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VCE Physics
Unit 3 - Topic 2
Electronics &
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 light
dependent resistors (LDR), photodiodes and light emitting diodes (LED);
simplify circuits comprising parallel and series resistance and unloaded voltage
dividers;
describe the operation of a transistor in terms of current gain and the effect of
biasing on the voltage characteristics in terms of saturation, cut-off and linear
operation, including linear gain (∆Vout/∆Vin) and clipping of a single stage npn
transistor voltage amplifier;
explain qualitatively how capacitors act as de-couplers to separate AC from DC
signals in transistor circuits;
use technical specifications related to voltage, current, resistance, power and
illumination for electronic components such as diodes, resistance, and optoelectronic converters including light dependent resistors (LDR), photodiodes and
light emitting diodes (LED), excluding current–voltage characteristic curves for
transistors, to design circuits to operate for particular purposes;
analyse simple electronic transducer circuits for transducers that respond to
changes in illumination and temperature including LDR, photodiode,
phototransistor and thermistor;
describe energy transfers and transformations in electrical–optical, and optical–
electrical conversion systems using opto-electronic converters;
describe the transfer of information in analogue form using optical intensity
modulated light;
use safe and responsible practices when working with electrical, electronic and
photonic equipment.
Chapter 1
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Topics covered:
Electric Charge.
Electric Current.
Voltage.
Electromotive Force.
Electrical Energy.
Electric Power.
1.0 Electric Charge
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The fundamental unit of electrical
charge is that carried by the electron
(& the proton).
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This is the smallest discrete charge
known to exist independently and is
called the ELEMENTARY CHARGE.
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Electric Charge (symbol Q) is
measured in units called COULOMBS
(C).
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The electron carries - 1.6 x 10-19 C.
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The proton carries +1.6 x 10-19 C.
If 1 electron carries 1.6 x 10-19 C
Then the number of electrons in 1 Coulomb of Charge
=
1C
1.6 x 10-19
= 6.25 x 1018 electrons
1.1 Flowing Charges
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When electric charges (in particular
electrons) are made to move or “flow”,
an Electric Current (symbol I) is said to
exist.
The SIZE of this current depends upon
the NUMBER OF COULOMBS of
charge passing a given point in a given
TIME.
Mathematically:
I = Q/t
where:
I = Current in Amperes (A)
Q = Charge in Coulombs (C)
t = Time in Seconds (s)
Section of Current Carrying Wire
If 1 Amp of current is flowing
past this point,
then 6.25 x 1018 electrons
pass here every second.
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1.2 Electric Current
Electric CURRENTS usually flow along
wires made from some kind of
CONDUCTING MATERIAL, usually, but
not always, a METAL.
Currents can also flow through a
Liquid (electrolysis), through a
Vacuum (old style radio “valves”), or
through a Semiconductor (Modern
Diodes or Transistors).
Typical Electric Circuit
A Current can only flow around a
COMPLETE CIRCUIT.
Battery
A break ANYWHERE in the circuit
Current
means the current stops flowing
EVERYWHERE, IMMEDIATLY.
The current does not get weaker as it
flows around the circuit, BUT
REMAINS CONSTANT.
A
It is the ENERGY possessed by the
Measures
electrons (obtained from the battery or
Current
power supply) which gets used up as
Resistor (consumes
Flow
the electrons move around the circuit.
energy)
In circuits, currents are measured with
Connecting
AMMETERS, which are connected in
Wires
series with the power supply.
1.3 Conventional Current vs
Electron Current
Well before the discovery of
the electron, electric currents
were known to exist.
It was thought that these
currents were made up of a
stream of positive particles and
their direction of movement
constituted the direction of
current flow around a circuit.
This meant that in a Direct Current
(D.C.) circuit, the current would flow
out of the POSITIVE terminal of the
power supply and into the NEGATIVE
terminal.
Currents of this kind are called
Conventional Currents, and ALL
CURRENTS SHOWN ON ALL
CIRCUIT DIAGRAMS EVERYWHERE
are shown as Conventional Current,
as opposed to the “real” or
ELECTRON CURRENT.
