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Rosshall Academy Physics Department
Standard Grade Physics
Unit 4: Electronics
Notes
Standard Grade Physics
Electronics
Notes
1
Section 1: Overview
G1.
State that an electronic system consists of three parts:
Input, process and output.
G2.
Distinguish between digital and analogue outputs.
G3.
Identify analogue and digital signals from waveforms viewed on an oscilloscope.
Electronic Systems
Electronics is a science that uses special electrical components such as transistors or silicon chips to
control the behaviour of electrons is a circuit. These circuits can then be used to form the electronic
systems that are all around us, for example, televisions and telephones.
All electronic systems consist of three parts: input  process  output.
Consider a loudhailer used to address large crowds.
Input -
The person speaks into the mouthpiece of the
loudhailer. The input energy is therefore
_______________ energy. The input device is
a ___________________. This changes the
____________ energy into ____________ energy.
Process
-
The signal from the _______________ is very weak and has to be made larger.
This process is achieved by an __________________.
Output
-
The purpose of the loudhailer is to broadcast a loud sound signal. The output device
therefore has to convert the __________________ signal from the ___________
into a _______________ signal. This is done by a __________________.
The loudhailer system contains two devices known as transducers. These are devices that take in one
form of energy and change it into another. An input transducer changes one form of energy into an
electrical signal and an output transducer changes an electrical signal into some other form of
energy.
For the loudhailer:
Input transducer
- _____________________________
Output transducer
- _____________________________
Standard Grade Physics
Electronics
Notes
2
Block Diagrams
Electronic systems can be shown on block diagrams.
Input
signal
Input transducer
Electrical
signal
Signal Processing
Electrical
signal
Output Transducer
Output
signal
The block diagram for the loudspeaker would be as follows:
Microphone
Input
Amplifier
Loudspeaker
Process
Output
Analogue or Digital?
There are two types of signals used within electronics – analogue and digital.

Analogue signals can have any value between a maximum and minimum.
Looked at on an oscilloscope an analogue signal may look like the following:
or
Digital signals on the other hand have only two possible values – these are known as ‘high’ and
‘low’ or ‘1’ and ‘0’.
On an oscilloscope a digital trace would look like the following.

High
Low
Standard Grade Physics
Electronics
Notes
3
Section 2: Output Devices
G1.
Give examples of output devices and the energy conversions involved.
G2.
Give examples of digital output devices and of analogue output devices.
G3.
Draw and identify the symbol for an LED.
G4.
State that an LED will light only if connected one way round.
G5.
Explain the need for a series resistor with an LED.
G6.
State that different numbers can be produced by lighting appropriate segments of a
7-segment display.
C7.
Identify appropriate output devices for a given application.
C8.
Describe by means of a diagram a circuit that will allow an LED to light.
C9.
Calculate the value of the series resistor for an LED.
C10. Calculate the decimal equivalent of a binary number in the range 0000 – 1001.
Output Devices
An output device takes in __________________ energy and converts it to some other form of
energy.

The Loudspeaker
Signal Generator
Loudspeaker
G
Energy Change
_____________ energy  _____________ energy
Analogue or digital device? ___________________________
Practical Use _______________________________________________________________

The Electric Motor
Variable Power
Supply
Energy Change
M
Motor
_____________ energy  _____________ energy
Analogue or digital device? ___________________________
Practical Use _______________________________________________________________
Standard Grade Physics
Electronics
Notes
4

The Relay
Variable Power
Power Supply
Supply
Energy Change
Motor
Relay
_____________ energy  _____________ energy
Analogue or digital device? ___________________________
Practical Use
A relay is an electrically operated switch. It can sometimes be useful to be able to operate a switch by
remote control; for example in a nuclear power station. This can be done by sending an electrical signal
to the relay which causes the electromagnet to become magnetized which in turn closes the switch
activating the remote circuit.

