Vce Unit 3 Elec&Photonics

8/11/2019 Vce Unit 3 Elec&Photonics

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VCE Phys icsUnit 3

Electron ics &

Photonics


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1.0 Unit Outline apply the concepts of current, voltage, power to the operation of electronic

circuits comprising diodes, resistance, and photonic transducers including lightdependent resistors (LDR), photodiodes and light emitting diodes (LED);

simplify circuits comprising parallel and series resistance and unloaded voltagedividers;

describe the operation of a transistor in terms of current gain and the effect ofbiasing on the voltage characteristics in terms of saturation, cut-off and linearoperation, including linear gain (Vout/Vin) and clipping of a single stage npntransistor voltage amplifier;

explain qualitatively how capacitors act as de-couplers to separate AC from DCsignals in transistor circuits;

use technical specifications related to voltage, current, resistance, power andillumination for electronic components such as diodes, resistance, and opto-electronic converters including light dependent resistors (LDR), photodiodes andlight emitting diodes (LED), excluding currentvoltage characteristic curves fortransistors, to design circuits to operate for particular purposes;

analyse simple electronic transducer circuits for transducers that respond tochanges in illumination and temperature including LDR, photodiode,

phototransistor and thermistor; describe energy transfers and transformations in electricaloptical, and optical

electrical conversion systems using opto-electronic converters;

describe the transfer of information in analogue form using optical intensitymodulated light;

use safe and responsible practices when working with electrical, electronic andphotonic equipment.


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Chapter 1

Topics covered:

Electric Charge.

Electric Current.

Voltage.

Electromotive Force. Electrical Energy.

Electric Power.


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1.0 Electric Charge The fundamental unit of electrical

charge is that carried by the electron(& the proton).

This is the smallest discrete charge

known to exist independently and is

called the ELEMENTARY CHARGE.

Electric Charge (symbol Q) is

measured in units called COULOMBS

(C).

The electron carries - 1.6 x 10-19C.

The proton carries +1.6 x 10-19C.

If 1 electron carries 1.6 x 10-19C

Then the number of electrons in 1 Coulomb of Charge

= 1 C

1.6 x 10-19

= 6.25 x 1018

electrons


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1.1 Flowing Charges When electric charges (in particular

electrons) are made to move or flow,

an Electric Current (symbol I) is said toexist.

The SIZE of this current depends upon

the NUMBER OF COULOMBS of

charge passing a given point in a given

TIME.

Section of Current Carrying Wire

Mathematically:

I = Q/twhere:I = Current in Amperes (A)

Q = Charge in Coulombs (C)

t = Time in Seconds (s)

If 1 Amp of current is flowing

past this point,

then 6.25 x 1018electrons

pass here every second.


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1.2 Electric Current Electric CURRENTS usually flow along

wires made from some kind ofCONDUCTING MATERIAL, usually, but

not always, a METAL. Currents can also flow through a

Liquid (electrolysis), through aVacuum (old style radio valves), orthrough a Semiconductor (ModernDiodes or Transistors).

A Current can only flow around a

COMPLETE CIRCUIT. A break ANYWHERE in the circuit

means the current stops flowingEVERYWHERE, IMMEDIATLY.

The current does not get weaker as itflows around the circuit, BUTREMAINS CONSTANT.

It is the ENERGY possessed by theelectrons (obtained from the battery orpower supply) which gets used up asthe electrons move around the circuit.

In circuits, currents are measured withAMMETERS, which are connected inseries with the power supply.

Typical Electric Circuit

Connecting

Wires

Resistor (consumes

energy)

BatteryCurrent

A

Measures

Current

Flow


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1.3 Conventional Current vs

Electron Current

Positive Terminal Negative Terminal

Conventional vs Electron Current

Resistor

Electron Current:

Never shown on

Circuit Diagrams

Conventional Current:

Alwaysshown on

Circuit Diagrams

Well before the discovery ofthe electron, electric currents

were known to exist.

It was thought that thesecurrents were made up of astream of positive particles and

their direction of movementconstituted the direction ofcurrent flow around a circuit.