Conventional vs Electron Current
Negative Terminal
Positive Terminal
Conventional Current:
Always shown on
Circuit Diagrams
Electron Current:
Never shown on
Circuit Diagrams
Resistor
1.4 Voltage
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To make a current flow around a
circuit, a DRIVING FORCE is required.
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This driving force is the DIFFERENCE
in VOLTAGE (Voltage Drop or
Potential Difference) between the
start and the end of the circuit.
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The larger the current needed, the
larger the voltage required to drive
that current.
VOLTAGE is DEFINED as the
ENERGY SUPPLIED TO THE CHARGE
CARRIERS FOR THEM TO DO THEIR •
JOB ie.TRAVEL ONCE AROUND THE
CIRCUIT.
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Mathematically;
Alessandro Volta
V = W/q
where:
V = Voltage (Volts)
W = Electrical Energy (Joules)
q = Charge (Coulombs)
So, in passing through a Voltage of
1 Volt, 1 Coulomb of Charge picks
up 1 Joule of Electrical Energy.
OR
A 12 Volt battery will supply each
Coulomb of Charge passing
through it with 12 J of Energy.
1.5 E.M.F.
Voltage is measured with a VOLTMETER.
Voltmeter
Circuit Symbol
V measures EMF
V
V
S
A
Voltmeters are placed in PARALLEL with
the device whose voltage is being
measured.
Voltmeters have a very high internal
resistance, so they have little or no effect
the operation of the circuit to which they are
attached.
Resistor
With S closed, a current begins to
flow and V drops and now
measures voltage available to
drive the current through the
external circuit
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
1.6 Electrical Energy
Electrical Energy (W) is
defined as the product of the
Voltage (V) across, times the
Charge (Q), passing through
a circuit element (eg. a light
globe).
Current and Charge are
related through:
Q = It.
substituting for Q, in
equation 1 we get:
Mathematically
W = VQ ………1,
where:
W = Electrical energy (Joule)
V = Voltage (Volts)
Q = Charge (Coulomb)
The conversion of Electrical
Energy when a current passes
through a circuit element (a
computer) is shown below.
W = VIt
In time t, W units of energy are transformed to heat and light
Q Coulombs of
Electricity enter
computer
Charges (Q) enter
with high energy
Charges (Q) leave
with low energy
Voltage
= V volts
Q Coulombs of
Electricity leave
computer
1.7 Electrical Power
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Electrical Power is DEFINED as the
Time Rate of Energy Transfer:
P = W/t
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where P = Power (Watts, W)
W = Electrical Energy (Joule)
t = Time (sec)
From W = VI t we get:
P = VI
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From Ohm’s 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 KilowattHours. (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
so
3,600,000 J = 1 kW.hour
or
3.6 MJ = 1 kW.h
1.8 A.C. Electricity
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There are two basic types of current
electricity:
(a) D.C. (Direct Current) electricity
where the current flows in one
A.C. ELECTRICITY - PROPERTIES
direction only.
Voltage
(b) A.C. (Alternating Current) where the
current changes direction in a
regular and periodic fashion.
VP
VPtoP
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The Electricity Grid supplies domestic
and industrial users with A.C.
electricity.
Time
T
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A.C. is favoured because:
(a) it is cheap and easy to generate
VP = “Peak Voltage”
(b) it can be “transformed”; its voltage
for Domestic Supply VP = 339 V
can be raised or lowered at will by
passage through a transformer.
VPtoP = “Peak to Peak Voltage”
for Domestic Supply VPtoP = 678 V
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The only large scale use of high
voltage D.C. electricity is in public
T = “Period”
transport, ie. trams and trains.
for Domestic Supply T = 0.02 sec
1.9 R.M.S. Voltage and Current
With an A.C. supply, the average values
for both voltage and current = 0,
so Vav and Iav cannot be used by the
Power Companies to calculate the
amount of electric power consumed by
its customers.
Yet, AC circuits do consume power,
so a method of calculating it had to
be found.
RMS values are DEFINED as:
The AC Voltage/Current which
delivers the same
voltage/current to an electrical
device as a numerically equal
D.C. supply would deliver.
An AC source operating at
240 V RMS delivers the same
power to a device as a DC
source of 240 V.
