‐sectional area of the material (m²)v = velocity of charge carriers (m/s)v = velocity of charge carriers (m/s)e = charge of an electron (  coulomb)

• The current is measured using an ammeter. O l h i t t lib ti i• Over large changes in temperature, calibration is necessary. 

• Over small changes in temperature, if the right semiconductor is used, the resistance of the material is li l ti l t th t tlinearly proportional to the temperature. 

• There are many different semiconducting thermistors with a range from about 0.01 kelvin to 2,000 kelvins (−273.14 °C 

1 700 °C)to 1,700 °C).

PTC ThermistorsPTC Thermistors• Most PTC thermistors are of the "switching" type, which means that 

their resistance rises suddenly at a certain critical temperature. • The devices are made of a doped polycrystalline ceramic• The devices are made of a doped polycrystalline ceramic 

containing barium titanate (BaTiO3) and other compounds. • The dielectric constant of this ferroelectric material varies with 

temperature.temperature. • Below the Curie point temperature, the high dielectric 

constant prevents the formation of potential barriers between the crystal grains, leading to a low resistance. 

• In this region the device has a small negative temperature coefficient. 

• At the Curie point temperature, the dielectric constant drops sufficiently to allow the formation of potential barriers at the grainsufficiently to allow the formation of potential barriers at the grain boundaries, and the resistance increases sharply. 

• At even higher temperatures, the material reverts to NTC behaviour.

• Another type of PTC thermistor is the polymer PTC, which is sold under brand names such as Polyswitch", "Semifuse", and "Multifuse". 

• This consists of a slice of plastic with carbon grains embedded in it. 

• When the plastic is cool the carbon grains are all in contact with eachWhen the plastic is cool, the carbon grains are all in contact with each other, forming a conductive path through the device. 

• When the plastic heats up, it expands, forcing the carbon grains apart and causing the resistance of the device to rise rapidlyapart, and causing the resistance of the device to rise rapidly. 

• Like the BaTiO3 thermistor, this device has a highly nonlinear resistance/temperature response and is used for switching, not for 

ti l t t tproportional temperature measurement.

• Yet another type of thermistor is a silistor, a thermally sensitive silicon resistor. 

• Silistors are similarly constructed and operate on the same principles as other thermistors, but employ silicon as the semiconductivecomponent material.

Shapes of ThermistorShapes of Thermistor

• Thermistor has different shapes such as DiscThermistor has different shapes such as, Disc type Thermistor, Washer type Thermistor, Bead type Thermistor Bulb type ThermistorBead type Thermistor, Bulb type Thermistor.

Advantages of ThermistorsAdvantages of Thermistors 

• Thermistor is chemically stableThermistor is chemically stable.

• It is used in nuclear environment.

h i i f i ll l• Thermistor is for series –parallel arrangement for using power handling capacity.

• It is also used to measure the temperature

Applications of ThermistorApplications of Thermistor

• PTC thermistors can be used as current‐limiting devices for circuit protection, as replacements for fuses. – Current through the device causes a small amount of resistive 

heating. f h l h h h h– If the current is large enough to generate more heat than the device can lose to its surroundings, the device heats up, causing its resistance to increase, and therefore causing even more heating.heating.  

– This creates a self‐reinforcing effect that drives the resistance upwards, reducing the current and voltage available to the device.

• NTC thermistors are used as resistance thermometers in low‐temperature measurements of the order of 10 K.

• NTC thermistors can be used as inrush‐current limiting devices in power supply circuits. p pp y– They present a higher resistance initially which prevents large 

currents from flowing at turn‐on, and then heat up and become much lower resistance to allow higher current flow during 

l tinormal operation. – These thermistors are usually much larger than measuring type 

thermistors, and are purposely designed for this application.• NTC thermistors are regularly used in automotive• NTC thermistors are regularly used in automotive 

applications. – For example, they monitor things like coolant temperature 

and/or oil temperature inside the engine and provide data toand/or oil temperature inside the engine and provide data to the ECU and, indirectly, to the dashboard.

• NTC thermistors can be also used to monitor the temperature of an incubatortemperature of an incubator.

• Thermistors are also commonly used in modern digital thermostats and to monitor the temperature of battery packs while chargingpacks while charging.

LDR (Light Dependent Resistor)

LDR or photoresistorLDR or photoresistor

• LDRs or Light Dependent Resistors are very useful especially in li ht/d k i itlight/dark sensor circuits. 

• Normally the resistance of an LDR is very high, sometimes as high as 10 Mega‐ohms, but when they are illuminated with light resistance drops dramaticallyresistance drops dramatically.