This meant that in a Direct Current(D.C.) circuit, the current would flowout of the POSITIVE terminal of thepower supply and into the NEGATIVEterminal.Currents of this kind are calledConventional Currents, and ALL

CURRENTS SHOWN ON ALLCIRCUIT DIAGRAMS EVERYWHEREare shown as Conventional Current,as opposed to the real orELECTRON CURRENT.


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1.4 Voltage To make a current flow around a

circuit, a DRIVING FORCE is required. This driving force is the DIFFERENCE

in VOLTAGE (Voltage Drop or

Potential Difference) between the

start and the end of the circuit.

The larger the current needed, thelarger the voltage required to drive

that current.

VOLTAGE is DEFINED as the

ENERGY SUPPLIED TO THE CHARGE

CARRIERS FOR THEM TO DO THEIRJOBie.TRAVEL ONCE AROUND THE

CIRCUIT.

So, in passing through a Voltage of1 Volt, 1 Coulomb of Charge picksup 1 Joule of Electrical Energy.

OR

A 12 Volt battery will supply eachCoulomb of Charge passing

through it with 12 J of Energy.

Mathematically;

V = W/qwhere:V = Voltage (Volts)

W = Electrical Energy (Joules)

q = Charge (Coulombs)

Alessandro Volta
http://www.cinemedia.net/SFCV-RMIT-Annex/rnaughton/VOLTA_BIO.html


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1.5 E.M.F.Voltage is measured with a VOLTMETER.

The term EMF (ELECTROMOTIVE FORCE)describes a particular type of voltage.

It is the VOLTAGE of a battery or power

supply when NO CURRENT is being drawn.

This is called the Open Circuit Voltage of

the battery or supply

V

Voltmeter

Circuit Symbol

With S closed, a current begins to

flow and V drops and now

measures voltage available to

drive the current through the

external circuit

Resistor

A

V

S

V measures EMF

Voltmeters are placed in PARALLEL with

the device whose voltage is being

measured.

Voltmeters have a very high internalresistance, so they have little or no effect

the operation of the circuit to which they are

attached.Resistor

A

V


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1.6 Electrical Energy

The conversion of Electrical

Energy when a current passesthrough a circuit element (a

computer) is shown below.

Mathematically

W = VQ1,

where:W = Electrical energy (Joule)

V = Voltage (Volts)

Q = Charge (Coulomb)

Current and Charge are

related through:

Q = It.substituting for Q, in

equation 1 we get:

W = VIt

Voltage= V volts

Charges (Q) enter

with high energy

Charges (Q) leavewith low energy

Q Coulombs ofElectricity enter

computer

Q Coulombs of

Electricity leave

computer

In time t, W units of energy are transformed to heat and light

Electrical Energy (W) is

defined as the product of the

Voltage (V) across, times theCharge (Q), passing through

a circuit element (eg. a light

globe).


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1.7 Electrical Power Electrical Power is DEFINED as the

Time Rate of Energy Transfer:

P = W/twhere P = Power (Watts, W)

W = Electrical Energy (Joule)

t = Time (sec)

From W = VI t we get:

P = VI From Ohms Law (V = IR) [see next

chapter] we get:

P = VI = I2R = V2/R

where: I = Current (Amps)R = Resistance (Ohms)

V = Voltage (Volts)

Electrical Power is sold to

consumers in units of Kilowatt-

Hours. (kW.h)

A 1000 W (1kW) fan heater operating

for 1 Hour consumes 1kWh of

electrical power.

Since P = W/t or W = P x t, we can say:

1 Joule = 1 Watt.sec

so

1000 J = 1kW.sec

so3,600,000 J = 1 kW.hour

or

3.6 MJ = 1 kW.h


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1.8 A.C. Electricity There are two basic types of current

electricity:

(a) D.C. (Direct Current) electricity

where the current flows in one

direction only.

(b) A.C. (Alternating Current) where the

current changes direction in a

regular and periodic fashion. The Electricity Grid supplies domestic

and industrial users with A.C.

electricity.

A.C. is favoured because:

(a) it is cheap and easy to generate

(b) it can be transformed; its voltage

can be raised or lowered at will by

passage through a transformer.