To get around this problem R.M.S. or
Root Mean Square values for AC
voltage and current were developed.
GRAPHICAL DEVELOPMENT OF THE RMS VOLTAGE FROM AN A.C. VOLTAGE
V
V2
339
5.8 x 104
t
0
-339
Mean V2
Mean V2
1.15 x 105
240
t
0
0
t
0
t
1.10 Peak versus RMS Values
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Voltage (V)
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+339 V
240 V
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VP
VP to P
- 339 V
Time (s)
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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 Domestic
Electricity 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
Chapter 2
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Topics covered:
Resistance.
Ohm’s Law.
Resistors in Series and Parallel.
Voltage Dividers
Impedance Matching
2.0 Resistance
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Electrical Resistance is a property of
ALL materials, whether they be
classed as conductors, insulators or
something in between. (ie
Semiconductors)
• The size of the resistance depends
upon a number of factors:
(a) The nature of the material. This is
measured by “resistivity” ()
(b) The length, L, of the material.
(c) The cross sectional area, A, of the
material.
Combining these mathematically:
R = L/A
where:
R = Resistance (Ohms) 
 = Resistivity (Ohm.m) .m
L = Length (m)
A = Cross Sectional Area (m2)
COMPARING RESISTANCE
L
A
1
A
2
Wires 1 and 2 are made from the
same material
Wire 1 has ½ the cross sectional
area of Wire 2
 Wire 1 has TWICE the resistance
of Wire 2
2.1 Ohm’s Law
Ohm’s Law - Graphically
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OHM’S LAW relates the Voltage
across, the Current through and the
Resistance of a conductor.
Mathematically:
V = IR
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where: V = Voltage (Volts)
I = Current (Amps)
R = Resistance (Ohms)
Any conductor which follows
Ohm’s Law is called an OHMIC
CONDUCTOR.
Georg Ohm
V
Slope = R
Device 1
Slope = R
Device 2
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 when
compared to Device 1.
2.2 Non Ohmic Devices
Component X
Voltage (V)
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Electrical devices which follow
Ohm’s Law (V = IR) are called
Ohmic Devices.
Electrical devices which do not
follow Ohm’s 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 individual
components covered in this
electronics course are Non
Ohmic Devices.
15
10
5
0
1
2
3
4 Current (A)
Component Y
Voltage (V)
15
10
5
0
2
4
6
8
Current (A)
2.3 Resistors in Series
Resistors in SERIES
R2
R1
R3
These three resistors can be replaced
by a single resistor of value
RT = R1 + R2 + R3
RT
Resistors in a Series Circuit
V1
V2
I1
R1
I2
R2
I
V
V3
I3
R3
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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
(b) The sum of the voltage drops across
the resistors = the voltage of the power
supply,
Thus: V = V1 + V2 + V3
The greater the number of resistors in a series network the greater the
value of the equivalent resistance (RT)
2.4 Resistors in Parallel
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Resistors in Parallel
Resistors connected “side by side”
are said to be connected in
“PARALLEL”.
The total resistance of a parallel
network is found from adding the
reciprocals of the individual
resistances.
Mathematically:
1/RT = 1/R1 + 1/R2 + 1/R3
R1
These three Resistors
can be replaced by a
single Resistor ( RT )
R2
RT
R3
Resistors in a Parallel Circuit
V1
IN A PARALLEL CIRCUIT:
(a) The current through each arm varies.
Thus: I = I1 + I2 + I3
(b) The voltage drop across each
arm is the same.
Thus: V = V1 = V2 = V3
The greater the number of resistors in a
parallel network the lower the value of the
equivalent resistance (RT).
R1
V2
I2
R2
V3
I3
R3
I
I1
V
2.5 Voltage Dividers - 1
Suppose you have a 12 V
battery, but you need only 4 V
to power your circuit. How do
you get around this problem ?
You use a Voltage Divider
Circuit.
They are made by using
combinations of fixed value
resistors or using variable
resistors called rheostats.
