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Topic 2.4 – Analogue Communications.
2.4.1 General Amplifier Systems.
Learning Objectives:
At the end of this topic you will be able to;
 understand that amplifiers increase the power (VI) of signals;
 describe the function of the following subsystems of a typical
amplifier system:
o signal source (e.g. microphone)
o preamplifier (i.e. voltage amplifier)
o mixer
o power (current) amplifier
o loudspeaker
 select and use the formula G 
VOUT
;
VIN
 understand what is meant by the bandwidth of an amplifier;
 measure the bandwidth of an amplifier from a graph of voltage
gain against frequency;
 explain and draw graphs to illustrate the meaning of output
clipping for a given input signal;
 understand the trade-off between gain and bandwidth and the
role of multiple stage voltage amplifiers in retaining bandwidth.
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GCSE Electronics.
Unit E2 : Applications of Electronics
2.4.1
General Amplifier Systems
In this topic we will consider the basic operation of a simple amplifier system,
such as might be used in a public address system, hi-fi or disco.
Introduction to waveforms
In this topic we are going to concern ourselves primarily with amplifier
systems designed to amplify the human voice, but what does the human voice
look like?
An oscilloscope can be used to demonstrate what a sound wave looks like just
by connecting it to a microphone and talking.
The oscilloscope will shown a very funny looking waveform as you talk, it won’t
stay still and is unlikely to repeat itself. Some examples are shown below.
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Topic 2.4 – Analogue Communications.
2.4.1 General Amplifier Systems.
The reason that the sound wave looks so strange is that it is a very complex
wave made up of waves of many different frequencies all added together.
Thinking of musical instruments, we know that they sound very different. If
we display the same note being played from four popular instruments we can
see why they sound different.
Each waveform repeats regularly, but the waveforms look completely
different. The range of frequencies in each waveform is completely
different. The study of sound waves is a university course in its own right. All
you need to know is that sound waves are complex waves made up of many
simple waves added together.
Consider a simple waveform called a sine wave. It has the following
appearance.
We will use this simple waveform to study the operation of a basic amplifier
system.
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GCSE Electronics.
Unit E2 : Applications of Electronics
Amplitude of a wave
An AC signal contains both positive and negative voltages. This is the first
time that we have come across this situation, and may lead to confusion
unless we adopt a consistent understanding of how to describe the waveform.
We will define amplitude as:
“the maximum ‘height’ of the positive part of a wave.”
It is sometimes referred to as peak value or maximum value. In electronics, it
is usually measured in volts.
The following diagram illustrates three waves with increasing amplitude.
Our definition of amplitude is consistent with the work carried out earlier in
Topic 2.1 when we considered astable waveforms. The output of an astable
circuit was a square wave which went from zero to a maximum positive
voltage. It had no negative half, so its amplitude was equal to the whole
positive height of the square wave.
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Topic 2.4 – Analogue Communications.
2.4.1 General Amplifier Systems.
Introduction to Amplifiers
In general the function of an amplifier is to produce an output which is an
enlarged copy of the input.
We are all familiar with amplifiers in everyday life. A pop group uses
amplifiers to boost the signal produced by input transducers in their
instruments. The amplified signal then drives the output transducer - a
system of loudspeakers.
Note:
No device can give out more power than is put into it. The additional
power provided by an amplifier comes from a power supply connected to
the amplifier.
Classification of Amplifiers
A wide variety of types of amplifier exist. They are usually described either
by their intended use (e.g. voltage amplifier) or frequency response (e.g. audio
frequency amplifier)
Voltage Amplifier
A voltage amplifier increases the magnitude of the voltage signal presented
at its input.
Input
Voltage
Input
Voltage
Amplifier
Output
Voltage
Output
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GCSE Electronics.
Unit E2 : Applications of Electronics
Current Amplifier
A current amplifier increases the magnitude of the signal current presented
at its input e.g. the current variations produced by a carbon microphone.
Input
Current
Input
Current
Amplifier
Output
Current
Output
Current and voltage amplifiers are generally grouped together and called
small signal amplifiers. Neither type is capable of providing a significant
amount of power output. The aim is that the output signal is an amplified and
undistorted copy of the input signal. The amplification produced is measured
by dividing the amplitude of the output signal by the amplitude of the input
signal.