The only large scale use of high

voltage D.C. electricity is in public

transport, ie. trams and trains.

Voltage

Time

VP VPtoP

T

A.C. ELECTRICITY - PROPERTIES

VPtoP= Peak to Peak Voltage

for Domestic Supply VPtoP= 678V

T = Period

for Domestic Supply T = 0.02 sec

VP= Peak Voltage

for Domestic Supply VP= 339 V


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1.10 Peak versus RMS Values In AC supplies, the Peak

and RMS values are related

through simple formulae:

For Voltage:

VRMS = VP/2

For Current:

IRMS = IP/2

In Australia DomesticElectricity is supplied at

240 V, 50 Hz

The Voltage quoted is the

RMS value for the AC

supply. Thus the Peak value for

voltage is

VP = VRMS x 2

= 240 x 1.414

= 339 V

Voltage (V)

Time (s)

VP

+339 V

- 339 V

VP to P

240 V


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Chapter 2

Topics covered:

Resistance.

Ohms Law.

Resistors in Series and Parallel.

Voltage Dividers Impedance Matching


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2.1 Ohms Law OHMS LAWrelates the Voltage

across, the Current through and theResistance of a conductor.

Mathematically:

V = IRwhere: V = Voltage (Volts)

I = Current (Amps)

R = Resistance (Ohms)

Any conductor which follows

Ohms Law is called an OHMIC

CONDUCTOR.

Ohms Law - Graphically

V

I

A graph of V versus I produces a

straight line with Slope = R

(Remember a straight line

graph has formula y = mx + c)

The graph is a straight line,the Resistance of Device 1 is

CONSTANT (over the range

of values studied).The slope indicates Device 2

has a lower (but still constant)

Resistance whencompared to Device 1.

Slope = RDevice 1

Slope = R

Device 2

Georg Ohm


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2.2 Non Ohmic Devices

Electrical devices which followOhms Law (V = IR) are called

Ohmic Devices.

Electrical devices which do not

follow Ohms Law are called

Non Ohmic Devices.

Non Ohmics show non linear

behaviour when a plot of V vs I

is produced, as can be seen in

the graphs for components X

and Y opposite.

Most of the individualcomponents covered in this

electronics course are Non

Ohmic Devices.

Voltage (V)

Current (A)

Component Y

0

5

10

15

2 4 6 8

Current (A)

Voltage (V)

Component X

0

5

10

15

1 2 43


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2.3 Resistors in Series Conductors which exhibit a

resistance to current flow are

generally called RESISTORS.

When connected end to end or in

SERIES, the total resistance of the

combination = the sum of the

individual resistances of the

resistors in the network.

Mathematically:RT= R1+ R2+ R3+

IN A SERIES CIRCUIT:

(a) Since only ONE pathway around the

circuit exists, the current through each

resistor is the same.

Thus: I = I1= I2= I3

Resistors in SERIES

These three resistors can be replaced

by a single resistor of value

RT= R1+ R2+ R3

R1 R

2

R3

V

V1 V2 V3

Resistors in a Series Circuit

(b) The sum of the voltage drops across

the resistors = the voltage of the power

supply,

Thus: V = V1+ V2+ V3

I

I1 I2 I3

The greater the number of resistors in a series network the greater the

value of the equivalent resistance (RT)

R1 R2 R3

RT


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2.4 Resistors in Parallel Resistors connected side by side

are said to be connected inPARALLEL.

The total resistance of a parallelnetwork is found from adding thereciprocals of the individualresistances.

IN A PARALLEL CIRCUIT:

(a) The current through each arm varies.

Thus: I = I1+ I2+ I3

R3

R2

R1

These three Resistors

can be replaced by asingle Resistor ( RT )

Resistors in Parallel

Resistors in a Parallel Circuit

R3

R2

R1

V

I3

I2

I1

I

V1

V3

V2

(b) The voltage drop across each

arm is the same.

Thus: V= V1= V2= V3The greater the number of resistors in a

parallel network the lower the value of the

equivalent resistance (RT).