I
R1
V1
R2
V2
V
For the circuit above:
V = V1 + V2
Voltage dividers are one of the most
Since this is a series circuit ,
important circuits types used in
the current ( I ) is the same
electronics.
everywhere:
Almost all sensor subsystems (eg
I = V1/R1
and
I = V2/R2
Thermistors, LDR’s), use voltage
divider circuits, there is just no other
So V1/V2 = R1/R2
way to convert the sensor inputs into
useful “electrical” information.
2.6 Voltage Dividers - 2
Using rheostats, the a voltage divider
can be set up as shown.
If the main voltage supply (V) is
connected across the ends of the
rheostat, then the voltage required
by RL is tapped between A and the
position of the slider.
A
RL
V
Rheostat
Slider
The further from A the slider moves the larger the
voltage drop across the load resistor , RL
Various
rotary
rheostats
Slider type rheostat
2.7 Voltage Divider Formula
The Voltage divider circuit is a SERIES circuit.
Thus, the SAME CURRENT flows EVERYWHERE
In other words, the SAME CURRENT flows through R1 AND R2
For the VIN circuit:
Applying Ohm’s Law
VIN = I (R1 + R2)
 I = VIN
…….(1)
(R1 + R2)
For the VOUT circuit:
I
VIN Circuit
R1
VOUT Circuit
VIN
VOUT = IR2
R2
VOUT
 I = VOUT ……..(2)
R2
Combining 1 and 2 we get:
VOUT =
VIN
R2
(R1 + R2)
so, VOUT =
VIN.R2
(R1 + R2)
This is the Voltage Divider Formula
2.8 Impedance Matching 1
IMPEDANCE is the TOTAL resistance to current flow
due to ALL the components in a circuit.
In Voltage Divider circuits we only have resistors,
so Total Impedance = Total Resistance.
I
7VV
R1
1
12
V
5VV
R2
2
In the circuit shown a supply of 12 V
700 Ω
is connected across 2 resistors of
500  and 700  in series.
The current (I) in the circuit is:
I = V/RT
= 12/1200
500 
= 0.01 A.
The Voltage Drop across R1
= I x R1
= 0.01 x 700
The Voltage Drop across R2
= 7.0 V
= I x R2
= 0.01 x 500
= 5.0 V
2.9 Impedance Matching 2
Suppose a load (RL), requires
5.0 V to operate.
Conveniently, 5 V appears
across R2.
Lets look at 2 cases where the impedance
of RL varies.
CASE (a):
Suppose RL has a total impedance of
50 
RL and R2 are in parallel,
so Total Resistance RT for the parallel
7V1V R1 700 Ω
network = (1/R2 + 1/RL)-1
-1
=
(1/500
+
1/50)
12
= 45.5 
V
I = V/RT
= 5.0/45.5
R2 500
500
5VV

R5000
2
L 50 
= 0.11 A.
This is an 110% increase in the
current in the circuit.
This will cause a dangerous heating
CASE (b): Now RL = 5000 ,
effect in R1 and also decrease the
Then RT = (1/500 + 1/5000)-1
Voltage across RL - both undesirable
= 454.5  and
events !
I = V/RT
In other words it is important to “match”
= 0.011 A.
the impedance of the load RL to that of
This is only a 10 % increase in
resistor R2 such that: RL  10R2
current.
I
Chapter 3.
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Topics covered:
Semiconductors
Diodes
p-n junctions
Forward & Reverse Bias
Capacitors
3.0 Semiconductors
N - Type Semiconductor
Si
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Si
P
Si
extra
electron
Si
Si
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Si
Si
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P - Type Semiconductor
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Si
Si
B
Si
Si
Si
hole
Si
Si
Most electronic devices, eg. diodes,
thermistors, LED’s and transistors are
“solid state semi conductor” devices.
“Solid State” because they are made up
of solid materials and have no moving
parts.
“Semiconductor” because these
materials fall roughly in the middle of
the range between Pure Conductor and
Pure Insulator.
Semiconductors are usually made from
Silicon or Germanium with impurities
deliberately added to their crystal
structures.
The impurities either add extra electrons
to the lattice producing n type
semiconductor material.
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 into
the hole leaving behind another hole.