• A photoresistor or light dependent resistor (LDR) is a resistor whose resistance decreases with increasing incident light intensity. 

• It can also be referred to as a photoconductor. • A photoresistor is made of a high resistance semiconductor. • If light falling on the device is of high enough frequency, photons g g g g q y, p

absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. 

• The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistancethereby lowering resistance

LDR properties

Resistance 

Lux• Symbol is as above

• It consists of 2 metal grids or electrodes that intersect each other whose space betweenis filled with a semiconductor material e.g. cadmium sulphide doped with copper

• When light is incident on the semiconductor material, the number of electrons in thei d h f d isemiconductor that are free to conduct increases

• Light intensity is measured in lux which is a unit used to measure the light powerincident per unit area of a surface

• The higher the intensity of light on the LDR the greater the number of electrons that• The higher the intensity of light on the LDR, the greater the number of electrons thatcan move freely hence as intensity of light increases, the resistance of the LDRdecreases i.e. more current, same p.d. R decreases

• Typical LDR data: (a) normal room lighting – 450 lux, LDR resistance 900 ohms

42

yp ( ) g g ,(b) sunlight – 28000 lux, LDR resistance 100 ohms

Resistance = Voltage ÷ Current

• At constant light intensity levels the line is straight, so the LDR resistance is constant

• If the light intensity increases the resistance decreases

• If the light intensity decreases the resistance increases

LDR

Bright light

Current

Bright light

Dim light

Potential Difference

g

Intrinsic & Extrinsic

• A photoelectric device can be either intrinsic or extrinsic. • An intrinsic semiconductor has its own charge carriers and g

is not an efficient semiconductor, e.g. silicon. • In intrinsic devices the only available electrons are in 

the valence band, and hence the photon must have enoughthe valence band, and hence the photon must have enough energy to excite the electron across the entire band gap. 

• Extrinsic devices have impurities, also called dopants added whose ground state energy is closer to the conductionwhose ground state energy is closer to the conduction band; since the electrons do not have as far to jump, lower energy photons (i.e., longer wavelengths and lower frequencies) are sufficient to trigger the device. q ) gg

• If a sample of silicon has some of its atoms replaced by phosphorus atoms (impurities), there will be extra electrons available for conduction. 

• This is an example of an extrinsic semiconductor.

• Photoresistors come in many different types• Photoresistors come in many different types. Inexpensive cadmium sulfide cells can be found in many consumer items such as camera light meters, street lights, clock radios, alarms, and outdoor clocks.

• They are also used in some dynamic compressors together with a small incandescentcompressors together with a small incandescent lamp or light emitting diode to control gain reduction.

• Lead sulfide (PbS) and indium antimonide (InSb) LDRs  are used for the mid infrared spectral region.

• Ge:Cu photoconductors are among the best far infrared detectors available and are used for infraredinfrared detectors available, and are used for infrared astronomy and infrared spectroscopy.

Photo CellPhoto Cell Solar Cell

Solar CellSolar Cell

• A solar cell (photovoltaic cell) is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect. 

• Assemblies of cells used to make solar modules which are used to capture energy from sunlight, are known as solar panels. 

• The energy generated from these solar modules, referred to as solar power, is an example of solar energy.

• Cells are described as photovoltaic cells when the light source is not necessarily sunlight. y g

• These are used for detecting light or other electromagnetic radiation near the visible range, for example infrared detectors, or measurement of light intensity., g y

• Solar cells are composed of various semiconducting t i lmaterials. 

• Semiconductors are materials, which become electrically conductive when supplied with light or heat, but which operate as insulators at low temperatures.

• Over 95% of all the solar cells produced worldwide areOver 95% of all the solar cells produced worldwide are composed of the semiconductor material Silicon (Si). 

• As the second most abundant element in earth`s crust, silicon has the advantage of being available insilicon has the advantage, of being available in sufficient quantities, and additionally processing the material does not burden the environment. T d l ll h i d i• To produce a solar cell, the semiconductor is contaminated or "doped". 

BIAS CircuitBIAS Circuit

Silicon Solar Cell manufacturingSilicon Solar Cell manufacturing

Working of PVWorking of PV• To understand the operation of a PV cell, we need to consider both the 

nature of the material and the nature of sunlightnature of the material and the nature of sunlight. • Solar cells consist of two types of material, p‐type silicon and n‐type 

silicon. • Light of certain wavelengths is able to ionise the atoms in the silicon and g g

the internal field produced by the junction separates some of the positive charges ("holes") from the negative charges (electrons) within the photovoltaic device. 

• The holes are swept into the positive or p‐layer and the electrons areThe holes are swept into the positive or p layer and the electrons are swept into the negative or n‐layer. 