Note:
Earlier we described resistance as the opposition to current flow.
Impedance is the total opposition to current flow in an AC circuit
caused by resistors, capacitors and inductors. It is an important factor
when matching one part of a system to another.
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Topic 2.4 – Analogue Communications.
2.4.1 General Amplifier Systems.
Power Amplifier
A power (or large signal) amplifier generates both large output currents and
voltages. The input to a power amplifier should ideally be a large voltage
signal (hence large signal amplifier), and is usually taken from the output of a
voltage amplifier (called a preamplifier in this application). The power
amplification produced is calculated by multiplying the voltage amplification
and current amplification together.
Voltage
Input
Input
Power
Amplifier
Output
Current
Input
Voltage
Output
Current
Output
Gain of an Amplifier
The ratio of the amplitudes of the output signal to the input signal of an
amplifier is called the ‘gain’ of the amplifier.
The formula for the voltage gain of an amplifier is:
Output Voltage
Input Voltage
V
G  OUT
VIN
Voltage Gain 
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GCSE Electronics.
Unit E2 : Applications of Electronics
Examples:
1.
A voltage amplifier has an output voltage of 200mV. What is the voltage
gain of the amplifier if the input voltage is 5mV?
G
2.
VOUT 200mV

 40
VIN
5mV
If VIN = 22mV and G = 40, What is the value of VOUT:
G
VOUT
VIN
VOUT
22mV
 40  22mV  880mV
40 
VOUT
3.
A voltage amplifier has an output voltage of 540mV. What is the input
voltage when the voltage gain of the amplifier is 20?
Output Voltage
Input Voltage
540mV
20 
Input Voltage
540mV
Input Voltage 
 27 mV
20
Voltage Gain 
Audio Frequency Amplifiers
Audio frequency (AF) amplifiers amplify AC signals in the range 20Hz to
20kHz.
The frequency response of an amplifier is best described by a graph showing
how the gain of an amplifier varies with different signal frequencies.
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Topic 2.4 – Analogue Communications.
2.4.1 General Amplifier Systems.
The typical voltage gain-frequency curve for an audio amplifier is shown
below.
The bandwidth of the amplifier is defined as the range of frequencies within
which the voltage gain does not fall below
1
(i.e. 0.7) of its maximum value,
2
as shown above.
Radio Frequency Amplifiers
Radio frequency (RF) amplifiers operate above 20kHz and are 'tuned' to
amplify a narrow band of frequencies around a centre frequency which could
be as high as 100 MHz.
Video (Wideband) Frequency Amplifiers
Video or wideband amplifiers can amplify a wide band of frequencies ranging
from DC to 50MHz.
We have described the basic properties of the basic types of amplifier, i.e.
Voltage, Current, Power and Audio. In this introductory course we will only be
considering the voltage amplifier in any detail.
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GCSE Electronics.
Unit E2 : Applications of Electronics
An Amplifier System
A simple public address system (or PA system) is shown in the following block
diagram. Sometimes each block is referred to as a “stage”.
Stage 1
Stage 2
Stage 3
Stage 4
We will take a short look at each stage in this simple system before looking
at a more sophisticated system.
Stage 1:
The microphone converts sound waves into tiny electrical signals that
can be processed by the rest of the system. It is important that the
microphone creates a faithful reproduction of the sound wave as an
electrical signal – no distortion!
Stage 2
The pre-amplifier is covered in Topic 2.4.2. Its purpose is to take the
small electrical signals from the microphone and increase the amplitude
of the signal voltage.
Stage 3
The power amplifier takes this enlarged voltage signal, and boosts the
current so that it is strong enough to drive the loudspeaker.
Stage 4
The loudspeaker is the final part of the system where the electrical
signal is transformed back into a sound wave. If the system has carried
out its function correctly the emerging sound wave will be an
undistorted but amplified version of the original.
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Topic 2.4 – Analogue Communications.
2.4.1 General Amplifier Systems.
A more sophisticated PA system would allow a number of inputs to be
connected
For example a band would have several microphone inputs and guitar pick-up
inputs. These inputs would need to be faded in or out individually. Consider
the following block diagram
You should notice there are two additions to the simple PA system.
The first is a music source and the second is a mixer.