Mathematically:1/RT= 1/R1+ 1/R2+ 1/R3

RT


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2.5 Voltage Dividers - 1

For the circuit above:

V= V1+ V2

Since this is a series circuit ,the current ( I ) is the same

everywhere:

I= V1/R1 and I = V2/R2

So V1/V2= R1/R2

R1 V1

R2 V2

V

I

Suppose you have a 12 V

battery, but you need only 4 V

to power your circuit. How doyou get around this problem ?

You use a Voltage Divider

Circuit.

They are made by using

combinations of fixed value

resistorsor using variableresistors called rheostats.

Voltage dividers are one of the most

important circuits types used inelectronics.

Almost all sensor subsystems (eg

Thermistors, LDRs), use voltage

divider circuits, there is just no other

way to convert the sensor inputs into

useful electrical information.


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2.6 Voltage Dividers - 2

If the main voltage supply (V) isconnected across the ends of the

rheostat, then the voltage required

by RLis tapped between A and the

position of the slider.

V

A

Rheostat

RL

Slider

The further from A the slider moves the larger thevoltage drop across the load resistor , RL

Using rheostats, the a voltage divider

can be set up as shown.

Slider type rheostat

Variousrotary

rheostats


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2.7 Voltage Divider Formula

For the VOUTcircuit:

VIN= I (R1+ R2)

VINCircuit

VOUTCircuitR1

R2

VIN

VOUT

I

For the VINcircuit:

Applying Ohms Law

The Voltage divider circuit is a SERIES circuit.

Thus, the SAME CURRENT flows EVERYWHERE

In other words, the SAME CURRENT flows through R1AND R2

I = VIN

(R1+ R2).(1)

VOUT= IR2

I = VOUTR2

..(2)

Combining 1 and 2 we get:

VOUT = VIN

R2 (R1+ R2)

so, VOUT = VIN.R2(R1+ R2)

This is the Voltage Divider Formula


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2.8 Impedance Matching 1IMPEDANCE is the TOTAL resistance to current flowdue to ALL the components in a circuit.

In Voltage Divider circuits we only have resistors,so Total Impedance = Total Resistance.

The current (I) in the circuit is:

I = V/RT

= 12/1200

= 0.01 A.

In the circuit shown a supply of 12 Vis connected across 2 resistors of

500 and 700 in series.

I

R2V2

V

R1V17 V

5 V 500

700

12

The Voltage Drop across R1= I x R1= 0.01 x 700

= 7.0 VThe Voltage Drop across R2

= I x R2= 0.01 x 500

= 5.0 V


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CASE (b): Now RL= 5000 ,Then RT= (1/500 + 1/5000)

-1= 454.5 and

I = V/RT= 0.011 A.

This is only a 10 % increase incurrent.

CASE (a):Suppose RLhas a total impedance of50

RLand R2 are in parallel,so Total Resistance RTfor the parallelnetwork = (1/R2+ 1/RL)

-1

= (1/500 + 1/50)-1= 45.5

I = V/RT= 5.0/45.5= 0.11 A.

This is an 110% increase in the

current in the circuit.This will cause a dangerous heatingeffect in R1 and also decrease theVoltage across RL - both undesirableevents !

Suppose a load (RL), requires

5.0 V to operate.

Conveniently, 5 V appearsacross R2.

2.9 Impedance Matching 2

I

R2V2

V

R1V1

500

700

12

7 V

5 V RL505 V

In other words it is important to matchthe impedance of the load R

L

to that ofresistor R2such that: RL 10R2

5 V 500 5000

Lets look at 2 cases where the impedance

of RLvaries.


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Chapter 3.

Topics covered:

Semiconductors

Diodes

p-n junctions

Forward & Reverse Bias

Capacitors


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3.0 Semiconductors Most electronic devices, eg. diodes,

thermistors, LEDs and transistors aresolid state semi conductor devices.

Solid State because they are made upof solid materials and have no movingparts.

Semiconductor because thesematerials fall roughly in the middle ofthe range between Pure Conductor andPure Insulator.

Semiconductors are usually made fromSilicon or Germanium with impuritiesdeliberately added to their crystalstructures.

The impurities either add extra electronsto the lattice producing n typesemiconductor material.