The Solenoid
Variable Power
Power Supply
Supply
Energy Change
Motor
Solenoid
_____________ energy  _____________ energy
Analogue or digital device? ___________________________
Practical Use
A solenoid consists of a coil of wire wrapped round a metal core. If there is no current flowing through
the coil a spring pushes the metal core away from the coil. When a current flows the coil becomes
magnetized and this attracts the metal core into the coil. Solenoids are used in the central locking
systems found in cars.
Standard Grade Physics
Electronics
Notes
5

The Filament Lamp
Motor
Variable Power
Power Supply
Supply
Energy Change
Lamp
_____________ energy  _____________ energy
Analogue or digital device? ___________________________
Practical Use _______________________________________________________________

The Light Emitting Diode (LED)
Motor
Variable
Power
Power Supply
Supply
Energy Change
LED
_____________ energy  _____________ energy
Analogue or digital device? ___________________________
Practical Use _______________________________________________________________
The LED in the circuit shown is connected the correct way round. What would happen if it was
connected the other way round?
_________________________________________________________________________
_________________________________________________________________________
Standard Grade Physics
Electronics
Notes
6

The 7-Segment Display
Motor
7-Segment
Display
Variable Power
Power Supply
Supply
Energy Change
_____________ energy  _____________ energy
Analogue or digital device? ___________________________
Practical Use
7-segment displays make use of 7 LEDs connected in parallel. They are arranged as shown below and
can be found in many electronic devices such as __________________ and _______________.
By illuminating different combinations of LEDs the numbers 0 – 9 can be
displayed.
LEDs Again
LEDs have many advantages over filament bulbs.
List 3 advantages that LEDs have when compared with filament bulbs.

________________________________________________________________

________________________________________________________________

________________________________________________________________
LEDs will only light if they are connected properly to the power supply.
Draw a circuit to show an LED connected correctly to a battery and a resistor.
Standard Grade Physics
Electronics
Notes
7
LEDs are semiconductor devices that require a very small current. For this reason they must be
protected by a series resistor when they are connected to a power supply. The series resistor limits
the amount of current flowing in the circuit and ensures that it doesn’t exceed the value that the LED
can handle safely.
In general the voltage across an LED should not exceed 2 or 3 V and the maximum current shouldn’t be
greater than 10 mA.
LEDs – Worked Example
The LED shown below operates from a voltage of 3V and a current of 10mA. What size of series
resistor is needed to allow the LED to be connected to a 5V supply?
5V
Standard Grade Physics
Electronics
Notes
8
Binary and Decimal Numbers

Decimal Numbers
All the numbers that we use everyday are made up of 10 digits from 0 to 9 this is known as a decimal
code.
Once we pass 9 we place a 1 in front of the number to represent 1 set of 10 – hence 11 is 10 + 1 and so
on.

Binary Numbers
Computers and other devices that use digital components count in a code known as binary. Binary code
made up of 1s and 0s.
It is possible to convert from decimal into binary and vice versa.
When working in binary it is easiest to think of columns of numbers and each of the columns has a
heading as shown below:
8s
4s
2s
1s
To convert a decimal number into binary always start at the left hand column.
For example: Convert the decimal number 5 into binary.
5 isn’t big enough to include a group of 8 so that column has a zero placed in it. 5 will contain 1 group of
4s, 0 2s and 1 1s.
8s
0
4s
1
2s
0
1s
1
Hence decimal 5 is equivalent to binary 0101.
To go from binary to decimal the process is reversed.
For example: Convert 1001 into a decimal number.
This number has 1 set of 8s, 0 4s, 0 2s and 1 1s. It is therefore 8 + 1 = 9.
8s
1
4s
0
2s
0
1s
1
Complete the table shown on the next page by converting from binary code into decimal code and vice
versa.
Standard Grade Physics
Electronics
Notes
9
Binary and Decimal Numbers – Worked Example
Complete the following table.
Decimal Number
Binary Number
0
0001
7
0101
2
0110
4
1000
0011
9
Standard Grade Physics
Electronics
Notes
10
Section 3: Input Devices
G1.
Describe the energy transformations involved in the following devices:
Microphone; thermocouple; solar cell
G2.
State that the resistance of a thermistor changes with temperature and
the resistance of an LDR decreases with increasing light intensity.
G3.
Carry out calculations using V = IR for the thermistor and LDR.
G4.
State that during charging the voltage across a capacitor increases with time.
G5.
Identify from a list an appropriate input device for a given application.
C6.
Carry out calculations involving voltages and resistances in a voltage divider.
C7.
State that the time taken to charge a capacitor depends on the values of the capacitance and
the series resistance.
C8.
Identify appropriate input devices for a given application.
Input Devices
An input device takes in a form of energy and converts it to ___________________ energy.