3.1 p-n junctions
Joining together p type and n type
material produces a so called “p-n
junction”
p
n
When brought together, electrons
from the n type migrate to fill holes
in the p type material.
As a result, a “depletion layer”, (an
insulating region containing very few
current carriers), is set up between
the two materials.
p
n
depletion layer
p
n
Note: undoped semiconductor
material, pure silicon or
germanium, is called “intrinsic
semiconductor material”.
The “majority” current carriers are
holes in p type material and electrons in
n type material.
However, each also has some
“minority” carriers (electrons in p,
holes in n) due to impurities in the
semiconductor and their dopeants
3.2 Forward and Reverse Bias
If an external supply is
now connected as
shown it draws the
charge carriers toward
the junction and makes
the depletion layer
smaller.
p
The current carriers now
have enough energy to
cross the junction which
now becomes “conducting”
or “forward biased”
n
depletion layer
If the external
supply is now
reversed,
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”
3.3 The Diode
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Diodes are electronic devices made by
sandwiching together n type and p
type semiconductor materials.
This produces a device that has a low
resistance to current flow in one
direction, but a high resistance in the
other direction.
Circuit Symbol
Anode (+)
Cathode (-)
Conventional
Current Flow
The “Characteristic Curve”
(the I vs V graph) for a
typical silicon diode is
shown.
Current (mA)
This diode will not fully conduct
until a forward bias voltage of 0.7
V exists across it.
Notice that when the diode
is reverse biased it does
still conduct - but the
current is in the pA or μA
range.
This current is due to
minority carriers crossing
what is for them a forward
biased junction.
V (μA)
0.7 V
Voltage (V)
Chapter 4
Topics Covered:
•Capacitors
•Capacitance
•Charge Storage
•Capacitors DC Blockers
4.0 Capacitors
Capacitors are devices with the
ability to temporarily store
electrical charge.
They are made from two plates
of conducting material
separated by a layer of
insulation material, called a
“dielectric”.
Each plate has a wire attached
which allows for the
capacitor’s connection into a
circuit.
There are many different types
of capacitors, some of which
are “polarised”- they must be
connected in a particular way.
Others are “non polarised”,
it does not matter which
way they are connected.
Polarised
Metal Charge
Storage Plates
Non Polarised
Connecting Wires
Dielectric or
Insulator
•The “sandwich” is then rolled
into a cylinder and covered with
a protective coating.
4.1 Capacitance
• The ability of capacitors to
store charge is called their
CAPACITANCE.
• This capacitance of any
capacitor is the ratio of the the
amount of Charge (Q) the
plates can carry to the
Potential Difference or Voltage
(V) between the plates.
• The unit of Capacitance is the
FARAD.
• This is a very large unit so
capacitance is often quoted in
microfarads F (10-6F) or
picofarads pF (10-12 F)
•
Mathematically:
C = Q/V
where:
C = Capacitance in Farads
Q = Charge in Coulombs
V = Potential Difference in Volts
4.2 Charge Storage
Capacitors store charge. How do
they perform in a circuit ? Let us set
up a circuit to study their operation.
As the charge on the capacitor builds ,
the current flow becomes less until the
capacitor becomes fully charged and
the current stops completely.
This process is shown graphically below.
As the charge builds on the plates the voltage
difference between the plates starts to rise until
it reaches a maximum value equal to the EMF of
the supply.
The charge on the plates mirrors the
voltage across the plates as shown
When the switch, S, is closed
the current (I ) rises to a
maximum rapidly. This forces
charge onto the plates of the
capacitor, as shown.
Voltage across Plates
Current
Supply
EMF
Time
Time
V
SS
A
I =I 0
R
Charge on Plates
Time
4.3 Capacitors – DC Blockers
What happens when the
As can be seen from the
previous slide, current will only
flow for a short time in a
capacitor circuit powered by a
DC supply.
Once the capacitor is fully
charged the DC Current then
stops flowing or is “blocked”
As far as the rest of the circuit elements are
concerned; resistor, ammeter and the
wires, it appears that the capacitor is not
even present – it has no effect on the
operation of the circuit.
A
C
R
IR
DC supply is replaced by
an AC supply ?
Since the AC supply
reverses direction
regularly, the capacitor will
not have time to fully
charge.