• Although these opposite charges are attracted to each other, most of them can only recombine by passing through an external circuit outside the material because of the internal potential energy barrierthe material because of the internal potential energy barrier. 

• Therefore if a circuit is made, power can be produced from the cells under illumination, since the free electrons have to pass through the load to recombine with the positive holes.

• The amount of power available from a PVThe amount of power available from a PV device is determined by;– the type and area of the material– the type and area of the material

– the intensity of the sunlight

the wavelength of the sunlight– the wavelength of the sunlight

• A typical single crystal silicon PV cell of 100 cm2 will produce about 1.5 watts of power at 0.5 volts DC and 3produce about 1.5 watts of power at 0.5 volts DC and 3 amps under full summer sunlight (1000Wm‐2). 

• The power output of the cell is almost directly proportional to the intensity of the sunlight. 

• For example, if the intensity of the sunlight is halved the power will also be halvedthe power will also be halved.

• An important feature of PV cells is that the voltage of the cell does not depend on its size and remains fairly constantcell does not depend on its size, and remains fairly constant with changing light intensity. 

• However, the current in a device is almost directly proportional to light intensity and sizeproportional to light intensity and size. 

• When people want to compare different sized cells, they record the current density, or amps per square centimetref llof cell area.

• The power output of a solar cell can be increased quite effectively by using a tracking mechanism to keep the PV d i di tl f i th b t ti thdevice directly facing the sun, or by concentrating the sunlight using lenses or mirrors. 

• However, there are limits to this process, due to the l it f th h i d th d t l thcomplexity of the mechanisms, and the need to cool the 

cells. • The current output is relatively stable at higher 

b h l i d d l di dtemperatures, but the voltage is reduced, leading to a drop in power as the cell temperature is increased. 

Equivalent CircuitEquivalent Circuit

• To understand the electronic behavior of a solar cell, it is useful to create a model which is electrically equivalent, and is based on discrete electrical components whose b h i i ll k A id l l ll b behavior is well known. An ideal solar cell may be modelled by a current source in parallel with a diode; in practice no solar cell is ideal, so a shunt resistance and a series resistance component are added to the model.The presulting equivalent circuit of a solar cell is shown above.

Construction

• A solar cell consists if P-type and N-type semi-conductor material ( silicon

Silicon is commonly used for Silicon is commonly used for fabricating solar cells, another fabricating solar cells, another construction consists of Pconstruction consists of P‐‐type type l i d ith l f Nl i d ith l f Nmaterial ( silicon,

germanium and selenium) forming a P-N junction.

• The bottom surface of the cell

selenium covered with a layer of Nselenium covered with a layer of N‐‐type cadmiumtype cadmium‐‐oxide to form Poxide to form P‐‐N N junction.junction.

The surface layer pf PThe surface layer pf P‐‐type material istype material is• The bottom surface of the cell (which is always away from light) covered with a continuous conductive contact to which a wire lead

The surface layer pf PThe surface layer pf P type material is type material is extremely thin (0.5 mm) so that light extremely thin (0.5 mm) so that light can penetrate to the junction.can penetrate to the junction.

Power solar cells are also fabricated Power solar cells are also fabricated i fl t t i t f ffi i ti fl t t i t f ffi i t

Another material used to Another material used to make solar cells are Thmake solar cells are Th‐‐Ar Ar ( h lli id ) G( h lli id ) Gattached.

• The upper surface has max: area exposed to light with a

in flat strips to form efficient in flat strips to form efficient coverage of available surface area.coverage of available surface area.

Indecently the maximum efficiency of Indecently the maximum efficiency of a solar cell in converting sunlight intoa solar cell in converting sunlight into

(Thallium Arsenide), Ga(Thallium Arsenide), Ga‐‐Ar Ar (Gallium Arsenide), In(Gallium Arsenide), In‐‐Ar Ar (Indium Arsenide).(Indium Arsenide).

p gsmall contact either a long the edge or around the perimeter.

a solar cell in converting sunlight into a solar cell in converting sunlight into electrical energy is nearly 15% at electrical energy is nearly 15% at present.present.

AdvantagesAdvantages

• Input of solar energy is free of costInput of solar energy is free of cost

• Power generation does not affect environmentenvironment

• Unlimited life span

• Easy to process

• Can be added in series and parallel to achieve pdesired voltage and current levels

ApplicationsApplications

• Power GenerationPower Generation

• Solar cars

C l l• Calculators

• Home Appliances

Cathode Ray Tube

Cathode Ray TubeCathode Ray Tube• The cathode ray tube (CRT) is a vacuum tube containing an electron 

gun (a source of electrons) and a fluorescent screen, with internal g ( ) ,or external means to accelerate and deflect the electron beam, used to create images in the form of light emitted from the fluorescent screen.fluorescent screen. 