Microphone
Microphone
Energy Change
Oscilloscope
_____________ energy  _____________ energy
Practical Use _______________________________________________________________

Thermocouple
-
Thermocouple
Energy Change
Voltmeter
+
V
_____________ energy  _____________ energy
When the junction of a thermocouple is heated it produces ________________ energy. The greater
the temperature of the junction the _______________ the _____________ energy produced.
Practical Use _______________________________________________________________
Standard Grade Physics
Electronics
Notes
11

The Solar Cell
Solar Cell
Energy Change
Voltmeter
V
_____________ energy  _____________ energy
When the solar cell is exposed to light energy it produces ________________ energy. The greater
the light intensity falling on the solar cell the _______________ the _____________ energy
produced.
Practical Use _______________________________________________________________

The Thermistor

Ohmmeter
T
Thermistor
When the thermistor is heated the ohmmeter reading _____________________.
The _________________ of a thermistor changes with temperature. As the temperature increases,
the ___________________ of the thermistor decreases.
Temperature 

Resistance ___
The Light Dependent Resistor (LDR)
Thermistor

Ohmmeter
When the LDR is exposed to light the ohmmeter reading ___________________.
The _________________ of an LDR changes with light intensity. As the light intensity increases, the
___________________ of the LDR decreases
Light  Resistance ___
Standard Grade Physics
Electronics
Notes
12
Thermistors and Light Dependent Resistors – Worked Examples
1.
Look at the circuit diagram and the data table for a thermistor.
Temperature (C)
20
30
40
3 k
Resistance ()
6000
4500
3000
T
12V
(a)
(b)
2.
What is the size of the current in the circuit at each temperature?
Estimate the resistance of the thermistor at a temperature of 45C.
In an experiment to find out how the resistance of an LDR changes with light intensity a
group of pupils obtained the following results.
Light Conditions
Voltage across LDR (V)
Current through LDR (A)
Darkness
Dim light
Bright light
10
10
10
0.001
0.025
0.500
(a)
(b)
Resistance of LDR ()
Complete the last column of the table.
What happens to the resistance of the LDR as the light intensity decreases?
Standard Grade Physics
Electronics
Notes
13

The Capacitor
A capacitor is a device that stores electric charge. It consists of two metal plates separated by a
layer of insulating material.
A capacitor has a capacitance – an ability to store charge – and the unit of capacitance is the farad
(___).
When there is no charge on the plate of the capacitor it is said to be uncharged. If a voltmeter is
connected across an uncharged capacitor it will read 0V.
1.5 V
V
0V
Capacitor is initially uncharged and voltage across the
plates of the capacitor is 0V.
If the switch is now closed the capacitor will begin to
charge up.
Capacitor is charging up. Electrons from the supply
are flowing onto one plate making it ___________
1.5 V
++ ++
charged. This __________ charge repels electrons
V
from the other plate which becomes positively
-- --
charged as a result.
The voltmeter reading increases.
Capacitor is now fully charged. The flow of electrons
has stopped and the voltmeter reading equals the
1.5 V
+++ +++
V
--- ---
S1
1.5 V
1.5 V
_______________ voltage. The capacitor will store
this charge until it is discharged.
S2
+++ +++
--- ---
V
When S2 is closed the electrons can flow off the
capacitor plates, lighting the bulb as they go. The
negative electrons continue to flow until there are no
more positive charges on the other plate.
The capacitor is now said to be discharged.
Standard Grade Physics
Electronics
Notes
14
If the voltage across the capacitor is monitored over a period of time then the graph of voltage
against time would look like the following trace:
Voltage/V
Time/s
It is normal when using a capacitor to include a series resistor.
If either the size of the resistor or the capacitor is
increased then the time taken to charge the
capacitor will _________________.
Add a second trace to the graph above to show the effect of increasing the size of either the
capacitor or resistor on the charging time.
Capacitors are input devices that can be used to introduce a time delay into a circuit.
Practical Use ________________________________________________________________
Standard Grade Physics
Electronics
Notes
15