So it cannot stop the
current flow before the
supply has switched
polarity and the current
begins flowing in the
reverse direction.
t
Under AC conditions the Capacitor appears
not to be there, i.e., it passes AC signals
without affecting or changing them. In other
words the capacitor acts like a short circuit.
Chapter 5
Topics Covered:
Input Transducers
5.0 Input Transducers
Transducers are devices which convert non
Examples of a few such devices
electrical signals into electrical signals.
Input Transducers convert mechanical and are shown here.
other forms of energy eg. Heat, Light or
Sound into Electrical Energy.
Light Emitting Diode (LED)
Light Dependent Resistor (LDR)
Symbol
Light is emitted when the diode
is forward biased
Thermistor
The resistance
changes as the
temperature
changes
The resistance changes as
light intensity varies
Photodiodes
Current flows when light of a
particular frequency illuminates
the diode
5.1 Light Emitting Diodes
flat edge
LEDs must be connected the correct way
round.
The diagram may be labelled a or + for
anode 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.
cathode (-)
anode (+)
a
k
Circuit Symbol
LEDs emit light
when an electric
current passes
through them.
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 VL values
varying from 1.7 V for red to 4.5 V for blue
5.2 Light Dependent Resistors (1)
A light sensor uses an LDR as
part of a voltage divider.
Suppose the LDR has a resistance
of 500Ω , (0.5 kΩ), in bright light,
and 200 kΩ in the shade (these
values are reasonable).
The light-sensitive part
of 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 sensor subsystem
which 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.
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.
5.3 Light Dependent Resistors (2)
The position of the LDR and the fixed
resistor are now swapped.
How does this change affect the
circuit’s operation ?
Remember the LDR has a resistance
of 500Ω , (0.5 kΩ), in bright light, and
200 kΩ in the shade.
In the light:
This sub system could be
thought of as a “light
sensor” and could be used
to automatically switch off
security lighting at sunrise.
Vout
=
10
x 9 = 8.57 V
10 + 0.5
=
10
x 9 = 0.43 V
10 + 200
In the dark:
Vout
5.4 Thermistors
A temperaturesensitive resistor is
called a thermistor.
There are several
different types:
The resistance of
most common
types of
thermistor
Different types of
decreases as the thermistor are
temperature rises. manufactured and each
has its own
They are called
characteristic pattern of
negative
resistance change with
temperature
temperature.
coefficient, or ntc,
The diagram shows
thermistors.
characteristic curve
Note the -t° next
for one particular
to the circuit
thermistor:
symbol.
Resistance (Ω)
100000
10000
1000
100
20
40 60 80
Temp (oC)
Note the log scale for resistance
5.5 Thermistor Circuits
How could you make a
sensor circuit for use
as a fire alarm?
R = 10 k
You want a circuit which
will deliver a HIGH
voltage when hot
conditions are detected.
You need a
voltage divider
At 80o RThermistor = 250 Ω (0.25 kΩ)
with the ntc
thermistor in the
10
=
V
x 9 = 8.78 V
out
position shown:
10 + 0.25
How could you make
You want a circuit
a sensor circuit to
which will give a
detect temperatures
HIGH voltage in
less than 4°C to warn
cold conditions.
motorists that there
At 4o RThermistor = 40 kΩ
may be ice on the
road?
40
You need a voltage Vout =
x 9 = 7.2 V
10 + 40
divider with the
thermistor in the
position shown:
R = 10 k
5.6 Photodiodes
Photodiodes are detectors
containing a p-n
semiconductor junction.
They are unique in that they
are the only device that can
take an external stimulus
and convert it directly to
electricity.
+V
RL
0V
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.
Photodiodes are
commonly used in
circuits in which there is
a load resistance in
series with the detector.
VOUT The output is read as a
change in the voltage
drop across the resistor.
The photovoltaic detector
may operate without
external bias voltage.
A good example is the
solar cell used on
spacecraft and satellites to
convert the sun’s light into
useful electrical power.
Chapter 6
Topics covered:
Transistors
6.0 Transistor Uses
The term 'transistor' comes from the phrase
'transfer-resistor' because of the way its input
current controls its output resistance.