N - Type Semiconductor

Si Si

Si Si

P Si

Si Si

extra

electron

P - Type Semiconductor

Si Si

Si Si

B Si

Si Si

hole

or create a deficit of electrons (called

holes) in the lattice producing p

type semiconductor material.

Holes are regarded as positive (+)

charge carriers, moving through the

lattice by having electrons jump intothe hole leaving behind another hole.


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3.2 Forward and Reverse Bias

p n

depletion layer

it draws the charge carriers away

from the junction and makes the

depletion layer bigger meaning

current is even less likely to flow

and the junction is now reverse

biased

p n

depletion layer

it draws thecharge carriers toward

the junction and makes

the depletion layer

smaller.

If an external supply is

now connected as

shown

The current carriers now

have enough energy to

cross the junction whichnow becomes conducting

or forward biased

If the externalsupply is now

reversed,

3 3 Th Di d


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3.3 The Diode Diodes are electronic devices made by

sandwiching together n type and ptype semiconductor materials.

This produces a device that has a lowresistance to current flow in onedirection, but a high resistance in theother direction.

Cathode (-)Anode (+)

Conventional

Current Flow

Current (mA)

Voltage (V)0.7 V

The Characteristic Curve

(the I vs V graph) for atypical silicon diode is

shown.

This diode will not fully conductuntil a forward bias voltage of 0.7V exists across it.

Notice that when the diodeis reverse biased it does

still conduct - but the

current is in the pA or A

range.

This current is due to

minority carriers crossingwhat is for them a forward

biased junction.V (A)

Circuit Symbol


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Chapter 4

Topics Covered:

Capacitors

Capacitance

Charge Storage

Capacitors DC Blockers


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C


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4.1 Capacitance

The ability of capacitors to

store charge is called theirCAPACITANCE.

This capacitance of any

capacitor is the ratio of the the

amount of Charge (Q) the

plates can carry to thePotential Difference or Voltage

(V) between the plates.

The unit of Capacitance is the

FARAD.

This is a very large unit socapacitance is often quoted in

microfarads

F (10-6F) or

picofarads pF (10-12 F)

Mathematically:

C = Q/Vwhere:

C = Capacitance in Farads

Q = Charge in Coulombs

V = Potential Difference in Volts

4 2 Ch St


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4.2 Charge Storage

Current

Time Time

Charge on Plates

When the switch, S, is closed

the current (I ) rises to a

maximum rapidly. This forces

charge onto the plates of thecapacitor, as shown.

As the charge builds on the plates the voltagedifference between the plates starts to rise until

it reaches a maximum value equal to the EMF of

the supply.

AVSS

IIII = 0

This process is shown graphically below.

Time

Voltageacross PlatesSupply

EMF

The charge on the plates mirrors the

voltage across the plates as shown

Capacitors store charge. How dothey perform in a circuit ? Let us setup a circuit to study their operation.

R

As the charge on the capacitor builds ,

the current flow becomes less until the

capacitor becomes fully charged and

the current stops completely.


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Chapter 5

Topics Covered:

Input Transducers


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5.0 Input TransducersTransducers are devices which convert non

electrical signals into electrical signals.

Input Transducers convert mechanical andother forms of energy eg. Heat, Light or

Sound into Electrical Energy.

Light Emitting Diode (LED)

Light is emitted when the diode

is forward biased

Light Dependent Resistor (LDR)

The resistance changes as

lightintensity varies

Symbol

Examples of a few such devices

are shown here.

Photodiodes

Current flows when light of a

particular frequency illuminates

the diode

Thermistor

The resistance

changes as the

temperature

changes


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5.1 Light Emitting Diodes

anode (+)

cathode (-)

flat edge

LEDs emit light

when an electric

current passes

through them.

LEDs must be connected the correct way

round.

The diagram may be labelled a or + foranode and k or - for cathode (yes, it really

is k, not c, for cathode!).

The cathode is the short lead and there

may be a slight flat region on the body of

round LEDs.