The Potentiometer
5V
is a sliding contact that can
be moved to any position between A and B.
The potentiometer consists of a coil of
wire and the sliding contact can tap into
different lengths of this wire, therefore
changing the resistance in the circuit.
B
A
V
At position A the resistance is ______  and the voltmeter will therefore read ______ V.
At position B the resistance is now at a maximum and the voltmeter will read _____ V.
A potentiometer makes it possible to ‘tap’ different voltages from a supply.
Standard Grade Physics
Electronics
Notes
16
Voltage Dividers
A voltage divider circuit usually consists of two resistive components in series with a power supply.
I
R1
Vs
In this circuit the supply voltage Vs is applied across
the resistors R1 and R2.
The resistors are connected in series and the current
I is the same through each resistor.
V1
From Ohm’s Law:
R2
V2
For a voltage divider
This can be rearranged
I
V1
R1
=
V2
R2
V1
V1
=
R2
R2
=
V1
R1
and
I
=
V2
R2
i.e., the ratio of the resistances is equal to the ratio of the voltages in the circuit.
Voltage and Resistance Ratios – Worked Example
Find the unknown in each of the following.
(a)
(b)
10
V1 30V
4
V1
20
V2
16
V2 28V
Standard Grade Physics
Electronics
Notes
17
Voltage Dividers 2
Look at the circuit below and calculate the reading on the voltmeter.
It is possible to use Ohm’s Law to solve this problem.
10
12V
1.
The total resistance should be calculated.
RT
20
2.
V
=
R1 + R2
=
10 + 20
=
30 
The current in the circuit should be found.
3.
V
=
IR
12
=
I x 30
I
=
12/30
=
0.4 A
The voltage across the appropriate resistor should be calculated.
V
=
IR
=
0.4 x 20
=
8V
Therefore the voltmeter reading is 8V.
By looking at the results above we can see that 20 is
2
3
of the total resistance ( 20
Looking at the voltages we see that the voltage across the 20 resistor is
(8
12
=
2
3
30
= 2 ).
3
of the total voltage
2 ). In other words, whatever fraction the resistor is of the total resistance it takes the
3
same fraction of the supply voltage.
We can use this fact to build another equation that allows us to calculate the voltmeter reading in the
above example in a much more straightforward manner.
R
x Vs
RT
V
=
V
= R x Vs
RT
Using this on the above example:
= 20 x 12
30
= 8V
The voltmeter reading is 8V.
Standard Grade Physics
Electronics
Notes
18
Voltage Dividers – Worked Example
Calculate the voltmeter reading in each of the following circuits.
(a)
10V
3
5
V
(b)
9V
5
10
V
Temperature Sensor in a Voltage Divider
T
V
In the circuit shown the thermistor forms part of a
voltage divider. The other element in the voltage divider
is a fixed resistor.
If the temperature of the thermistor increases then its
resistance decreases. Its share of the supply voltage
will therefore decrease and hence the voltmeter reading
will go down.
Light Sensor in a Voltage Divider
V
In the circuit shown the light dependent resistor forms
part of a voltage divider. The other element in the
voltage divider is a fixed resistor.
If the light intensity falling on the LDR increases then
its resistance decreases. Its share of the supply voltage
will therefore decrease and hence the voltmeter reading
will go down.
Standard Grade Physics
Electronics
Notes
19
Sensors – Worked Example
(a)
Explain what will happen to the reading on the voltmeter if the temperature of the circuit
shown below increases.
T
V
(b)
Explain what will happen to the reading on the voltmeter if the light intensity increases.
V
Switch as an Input Device
A switch can be used as an input device. It provides a simple way of changing the voltage in a circuit.
5V
5V
V
V
Voltmeter Reading = ___ V
Voltmeter Reading = ___ V
Using a switch like this allows us to create a digital input – 5 V is ‘high’ or ‘1’ and 0 V is ‘low’ or ‘0’.
Standard Grade Physics
Electronics
Notes
20
Section 4: Digital Processes
G1.
State that a transistor can be used as a switch.
G2.
State that a transistor may be conducting or non-conducting, i.e. On or Off.
G3.
Draw and identify the circuit symbol for an NPN transistor.
G4.
Identify from a circuit diagram the purpose of a simple transistor switching circuit.
C15. Explain the operation of a simple transistor switching circuit.
Up until this point we have looked at input and output devices. To make any use of these we need a
process device. The first process device we shall examine in the transistor.
Transistors
Transistors can be used as an electronic switch. The symbol for a transistor is shown below:
Collector
Base
Emitter
Transistors have three terminals, known as the base, the emitter and the collector. The particular
transistor shown above is known as an NPN transistor.
Consider the electronic system shown below:
Potentiometer
6V
Input
Transistor Switch
Process
LED
Output
0V
The potentiometer is used to change the voltage across the base-emitter junction. When the voltage
across this junction is greater than ___________V the transistor switches on it is said to be
conducting. If the voltage falls below _________ V then the transistor switches off – it is said to be
non-conducting. The transistor switch is an example of a digital process – it can either be on or off.
Standard Grade Physics
Electronics
Notes
21