Transistors are used to perform three basic functions.
They can operate as either
There are over 50
(a) a switch; or
million transistors
(b) an amplifier;
or (c) an oscillator 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 1947
at Bell Labs in the US
6.1
Transistor
Construction
There are two general groups of
transistors:
•BJT (Bipolar Junction Transistors)
•FET (Field Effect Transistors)
This course deals only with BJT’s.
There are two basic types of BJT’s:
•NPN Transistors
•PNP Transistors
This course deals only with NPN’s
Collector
The Construction
of a BJT npn type
transistor is:
Collector
Base
N
Base
P
N
An npn type transistor
Emitter
Emitter
Circuit symbol
The arrow points in the
direction of conventional
current flow
Note: npn transistors have
the arrow:
Not Pointing iN
6.2 Transistor Biasing
C
A transistor can be regarded as
two diodes connected such that
they share a common anode
B IB
Small
Current
Collector
IC
IE
Large
Current
E
For any transistor to conduct, two things must occur:
The base - emitter junction must be forward biased.
The base - collector junction must be reverse biased.
Base
Emitter
The “secret” to the operation of
the transistor is the movement of
minority carriers across, what is
for them, the forward biased base
collector junction.
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.
The miracle of transistor action :
A small current injected into the
forward biased base-emitter
junction
causes a large current to flow
across the collector-emitter, even
though the base-collector junction
is reverse biased!!
6.3 Transistor Parameters
For a transistor to operate
in any of its modes it needs
to be “powered up” i.e.,
connected to a voltage
source.
Positive rail
+V
IC
IB
B
C
E
VBE
VCE
IE
0V
Negative or Neutral rail
I E = I C + IB
IC = βIB
where β is the DC current
gain sometimes labelled hFE
This powering up
results in a number
of voltage drops
and current flows;
VBE – the voltage
drop between Base
and Emitter – must
be at last +0.6 V for
the transistor to
operate.
IB – the base
current – controls
the transistor’s
operation - usually
very small, in the
μA range.
Firstly the transistor
is connected
between the Positive
and Neutral “rails”.
VCE – the voltage drop
between Collector and
Emitter. VCE is high
when the transistor is
off and gets lower as Ic
grows falling to about
0.2 V at “saturation.
Ic – the collector
current – larger than
(but controlled by)
base current - in the
mA or A range.
IE – the emitter current – the sum of base
and collector currents
β can vary from a few tens to a few hundreds
6.4 Transistor Operation
The operation of the transistor is shown below:
Notice:
1. IB will not flow
until VBE reaches
0.6 V
2. Once IB flows IC
begins to flow
3. As IC rises VCE
falls
IC
IB
VCE
VBE
6.5 Transfer Characteristics
Transistor parameters can
shown on graphs called the
transistor’s transfer
characteristics.
VCE is the collector – emitter
voltage and VBE is the baseemitter voltage.
With VBE below about
0.6 volts, there is no
current flowing, and
the transistor is turned
off. This is called the
cut off region, here VCE is
high, just like the voltage
across an open switch.
With VBE above 0.7 V the
transistor is “saturated” or
fully turned on and VCE is
almost zero like the voltage
across a closed switch
Collector
VCE (V)
Cut off
region
Linear
Amplification
Region
Base
Emitter
Saturated
Region
0.65 V
VBE (V)
With VBE between 0.6 and 0.7 volts,
current starts to flow, and there is a
linear region where VBE is
proportional to the current flowing
into the base.
When operated in this region the
transistor can be used as an amplifier.
6.6 The Q Point
IC(mA)
35
A number of performance curves are
published on any particular transistor.
The Collector Characteristic Curves
are among the most useful.
This set of curves plots the CollectorEmitter Voltage (VCE ) and the
Collector Current ( IC ) for various
values of Base Current ( Ib )
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)
Load Line
30
25
IB = 25 μA
20
Q Point
15
IB = 15 μA
10
IB = 5 μA
5
0
5
10
15
20
25
VCE (V)
Setting IB at 15 μA,
the ideal Q point will be at VCE = 10
V, the green dot, giving an IC of 15 mA
Why this Q point ?