Circuit Symbol

a k

LEDs must have a

resistor in series

to limit the current

to a safe value

Notice this is a voltage

divider circuit

Most LEDs are limited to a maximum

current of 30 mA, with typical VLvalues

varying from 1.7 V for red to 4.5 V for blue


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5.2 Light Dependent Resistors (1)

The light-sensitive partof the LDR is a wavy

track of cadmium

sulphide.

Light energy triggers

the release of extra

charge carriers in this

material,

so that its resistance

falls as the level of

illumination increases.

A light sensor uses an LDR as

part of a voltage divider.

Suppose the LDR has a resistance

of 500, (0.5 k), in bright light,

and 200 kin the shade (these

values are reasonable).

When the LDR is in

the light, Vout will be:

When the LDR is in

the dark, Vout will be:

In other words, this circuit gives a LOW voltage

when the LDR is in the light,

and a HIGH voltage when the LDR is in the shade.

A sensor subsystemwhich functions like this

could be thought of as a

'dark sensor' and could

be used to control

lighting circuits which

are switched on

automatically in the

evening.


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5.3 Light Dependent Resistors (2)The position of the LDR and the fixed

resistor are now swapped.

Remember the LDR has a resistance

of 500, (0.5 k), in bright light, and

200 kin the shade.

In the light:

In the dark:

This sub system could be

thought of as a light

sensor and could be used

to automatically switch offsecurity lighting at sunrise.

How does this change affect the

circuits operation ?

Vout 10

10 + 0.5= x 9 = 8.57 V

Vout 10

10 + 200= x 9 = 0.43 V


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5.4 ThermistorsA temperature-

sensitive resistor is

called a thermistor.There are several

different types:

The resistance of

most common

types of

thermistor

decreasesas the

temperature rises.

They are called

negative

temperaturecoefficient, or ntc,

thermistors.

Note the -t next

to the circuit

symbol.

Different types of

thermistor are

manufactured and each

has its own

characteristic pattern of

resistance change withtemperature.

Resistance ()

Temp (oC)20 40 60 80

100

1000

10000

100000

Note the log scale for resistance

The diagram shows

characteristic curve

for one particular

thermistor:


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5.5 Thermistor Circuits

R = 10 k

How could you make a

sensor circuit for use

as a fire alarm?

At 80oRThermistor= 250 (0.25 k)

10

10 + 0.25= x 9 = 8.78 VVout

R = 10 k

You want a circuit which

will deliver a HIGH

voltage when hotconditions are detected.

You need a

voltage divider

with the ntc

thermistor in the

position shown:

How could you make

a sensor circuit to

detect temperatures

less than 4C to warn

motorists that theremay be ice on the

road?

You want a circuit

which will give a

HIGH voltage in

cold conditions.

You need a voltage

divider with the

thermistor in the

position shown:

At 4oRThermistor= 40 k

40

10 + 40= x 9 = 7.2 VVout


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5.6 Photodiodes

The photovoltaic detector

may operate without

external bias voltage.

A good example is thesolar cell used on

spacecraft and satellites to

convert the suns light into

useful electrical power.

Photodiodes are detectors

containing a p-nsemiconductor junction.

Photodiodes are

commonly used in

circuits in which there isa load resistance in

series with the detector.

The output is read as a

change in the voltage

drop across the resistor.

The magnitude of the

photocurrent generated by a

photodiode is dependent upon

the wavelength of the incident

light.Silicon photodiodes respond

to radiation from the ultraviolet

through the visible and into the

near infrared part of the E-M

spectrum.

RL VOUT

+V

0 V

They are unique in that they

are the only device that can

take an external stimulus

and convert it directly to

electricity.


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Transistor Uses

Transistors are used to perform three basic functions.

They can operate as either

(a) a switch; or

(b) an amplifier;

There are over 50

million transistors

on a single

microprocessor

chip.(The Intel

Pentium 4 has 55

million transistors)

This is first ever solid state amplifier

(transistor) and was created in 1947at Bell Labs in the US

or (c) an oscillator

The term 'transistor' comes from the phrase

'transfer-resistor' because of the way its input

current controls its output resistance.

C


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Transistor ConstructionThere are two general groups of

transistors:

BJT (Bipolar Junction Transistors)

FET (Field Effect Transistors)This course deals only with BJTs.