• The image may represent electrical waveforms (oscilloscope), pictures (television, computer monitor), radar targets and others.

Th CRT d l l hi h i l d• The CRT uses an evacuated glass envelope which is large, deep, heavy, and relatively fragile.

• Cathode‐ray tubes use an electron beam; before the basic nature of the beam was understood, it was called a cathode ray because it emanated from the cathode (negative electrode) of a vacuum tube

• The interior of the tube is a very good vacuum, with a pressure of around 0 01 Pa (10 7 atm) or lesspressure of around 0.01 Pa (10−7 atm) or less. 

• At any greater pressure, collisions of electrons with air molecules would scatter the electron beam excessively. 

• The cathode, at the left end in the figure, is raised to a high temperature by the heater, and electrons evaporate from the surface of the cathode. 

• The accelerating anode, with a small hole at its center, is maintained at a high positive potential V1, of the order of 1 to 20 kV, relative to the cathode. 

• This potential difference gives rise to an electric field directed from right to left in the region between the accelerating anode and the cathode. 

• Electrons passing through the hole in the anode form a narrow beam and travel with constant horizontal velocity from the anode to the fluorescent screen. 

• The area where the electrons strike the screen glows brightly

• The control grid regulates the number of electrons that reach the anode and hence the brightness of the spot onreach the anode and hence the brightness of the spot on the screen. 

• The focusing anode ensures that electrons leaving the cathode in slightly different directions are focused down tocathode in slightly different directions are focused down to a narrow beam and all arrive at the same spot on the screen. 

• The assembly of cathode control grid focusing anode and• The assembly of cathode, control grid, focusing anode, and accelerating electrode is called the electron gun.

• The beam of electrons passes between two pairs of deflecting platesdeflecting plates. 

• An electric field between the first pair of plates deflects the electrons horizontally, and an electric field between the second pair deflects them verticallysecond pair deflects them vertically. 

• If no deflecting fields are present, the electrons travel in a straight line from the hole in the accelerating anode to the center of the screen where they produce a bright spotcenter of the screen, where they produce a bright spot

Vacuum Diode

IntroductionIntroduction

• In 1904, Sir J.A. Fleming (1849-1945), an , g ( ),English Physicist, invented first vacuum diode called Fleming’s Valve.

• Fleming’s valve was so sensitive that it found little immediate applications.little immediate applications.

• Many improvements have been made in the y pvacuum diode since the invention of the first crude model

ConstructionConstruction• A vacuum diode consists of two

electrodes, a cathode and an d ( l ) l d i anode (plate), enclosed in a

highly evacuated glass envelope.

• The cathode is in the form of nickel cylinder coated with b i d t ti t id barium and strontium outside an is heated indirectly to provide electron emission.

• The anode is generally hallow cylinder made of nickel or

l bd d d th molybdenum and surrounds the cathode.

OperationOperation

• When the cathode is heated by passing the electric current passing the electric current through the heater, it emits a large number of electrons.

• The behavior of these emitted electrons will depend upon the anode potential with respect to the cathode.

• If the anode is at zero potential with respect to cathode, the emitted electrons simply can’t emitted electrons simply can’t go to plate as the latter is neutral. Therefore the circuit current is zero.

• The emitted electrons start accumulating near the cathode and form a cloud of electrons, this is k hknown as space charge.

• If the plate is made positive with respect to cathode, then electrons from the space charge are

tt t d t th l t attracted to the plate. These electrons flow from cathode to plate to constitute which is known constitute which is known as plate current.

ConclusionConclusion

• The current flows in the diode only when plate is made positive with respect to the cathode, no current can flow when plate is negative with respect to cathode.

• Electron flow within a diode takes place only from cathode to plate and never from plate to cathode.

• This unidirectional conduction enables the diode to act like a switch or valve.

• This property permits the diode to act as a rectifier, changing alternating current into direct current.

Characteristics of Vacuum DiodeCharacteristics of Vacuum Diode

• the most important characteristic of a vacuum diode is the plate characteristic which gives the relation between plate voltage and plate current for a given cathode temperature.