The Mixer is covered in Topic 2.4.3. Its function is to add together electrical
signals from microphones or pick-ups from electric guitars or backing tracks
from a CD player. Most music sources produce a much larger signal than a
microphone and do not need a pre-amplifier. In a real system each microphone
would have its own pre-amplifier.
If we try to amplify the signal too much the system will not be able to
provide the voltage required. This results in distortion of the output signal,
called clipping distortion.
Typically the output voltage maximum is between 1-2V less than that of the
power supply. For example if the power supply was ±15V, then the maximum
output would be limited to around ±13V. If the same amplifier was then
connected to a ±5V supply, without making any changes to the circuit, the
maximum output would then be limited to just ±3V. We call this effect
saturation. In exam questions you will be provided with the saturation values.
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GCSE Electronics.
Unit E2 : Applications of Electronics
The effect of saturation is shown below.
Example:
An amplifier has a gain of +200, and its output saturates at ±12V, when
connected to a ±14V power supply. The following signal is applied to the input.
VIN /mV
100
-100
Required Output
Actual Clipped Output
VOUT /V
15
10
5
-5
-10
-15
The output signal has been clipped at the saturation values of ±12V, when the
required output should be at ±20V. The result is distortion of the waveform.
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Topic 2.4 – Analogue Communications.
2.4.1 General Amplifier Systems.
There are two ways in which this type of distortion can be avoided:
i.
increase the power supply voltage range to at least ±22V for
example in this case. {However this is only possible if the amplifier
can operate at this voltage level.}
ii.
decrease the gain of the amplifier so that the maximum signal
output is limited to ±12V.
Bandwidth
The majority of amplifier designs are built to amplify AC signals. One of the
features of an AC signal is that not only can its amplitude change, but also it’s
frequency. An amplifier must boost the amplitude of the signal but leave the
frequency of the signal unchanged.
Amplifiers contain components that respond differently to different
frequencies, so there is a problem trying to design one amplifier to cover the
entire frequency range.
Amplifiers are designed to allow a specific range of frequencies to be
amplified, e.g.
 a telephone amplifier is designed to accept frequencies from 300Hz to 3KHz;
 a music amplifier is designed to accept frequencies from 20Hz to 20kHz
If we were to pass a video signal, with frequencies as high as 6MHz, through
an audio amplifier, designed for frequencies up to 20kHz, then we would not
obtain the correct output.
The range of frequencies that can be amplified correctly is defined as the
bandwidth of the amplifier.
The bandwidth of an amplifier is the range of frequencies that can be
amplified to more than 1
(70%) of the maximum gain.
2
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GCSE Electronics.
Unit E2 : Applications of Electronics
This is easier to see if we look at the typical response of an amplifier as the
frequency is increased.
Voltage Gain (%)
100
80
60
Bandwidth
40
20
1
10
100
1000
10k
100k
1M
10M
frequency (Hz)
A common mistake made by candidates in examinations is to think of the
bandwidth as being the range of frequencies where 100% gain is achieved.
The 70% or 1
2
comes from a mathematical analysis of the point where half
the original signal power is lost.
Fortunately in an examination you will not be asked to prove this.
Here are some examples to show you what is required.
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Topic 2.4 – Analogue Communications.
2.4.1 General Amplifier Systems.
Examples:
1.
An amplifier has the following frequency response. Use the graph to
estimate the bandwidth of this amplifier. Show on the graph how you
obtain your result.
Solution :
Step 1: Work out 1
2
or 70% of the maximum gain.
70
 400  70  4  280
100
Step 2: Now draw a horizontal line across the graph from a gain of 280, as
shown by the red line below:
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GCSE Electronics.
Unit E2 : Applications of Electronics
Step 3 : Now draw a vertical line down to the frequency axis from the
intercept of the original graph and the red line you have just drawn.
This is shown in blue on the graph:
Step 4:
2.
Read off the bandwidth from the intercept with the frequency
axis, in this case 25 kHz.
A pre-amplifier with a voltage gain of 200, has a bandwidth of 20kHz.
Use the axes provided to sketch the frequency response of the
amplifier.
Step 1: From the question we know the maximum gain will be 200. If the
bandwidth is 20 kHz then we know that at 20 kHz the gain will be
1
or 70% of 200 = 140.