There are two basic types of BJTs:

NPN TransistorsPNP Transistors

This course deals only with NPNs

The Construction

of a BJT npn typetransistor is:

Emitter

Collector

Base

Base

Collector

Emitter

Circuit symbol

N

P

N

Note: npn transistors have

the arrow:

Not Pointing iN

The arrow points in the

direction of conventional

current flow

An npn type transistor

T i Bi i


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Transistor Biasing

For anytransistor to conduct, two things must occur:

The base - emitter junction mustbe forward biased.

The base - collector junction mustbe reverse biased.

The miracle of transistor act ion :

A smal l current injected into th e

forward b iased base-emit ter

junct ion

B

C

E

The secret to the operation of

the transistor is the movement of

minority carriers across, what is

for them, the forward biased basecollector junction.

A transistor can be regarded as

two diodes connected such that

they share a common anode

IB IC

IE

Base

Collector

Emitter

Biasing is achieved by connecting

the transistor to a DC supply and it is

used to make sure it is switched

on, ie, ready for work.

Small

Current

Large

Current

causes a large current to f low

acros s the co l lector-emitter, even

thoug h the base-col lector jun ct ion

is reverse biased!!

T i t P t


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Transistor Parameters

VBEVCE

IBB

C

E

IC

IE

For a transistor to operate

in any of its modes it needs

to be powered up i.e.,

connected to a voltage

source.

+VPositive rail

0VNegative or Neutral rail

This powering up

results in a number

of voltage drops

and current flows;

Firstly the transistor

is connected

between the Positiveand Neutral rails.

VBEthe voltage

drop between Base

and Emittermust

be at last +0.6 V forthe transistor to

operate.

IBthe base

currentcontrols

the transistors

operation - usuallyvery small, in the

A range.

VCEthe voltage drop

between Collector and

Emitter. VCEis high

when the transistor isoff and gets lower as Ic

grows falling to about

0.2 V at saturation.

Icthe collector

currentlarger than

(but controlled by)base current - in the

mA or A range.

IEthe emitter currentthe sum of base

and collector currents

IE = IC+ IB

IC= IB

where is the DC current

gain sometimes labelled hFE can vary from a few tens to a few hundreds

T i t O ti


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Transistor Operation

The operation of the transistor is shown below:

IC

IB

VCE

VBE

Notice:1. IBwill not flow

until VBEreaches

0.6 V

2. Once IBflows IC

begins to flow

3. As ICrises VCEfalls

T f Ch t i ti


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Transfer CharacteristicsTransistor parameters can

shown on graphs called the

transistors transfercharacteristics. VCE(V)

VBE(V)0.65 V

Cut off

region

Linear

Amplification

Region

SaturatedRegion

With VBEbetween 0.6 and 0.7 volts,

current starts to flow, and there is a

linear region where VBEisproportional to the current flowing

into the base.

here VCEis

high, just like the voltage

across an open switch.

Base

Collector

EmitterWith VBEbelow about

0.6 volts, there is nocurrent flowing, and

the transistor is turned

off.

VCEis the collectoremitter

voltage and VBEis the base-

emitter voltage.

With VBEabove 0.7 V the

transistor is saturated or

fully turned on and VCEis

almost zero like the voltage

across a closed switch

This is called the

cut off region,

When operated in this region the

transistor can be used as an amplifier.

Th Q P i t


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The Q Point

A number of performance curves arepublished on any particular transistor.

The Collector Characteristic Curves

are among the most useful.

This set of curves plots the Collector-

Emitter Voltage (VCE) and the

Collector Current ( IC) for variousvalues of Base Current ( Ib)

VCE(V)

IC(mA)

0 5 10 15 20 25

35

30

25

20

15

10

5 IB= 5 A

IB= 15 A

IB= 25 A

A Load Line needs to be produced.

This connects the maximum Applied

Voltage (VCE) (red dot) with the

Maximum allowed Collector

Current (IC) yellow dot.The load line allows the selection of

the ideal conditions (voltage and

current values) for the transistor to

operate as an amplifier by setting

the Quiescent Point (Q point)

the ideal Q point will be at VCE= 10

V, the green dot, giving an ICof 15 mA

IB= 15 A

Why this Q point ?