Light Controlled Switches
Light Sensor
6V
Input
Transistor Switch
Process
LED
Output
0V
If the light intensity decreases then the resistance of the LDR will ________________. This means
that the voltage across the LDR will also _________________. If the voltage across the LDR
________________ then the voltage across the variable resistor must _____________________.
The voltage across the variable resistor is the voltage across the base-emitter junction of the
transistor. When this voltage falls below _________ V the transistor switches ______ and the LED
switches ____.
Practical Use _________________________________________________________________
Explain what happens in the circuit shown below if the light intensity decreases.
6V
0V
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
Practical Use __________________________________________________________________
Standard Grade Physics
Electronics
Notes
22

Temperature Controlled Switches
Temperature Sensor
Transistor Switch
LED
T
6V
0V
Explain what happens in this circuit if the temperature decreases.
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
Practical Use __________________________________________________________________
Explain what happens in this circuit if the temperature decreases.
T
6V
0V
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
Practical Use _________________________________________________________________
Standard Grade Physics
Electronics
Notes
23

Time Delayed Switch
6V
0V
Explain what happens in this circuit when the switch is closed.
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
Practical Use _________________________________________________________________
Standard Grade Physics
Electronics
Notes
24
G5.
Draw and identify the symbols for two-input AND and OR gates,
and a NOT gate.
G6.
State that logic gates may have one or more inputs and that a truth table
shows the output for all possible input combinations.
G7.
State that
High voltage = logic ‘1’
Low voltage = logic ‘0’.
G8.
Draw the truth tables for two-input AND and OR gates and a NOT gate.
G9.
Explain how to use combinations of digital logic gates for control in simple situations.
C16. Identify the following gates from truth tables:
Two-input AND
Two-input OR
NOT (inverter).
C17. Complete a truth table for a simple combinational logic circuit.
Logic Gates
Logic gates are tiny integrated circuits built on a single silicon chip. They are digital devices and both
the input to and the output from a logic gate is in the form of a digital signal i.e. a ‘1’ or ‘0’.
We will look at three types of gates: AND gates, OR gates and NOT gates. AND and OR gates have
two inputs and one output whereas a NOT gate has one input and one output. A table, known as a truth
table, shows the possible outputs for all possible combinations of inputs.