2
This gives us the critical parts of the response graph, as shown below;
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Topic 2.4 – Analogue Communications.
2.4.1 General Amplifier Systems.
Maximum Gain
70% of max gain, at
maximum bandwidth
Step 2: Now it is just a case of completing the graph to show a decrease in
gain from the maximum which passes through the point at 20 kHz as
shown below:
Maximum Gain
70% of max gain, at
maximum bandwidth
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GCSE Electronics.
Unit E2 : Applications of Electronics
Gain-Bandwidth Product
The bandwidth of an amplifier is closely linked to its voltage gain. The higher
the voltage gain, the smaller its bandwidth will be. This leads to another
measure of the performance for amplifiers called the gain-bandwidth
product. For example if a pre-amplifier has a gain-bandwidth product of
1MHz, this means that:
i.
ii.
iii.
iv.
For a gain of 1, the bandwidth will 1MHz
For a gain of 10, the bandwidth will be 100kHz
For a gain of 100, the bandwidth will be 10kHz
For a gain of 1000, The bandwidth will be 1kHz etc
In each example the gain x bandwidth = 1 MHz. As the voltage gain gets
bigger, the bandwidth gets smaller.
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Topic 2.4 – Analogue Communications.
2.4.1 General Amplifier Systems.
Student Exercise 1:
1.
An amplifier has a gain-bandwidth product of 4 MHz. Calculate the
missing values in the table below for the amplifier.
Gain
Bandwidth
1
40 kHz
400 Hz
500
25
Space for calculations:
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GCSE Electronics.
Unit E2 : Applications of Electronics
2.
Here is some data about two amplifiers, X and Y.
Property
Gain bandwidth product / MHz
Max. output voltage / V
(i)
X
Y
3
16
0.6
5
Which amplifier would have a bandwidth of 6 kHz when configured
to have a gain of +500?
.......................................................................................................................
(ii)
Give a reason for your answer:
.......................................................................................................................
.......................................................................................................................
3.
An amplifier has the following frequency response. Complete the scales
and use the graph to estimate the bandwidth of this amplifier. Show on
the graph how you obtain your result.
Bandwidth = .................... kHz
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Topic 2.4 – Analogue Communications.
2.4.1 General Amplifier Systems.
Multi-Stage Amplifiers
What if we want high gain and a large bandwidth?
The gain-bandwidth issue is a problem for a single amplifier, but we can link
together two low gain amplifiers (which therefore have a wide bandwidth) to
provide a larger amplification overall whilst maintaining a wide bandwidth.
In the following diagram, the overall gain of the system is the product of the
two individual amplifier stages.
Overall Gain 
Vout
 A1  A2
Vin
Keeping the gain of each stage small, maintains a good overall bandwidth.
Example:
Overall gain = A1 x A2 = 10 x 10 = 100
Bandwidth of each amplifier =
GBP 1000000

 100kHz
Gain
10
(So, high gain and bandwidth overall!)
Note
We assume that the individual amplifiers have the same gainbandwidth product but they may have different voltage gains.
To find the overall bandwidth divide the gain-bandwidth product by the
highest individual gain.
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GCSE Electronics.
Unit E2 : Applications of Electronics
Homework Questions 1
1.
For the amplifier shown below, calculate Vout, when Vin = 1mV and the
gain of the amplifier = 35.
Vout =
2.
For the amplifier shown below, calculate the gain of the amplifier if
Vout=50mV and Vin=2mV.
Gain =
3.
Complete the following table with the missing values.
Vin
Gain G
5mV
2mV
4.
60mV
100
30
120mV
10
1V
6mV
25
20µV
400
A multistage amplifier consists of 2 amplifiers, one having a gain of 10
and the second having a gain of 20. They each have a gain-bandwidth
product of 2MHz.
Overall gain = ………………;
22
Vout
Overall bandwidth =………………
Topic 2.4 – Analogue Communications.
2.4.1 General Amplifier Systems.
Solutions to Student Exercises
Exercise 1:
1.
Gain
Bandwidth
1
4MHz
100
40 kHz
10000
400 Hz
500
8 kHz
25
160 kHz
2.