Because this will allow the

transistor to produce an amplified

AC output signal that can swing

by the maximum amount aroundthis D.C. Q point.

Load Line

Setting IBat 15 A,

Q Point

T i t A lifi


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Transistor AmplifiersThe course requires the

study of only type of

transistor amplifier:the single stage common

emitter amplifier.

+V

0 V

R1

R2VIN

VOUT

Single stage because

it has only 1 transistor

Common emitterbecause the emitter is

common to both input

and output.

The voltage divider consisting of R1and R2provides the forward bias so the base will be

positive with respect to the emitter.

Resistors are sized to set the quiescent or

steady state operating point at the middle of the

load line (shown by the green dot on load line).

RL

RE C2

RLis chosen to limit the collector current to

the maximum allowed value (the yellow dot).REis chosen to set VCEat the voltage which

will allow the biggest swing in the output

signal to occur.

C1 is placed in the circuit to

block any DC component of

the input signal.

C2is placed in the output

to provide a resistance

free path for an AC output

signal.

C1

So this amplifier is now correctly biased and can operate to produce an enlarged(amplified), inverted output.

Cli i


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Clipping+V

0 V

R1

R2VIN

VOUT

C1

C2RE

RLSetting the Q point of the

amplifier at an incorrect level

can lead to the output signal

being distorted, cut off or

clipped

VCE(V)

VBE(V)

Q

VIN

VOUT

Q set too low

bottom of

signal clipped

Q

VIN

VOUT

Q set correctly

no clipping

Q

VIN

VOUT

Q set too high

top of signal

clipped

Single stage NPN TransistorCommon Emitter Amplifier

The gain of the

amplifier can be

calculated from:Gain = VOUT/VIN


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Ph t t i t


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Phototransistors

Like diodes, all transistors are

light-sensitive.Phototransistors are designed

specifically to take advantage of

this fact.

The most-common variant is an

NPN bipolar transistor with an

exposed base region.Here, light striking the base

replaces what would ordinarily be

voltage applied to the base -- so, a

phototransistor amplifies

variations in the light striking it.

Phototransistors may or may nothave a base lead (if they do, the

base lead allows you to bias the

phototransistor's light response.

Note that photodiodes also

can provide a similar

function, although withmuch lower gain (i.e.,

photodiodes allow much

less current to flow than

do phototransistors).

Phototransistors are used

extensively to detect light

pulses and convert them

into digital electrical

signals.

In an optical fibre network

these signals can be used

directly by computers orconverted into analogue

voice signals in a

telephone.

Ph t t i t A li ti


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Phototransistor Applications

RL

+V

0V

VOUT

RL

+V

0V

VOUT

When light is on- VOUT is High

When light is on- VOUT is Low

Phototransistors can be used as light activated switches.

Further applications

1. Optoisolator- the optical

equivalent of an electrical

transformer. There is no

physical connection

between input and output.

2. Optical Switchan

object is detected when it

enters the space between

source and detector.


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Chapter 7

Topics Covered:

Opto - Electronic Devices

CD R d


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CD Readers

CD pits

digital

signal

analogue

signalphotodiode

DAC

digital to

analogueconverter

amplifier speaker

Compact discs store information in Digital form.

This information is extracted by a laser and

photodiode combination.

The data is passed through a series of electronic

processes to emerge from the speaker as sound

Optoisolator Circ it


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Optoisolator CircuitHow does VOUTrespond to

changes to VIN?

As the input signal changes,

IFchanges and the light level

of the LED changes.This causes the base current

in the phototransistor to

change causing a change inboth ICand hence VOUT

The response of the phototransistor is not

instantaneous, there is a lag between a

change in VIN

showing up as a change in VOUT

IF

tIC

t

Assume VINvaries such that the LED

switches between saturation (full on) and

cut off (full off), producing a square wave

variation in IFICwill respond showing a slight time lag

every time IFchanges state


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Documents

Vce Unit 3 Elec&Photonics