NOT gate
Symbol
Truth Table
Input
0
1

Output
AND Gate
Symbol
Truth Table
Inputs
A
B
A
0
0
1
1
Output
B
0
1
0
1
Standard Grade Physics
Electronics
Notes
25

OR gate
Symbol
Truth Table
A
B
Inputs
Output
A
0
0
1
1
B
0
1
0
1
Combinational Logic Circuits
AND, OR and NOT gates can be combined to form more complicated circuits that can be used to
switch something on or off.
When logic gates have been combined, completing the truth table should be taken in steps.
A
C
F
D
B
A
0
0
1
1
E
B
0
1
0
1
C
D
E
F
Standard Grade Physics
Electronics
Notes
26
Consider the following examples. In these examples asssume that a light sensor gives out a logic 1 in
light conditions and a logic 0 in dark conditions;and that a temperature sensor gives out a logic 1 when
the temperature is high and a logic 0 when the temperature is low.
Example 1
Draw a logic diagram and the truth table for a warning LED to light when a motorbike’s engine gets too
hot. The LED should only go on when the ignition of the motorbike is switched on (logic level 1) and the
engine is too hot.
Ignition Switch
LED
Temperature
Switch
Inputs
Temperature Sensor
Cold (0)
Cold (0)
Hot (1)
Hot (1)
Output
Ignition Switch
Off (0)
On (1)
Off (0)
On (1)
Example 2
Draw a logic diagram and truth table which will switch on the pump of a central heating system when
the house is cold and the central heating is switched on (logic level 1)
Standard Grade Physics
Electronics
Notes
27
G10. State that a digital circuit can produce a series of clock pulses.
G11. Give an example of a device containing a counter circuit.
G12. State that there are circuits which can count digital pulses.
G13. State that the output of a counter circuit is binary.
G14. State that the output of a binary counter can be converted to decimal.
C18. Explain how a simple oscillator built from resistor, capacitor and inverter operates.
C19. Describe how to change the frequency of the clock.
Counting Systems
In the past, clocks and watches used a mechanical system
of cogs and wheels to measure the passage of time.
Digital clocks and watches have no moving parts and must
measure the passage of time electronically. To do this they use
a counting circuit.
Clock pulses are voltage pulses that have steady frequency. By counting these pulses it is possible to
measure the passage of time.
Counting circuits are found in many devices that have timers. Give three examples of devices which
would have a digital counting circuit.

________________________________________________________________

________________________________________________________________

________________________________________________________________
Standard Grade Physics
Electronics
Notes
28
The Clock Pulse Generator

Z
V1
5V
R
X
Y
C
W
Before trying to analyse the circuit fully we need to take a closer look at the operation of the LED.
To switch on the LED there must be a voltage across it. Point Z is
attached to the positive terminal of the power supply and is
therefore always held at 5V. For a current to flow through the
LED there must be a voltage difference between Z and Y.
If the output from the NOT gate is ‘1’ this corresponds to 5V.
This means that the voltage difference between Y and Z is 0V and
the LED will be off.
Z
Y
If the output from the NOT gate is ‘0’ this corresponds to 0V. The voltage difference between Y
and Z is now 5V and the LED will come on.
To light the LED the output from the NOT gate must be ‘0’.
Operation of Clock Pulse Generator
Capacitor
Uncharged
VWX/V
0
Input to
Output from
NOT gate
NOT gate
0
1
V1/(V)
LED
0
OFF
Capacitor starts to charge up. Voltage across the capacitor increases until it is
fully charged.
Charged
5
1
0
5
ON
Capacitor starts to discharge through the resistor. Voltage across the capacitor
decreases until the capacitor is fully discharged.
Uncharged
0
0
1
0
OFF
Capacitor can now start to charge once more and the process will be repeated.
Charged
5
1
0
5
ON
If the output of the clock pulse generator was monitored using an oscilloscope then the following
pattern would be obtained.
Standard Grade Physics
Electronics
Notes
29
Changing the Frequency of the Clock Pulses
The frequency of the pulses can be changed by changing the values of either the resistor or the
capacitor in the clock pulse generator.
Increasing the size of either the resistor or the capacitor will _____________ the time taken to
charge the capacitor. This will cause the frequency of the clock pulses to __________________.
Decreasing the size of either the resistor or the capacitor will _____________ the time taken to
charge the capacitor. This will cause the frequency of the clock pulses to __________________.