Following points may be noted from the plate characteristics :characteristics :

1. All the curves are coincident at low plate voltage where the negative space charge is voltage, where the negative space charge is most effective in limiting plate current. This low plate region is known as space charge limited region.g

In the space charge limited region the plate current is given by the relation :current is given by the relation :

Ib = KEb3/2

Where ‘K’ is constant whose value depends upon the shape of electrodes and geometry of tube, this relation is known as ‘Child’s Law’

2. As plate voltage is made progressively p g yhigher greater portion of electrons from space charge is attracted to gplate and eventually at some plate voltage, the space charge is 3.3. If the cathode temperature is raised, the If the cathode temperature is raised, the p gcompletely eliminated, under such conditions the entire supply of

p ,p ,rate emission is increased .Consequently, rate emission is increased .Consequently, the saturation point is raised.the saturation point is raised.

pp y femitted electrons is attracted to the plates.

This maximum plate current is called saturation currentsaturation current.

Plate Resistance of DiodePlate Resistance of Diode

• The internal resistance offered by the diode in ff yknown as its plate resistance.

Vacuum tube diode has two types of resistances:Vacuum tube diode has two types of resistances:

1. DC plate resistance.

2. AC plate resistance.2. AC plate resistance.

1. DC plate resistance:1. DC plate resistance:

The resistance offered by the diode to Direct t i k DC Pl t R i tcurrent is known as DC Plate Resistance.

2. AC Plate Resistance:2. AC Plate Resistance:

It is the resistance offered by the diode to lt ti talternating current.

ORThe ratio of a small change in plate voltage f g p gacross a diode to the resulting change in plate current is known as AC Plate Resistance.

Vacuum TriodeVacuum Triode

IntroductionIntroduction

• In 1906 Dr. Lee De Forest (1873-1961) an American Scientist placed a third electrode in the form of wire mesh between the cathodes and the plate of vacuum diode, the resulting device the plate of vacuum diode, the esulting device was called Triode.

SymbolSymbol

ConstructionConstruction• As the name implies, a triode

has three electrodes namely cathode, anode and control grid.

• The cathode is located at the centre of the tube and is surrounded by the control grid which is in turn surrounded by

l t plate,

• The cathode and plate have similar construction as for a diode.

• The control grid consists of a fine wire mesh placed very close to the cathode.

• The spacing between the turns • The spacing between the turns of the mesh are wide enough so that the passage of the electrons from cathode to the plate is not obstructed by the grid g

Cut‐Away view

Action of Control GridAction of Control Grid

• The electron emitted by the cathode pass y pthrough the opening of the control grid to reach the plate.

• As the control grid is much closer to the cathode than the plates. Therefore a small voltage on the than the plates. he efo e a small voltage on the control grid has much more control on the electron flow than a comparatively high voltage on the plateon the plate.

1. When the control grid is at the zero potential with respect to the cathode the triode valve just behave like respect to the cathode, the triode valve just behave like a diode.

Fig: (1)

2. If the control grid is placed at some negative potential f g p g p(say -5v) with respect to the cathode, it has repelling effect on electrons, flowing towards the plates. Consequently, fewer electrons reach the plate, there by reducing the plate current.

Fig: (2)

3. As the negative potential on the grid (called Grid Bias) is increased grid (called Grid Bias) is increased, the plate current decreases continuously. If sufficient negative voltage (say -20v) is placed on the voltage (say 20v) is placed on the grid, all the electrons are repelled towards cathode. Consequently the plate current becomes zero and the ptriode is said to be cut off.

Grid Cut Off / Cut Off Bias:Grid Cut Off / Cut Off Bias:

The smallest negative grid voltage, for a given plate voltage at which

Fig: (3)

for a given plate voltage at which plate current becomes zero is known as grid cut off.

4. If the control grid is made slightly positive (say 1v) with respect to cathode, the helping electrostatic fields of plate espect to cathode, the helping elect ostatic fields of plate and grid will accelerate the electrons towards the plate therefore the plate current is increased and at the same time some of the electrons are attracted to the grid to constitute the grid current. The grid current is undesirable because it causes power loss in the grid circuit. Therefore grid is always kept at negative

i l i h h dpotential with respect to cathode.

Fig: (4)

ConclusionConclusion

• From the previous discussion it is concluded that the slight change in grid potential brings about the large change in plate currentabout the large change in plate current.

Triode Characteristics

• The graphical representation of relationship b t l t t l t lt d id between plate current, plate voltage and grid voltage under normal operating conditions are known as triode characteristics.

Ib = f (Eb, Ec)

There are three variables and therefore we require a three dimensional surface to represent the f prelation among all the three quantities at a time.

A di l l t h t i ti i I /E C • Accordingly plate characteristic i.e. Ib /Eb. Curve at constant Ec, mutual characteristic i.e. Ib/Ec

curve at constant Eb and constant current h i i i E /E Icharacteristic i.e. Eb/Ec curve at constant Ib.