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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
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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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