The Counting Circuit
A counting circuit is an electronic circuit that enables us to count pulses such as those produced by a
clock pulse generator.
The output from the counting circuit is binary and therefore, to be of use to people, it has to be
converted into a digital display.
Clock Pulse
Generator
Outputs
Counting Circuit
0101
7-Segment Display
5
Standard Grade Physics
Electronics
Notes
30
Section 5: Analogue Processes
G1.
Identify from a list, devices in which amplifiers play an important part.
G2.
State the function of an amplifier in devices such as radios, intercoms
and music centres.
G3.
State that the output signal of an audio amplifier has the same frequency as, but a larger
amplitude than, the input signal.
G4.
Carry out calculations involving input voltage, output voltage, and voltage gain of an amplifier.
C5.
Describe how to measure the voltage gain of an amplifier.
C6.
State that power can be calculated from V2/R where V is the voltage and R is the
resistance(impedance) of the circuit.
C7.
State that the power gain of an amplifier is the ratio of power output to power input.
C8.
Carry out calculations involving the power gain of an amplifier.
Amplifiers
Amplifiers are found in a wide range of modern devices. They take in a small electrical input signal and
produce a larger output signal.
A loudhailer is an example of a device that contains an amplifier.
I Love Physics
A person speaks into a microphone and the microphone
changes the ________ signal into a small ___________
signal. This __________ signal is passed onto the
amplifier which boosts the signal. The amplified signal is
then fed onto a loudspeaker which changes the
_____________ signal into a __________ signal.
Give three examples of devices which contain an amplifier.

____________________________________________________________________

____________________________________________________________________

____________________________________________________________________
Standard Grade Physics
Electronics
Notes
31
The circuit below will allow us to both look at and measure the input and output signals from an
amplifier.
Amplifier
Oscilloscope –
measuring the
input signal
Oscilloscope –
measuring the
output signal
Signal
Generator
Output Signal
Input Signal
Note: The amplifier changes the amplitude of the signal (the amplitude of the output signal is
_______________ than that of the input signal). The frequency of the output signal is ____
____________ as that of the input signal.
Voltage Gain of an Amplifier
The purpose of an amplifier is to ____________ the size of an input signal. It is important that we
know how much bigger the output signal will be when compared with the input signal. We can find this
out if we know the voltage gain of the amplifier.
Voltage gain =
Output Voltage
Input Voltage
Note: Voltage gain is one of the few quantities in physics that has no unit.
Standard Grade Physics
Electronics
Notes
32
Power Gain of an Amplifier
The input power of a amplifier can be calculated if the input voltage and the input resistance are
known.
Voltage2
Power =
P =
Resistance
V2
R
The output power can be found in a similar manner.
Once the input and output powers are known it is possible to calculate the power gain of the amplifier.
This gives a measure of how the output power compares with the input power.
Power Gain
=
Output Power
Input Power
Note: Resistance is sometimes referred to as impedance. (If you say that a person’s path has been
impeded this means that there is an obstacle in his way – there is something opposing his motion – in
other words he has encountered resistance.)
Gain of an Amplifier – Worked Examples

Voltage Gain
If the input voltage to a particular amplifier is 0.5 V and the output voltage is 2.5 V
the voltage gain of the amplifier?
what is
Standard Grade Physics
Electronics
Notes
33

Power Gain
A boy connects a set of headphones of resistance 20 to his MP3 player. The amplifier of the
player produces 0.06W of power in the headphones.
(a)
(b)
What is the voltage applied to the headphones?
Calculate the input power to the MP3 amplifier when the power gain is 20
The Finishing Line
Standard Grade Physics
Electronics
Notes
34