(i)
Amplifier X
(ii)
6kHz x 500 = 3MHz = gain-bandwidth product of the amplifier,
and this corresponds to amplifier X. Amplifier Y only has a GBP of
0.6MHz and therefore could not have this gain for this bandwidth.
3.
Bandwidth = .......50............. kHz
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GCSE Electronics.
Unit E2 : Applications of Electronics
Homework Questions 1
1.
Vout  A  Vin
 35  1mV
 35mV
2.
A
3.
4.
Vout 50mV

 25
Vin
2mV
Complete the following table with the missing values.
Vin
A
Vout
5mV
12
60mV
2mV
100
200mV
4mV
30
120mV
100mV
10
1V
6mV
25
150mV
20µV
400
8mV
Overall gain = G1  G2 = 10  20 = 200
Overall bandwidth 
GBP
1600000

 80000Hz  80kHz
Highest Gain
20
Now for some examination style questions.
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Topic 2.4 – Analogue Communications.
2.4.1 General Amplifier Systems.
Examination Style Questions
1.
The block diagram for a public address system used in a school hall is shown below.
(a)
Write the names of the four blocks in the boxes on the above diagram.
Choose from the following list.
Power Amplifier
Loudspeaker
Pre-amplifier
Comparator
Microphone
[4]
(b)
The deputy headteacher of the school plays a guitar during assembly, and wants to add a
guitar output into the public address system. Redraw the PA system to show how this
second input can be added to the system. You may add any additional blocks you may
need.
[2]
(c)
The pre-amplifier block consisted of a multi-stage amplifier made up of two amplifiers
each having a gain of 20 and gain bandwidth product (GBP) of 1MHz. calculate:
(i)
the overall gain……………………………………………………………………..
(ii)
the overall bandwidth
……………………………………………………………
[3]
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GCSE Electronics.
Unit E2 : Applications of Electronics
2.
The block diagram for a disco sound system is shown below.
(a)
(i)
Name one possible signal source.
...............................................................................................................................................
[1]
(ii)
Why is a pre-amplifier needed?
...............................................................................................................................................
...............................................................................................................................................
[1]
(iii) Why is a power amplifier needed?
...............................................................................................................................................
(b)
26
...............................................................................................................................................
[1]
The gain of the pre-amplifier is measured as the frequency of a signal source is varied.
The results are shown in the following table.
Topic 2.4 – Analogue Communications.
2.4.1 General Amplifier Systems.
(i)
On the grid below, draw a graph of gain against frequency.
[3]
(ii)
Mark carefully on the graph, the points at which the bandwidth of the amplifier
would be measured.
[2]
(iii)
Hence estimate the bandwidth of this preamplifier.
Bandwidth is ....................................................... kHz
[1]
(c)
The amplitude of the input voltage at a frequency of 5kHz was found to be 5mV. Calculate
the amplitude of the output voltage at this frequency.
...............................................................................................................................................
...............................................................................................................................................
[2]
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GCSE Electronics.
Unit E2 : Applications of Electronics
3.
(a)
A public address system contains some of the following sub-systems:
loudspeaker
power amplifier
preamplifier
(i)
demodulator
microphone
Here is an outline of the block diagram for the system. Complete it by adding the
names of the appropriate subsystems in the list to the correct block.
[4]
(ii)
How does the job of the preamplifier differ from that of the power amplifier?
................................................................................................................................................
................................................................................................................................................
................................................................................................................................................
[2]
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Topic 2.4 – Analogue Communications.
2.4.1 General Amplifier Systems.
Self Evaluation Review
Learning Objectives
My personal review of these objectives:



understand that amplifiers increase the
power (VI) of signals;
describe the function of the following
subsystems of a typical amplifier system:
signal source (e.g. microphone), preamplifier
(i.e. voltage amplifier), mixer, power (current)
amplifier, loudspeaker
select and use the formula G 
VOUT
;
VIN
understand what is meant by the bandwidth
of an amplifier;
measure the bandwidth of an amplifier from
a graph of voltage gain against frequency;
explain and draw graphs to illustrate the
meaning of output clipping for a given input
signal;
understand the trade-off between gain and
bandwidth and the role of multiple stage
voltage amplifiers in retaining bandwidth.
Targets:
1.
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
2.
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
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