Because this will allow the
transistor to produce an amplified
AC output signal that can “swing”
by the maximum amount around
this D.C. Q point.
6.7 Transistor Amplifiers
+V
The course requires the
study of only type of
transistor amplifier:
the single stage common
emitter amplifier.
RL
C1 R1
VOUT
VIN
R2
R
E
C2
Single stage because
0V
it has only 1 transistor
Common emitter
The voltage divider consisting of R1and R2
because the emitter is
provides the forward bias so the base will be
common to both input
positive with respect to the emitter.
and output.
Resistors are sized to set the quiescent or
steady state operating point at the middle of the
C1 is placed in the circuit to
load line (shown by the green dot on load line).
block any DC component of
the input signal.
RL is chosen to limit the collector current to
C2 is placed in the output
the maximum allowed value (the yellow dot).
to provide a resistance
RE is chosen to set VCE at the voltage which
free path for an AC output
will allow the biggest “swing” in the output
signal.
signal to occur.
So this amplifier is now correctly biased and can operate to produce an enlarged
(amplified), inverted output.
6.8 Clipping
+V
Setting the Q point of the
amplifier at an incorrect level
can lead to the output signal
being distorted, cut off or
“clipped”
RL
C1 R1
VOUT
VIN
R2
RE
C2
0V
Single stage NPN Transistor
Common Emitter Amplifier
VCE (V)
Q
Q
Q
Q set too high –
VOUT top of signal
Q set correctly –
VOUT clipped
no clipping
Q set too low –
VOUT bottom of
signal clipped
VBE (V)
VINVIN
VIN
The gain of the
amplifier can be
calculated from:
Gain = VOUT/VIN
6.9 The Transistor as a Switch
+V
ITh
Thermistor
R1, R2 are
protection
resistors
R1
LEDload
-to
c
When a transistor is used
as a switch it must be
either OFF (at Cut Off) or
fully ON (at Saturation).
The output device switched by
the transistor is usually called
the 'load’ e.g. an LED
b
R3
R2
e
0V
Initially,
LED is OFF
Temp LOW
Therefore,
RTHERMISTOR is HIGH
ITh LOW
VR3 below 0.6 V
Transistor is OFF
This circuit could be used
to operate a temperature
warning light is a car
When
Temp RISES
ITh RISES
VR3 RISES above 0.7 V
Transistor switches ON
LED switches ON
6.10 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 or
converted into analogue
voice signals in a
telephone.
Note that photodiodes also
can provide a similar
function, although with
much lower gain (i.e.,
photodiodes allow much
less current to flow than
do 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 not
have a base lead (if they do, the
base lead allows you to bias the
phototransistor's light response.
6.11 Phototransistor Applications
Phototransistors can be used as light activated switches.
+V
When light is on
VOUT is High
RL
+V
RL
When light is on
VOUT is Low
VOUT
VOUT
0V
0V
Further applications
1. Optoisolator- the optical
equivalent of an electrical
transformer. There is no
physical connection
between input and output.
2. Optical Switch – an
object is detected when it
enters the space between
source and detector.
Chapter 7
Topics Covered:
Opto - Electronic Devices
7.0 CD Readers
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
CD pits
DAC
photodiode
digital
signal
digital to analogue
analogue
signal
converter
amplifier speaker
7.1 Optoisolator Circuit
How does VOUT respond to
changes to VIN ?
As the input signal changes,
IF changes and the light level
of the LED changes.
This causes the base current
in the phototransistor to
change causing a change in
both IC and hence VOUT
IF
The response of the phototransistor is not
instantaneous, there is a lag between a
change in VIN showing up as a change in VOUT
Assume VIN varies such that the LED
switches between saturation (full on) and
cut off (full off), producing a square wave
variation in IF
IC will respond showing a slight time lag
every time IF changes state
t
IC
t
7.2 Opto-electronic Devices
An op amp (operational amplifier)
is a high gain, linear, DC amplifier
The inputs marked as (+) and (-)
do not refer to power supply
connections but instead refer to
inverting and non inverting
capabilities of the amplifier.
The End
Ollie Leitl 2005
c