INTRODUCTION TO ELECTRONIC AMPLIFIERS
An amplifier is an electronic block that magnifies either potential signals (voltage
amplifier), intensity signals (current amplifier) or both (power amplifier). The
amplifier consists of two inputs which are the signal to amplify (see Input Signal
on figure 1) and the supply of energy (see Power Supply). The output (see Output
Signal) is the input signal that has been amplified of a certain gain.
fig 1 : Flowchart of an electronic amplifier
Amplifiers are found at most of the stages of many electronic applications :

On input stages they bring the signals up to a value where they can be
exploited by the circuit.

On intermediary stages they maintain and correct the magnitude of the
signals.

On output stages they normalize the amplitude to meet standards for the
connections.
The main difference between common passive components such as resistors,
capacitors or inductors is that an electronic amplifier is an active component since
its made of more basics active components. An active component by definition
contains internal sources of energy as showed in the following diagrams.
fig 2 : Thevenin representation and Norton representation of sources
There are two different types of sources : the voltage and current sources. A
voltage source provides a constant voltage for a certain range of current. A current
source delivers a constant current for a certain range of voltage.
Typical active components used for amplification are electronic tubes and
transistors. Historically, the tubes are older and are still used for some specific
applications that require high power. Since the 60’s however, bipolar or MOSFET
transistors have become cheaper, faster, more efficient and require less supply to
amplify signals in most of the daily electronics we use : TV, phone, computers etc
…
THE QUADRUPOLE REPRESENTATION
To precisely represent a voltage amplifier, the Thevenin representation is more
adapted since it describes directly the relation between the output voltage and the
source. For the same reason, the Norton representation is more adapted for current
amplifiers.
If we consider the power supply input to be independent from the input and output
signals, we can represent the amplifier according to the quadrupole model :
fig 3 : Quadrupole model of a voltage amplifier with common ground
Only four parameters can here fully describe how the amplifier works : the input
impedance Zinput, the output impedance Zoutput, the transconductance G and the
reaction parameter G12. As mentioned previously, the transconductance
block G acts precisely as a voltage source. The general relation between these
parameters and the signals Vinput, Iinput, Voutput and Ioutput is given by the following
equations :
Equation 1 : General relations for a
voltage amplifier
THE IDEAL MODEL
An amplifier is considered to be ideal when the shape of the signal is not modified
by the process of amplification, no matter what is the shape or the frequency of the
input signal. Moreover, the gain should be a constant value, regardless again of the
shape or frequency of the signal and noises should not be amplified.
When considering the amplifier to be ideal, the output current Ioutput does not
influence the input Vinput, hence G12=0. The output impedance Zoutput is also
considered to be equal to zero in the case of an ideal voltage amplifier since the
output current does not influence the output voltage.
The relation between these parameters and the
signals Vinput, Iinput, Voutput and Ioutput for an ideal voltage amplifier is given by the
following equations :
Equation 2 : General relations
for an ideal voltage amplifier
It is easy to find from the equation 2 that the gain (A) of the ideal voltage amplifier
can be written as the fractionA=Voutput/Vinput=G/Zinput. We can note that this model
can be adapted to the current amplifier by replacing the output impedance Zoutput by
a parallel output admittance Youtput.
fig 4 : Quadrupole model of a current amplifier with common ground
Usually, the gain of an electronic amplifier is written in decibels (dB). For
example, if an amplifier has a gain of A=106 , we can convert it in dB by using the
formula:
LIMITATIONS OF REAL AMPLIFIERS
Most of the time, this ideal model can be used for simple calculations. However,
ideal amplifiers cannot be built due to physical and technological constraints. Real
amplifiers tend to have a constant gain A only on a certain range of
frequency f1 to f2 called bandwidth (BW). These cutting frequencies correspond
where a loss of 50 % to the maximal gain appears. In dB scale, this corresponds to
a loss of10log(0.5) = -3 dB. Moreover as shown in Figure 5, the output voltage
cannot exceed the supply voltage leading to a saturation effect of the amplification
process.
fig 5 : Limitations in frequency and linearity of the real amplifier
As we will discuss it more in details in the next tutorial about Common Emitter
Amplifier, the supply voltage Vsupply controls the flow of electrons in the active
bipolar transistors found in amplifiers. The saturation effect precisely occurs when
the flow of electrons can not be greater than the command voltage. A good analogy
of this phenomenon is a tap water system : the flow of water can not exceed a
certain limit set by when the tap is fully opened.
Another limitation to consider for real amplifiers is the distortion of the output
signal. Due to intrinsic non-linearities of the active components the output signal
can present a different shape than the input signal.Distortions can have many
causes, one of the most visual and common type is the amplitude distortion. The
cause of this distortion is directly due to the bandwidth (BW) of the amplifier.
As an example, let’s consider a square signal S(t) of frequency f=10 kHz to be
amplified by a limited fc=50 kHz BW amplifier of maximal
gain Amax=10. According to Fourier’s theory, every periodic signal can be written
as an infinite sum of pure sine signals called harmonics.
For a square signal, the Fourier serie can be written such as :
Equation 3 : Fourier serie of a square signal
An ideal amplifier would multiply all the terms of the Equation 3 sum by a
constant value Amax. However, a real amplifier as described previously would
indeed amplify the first term and second term sin(2πft) and sin(6πft) of Amax but
the third term sin(10πft) would be amplified only of 0.5Amax since this term
correspond to a harmonic of frequency fc=50 kHz. The other harmonics of higher
frequencies would be less amplified since the gain of the amplifier continues to
decrease (see Figure 6).
fig 6 : Amplitude distortion of a square signal. Plotted using MatLab®
As a conclusion the output of a square signal S(t) amplified by an amplifier of 50
kHz bandwidth and gain Amax=10 would look distorted such as presented in the
figure 6.
Mathematically speaking, this type of distortion is equivalent to only keeping the
harmonics of the input signal that are under the cutting frequency of the amplifier.
The output signal does not remain thus as an infinite sum of sine but becomes a
finite sum.
NOISE CONSIDERATIONS
The noise N is another undesired effect that often affects electronic amplifier.
Many types of noises exist and their cause is never easy to understand and
commonly due to the microscopic structure of the semiconductors used in
electronic components or quantum phenomena. Their consequences are however
very visual since it adds a random parasite signal to the expected ideal output. The
ratio signal/noise (S/N) is usually given in dB because on an oscilloscope figure for
example, the signal and the noise can simply be substracted since:
It is a quantity that one wants to maximize to get a proper amplification. If the ratio
is much higher than 1 (in linear scale) the noise is negligible, if the ratio is close to
0 (in linear scale) the amplitude of the noise is higher than the amplitude of the
signal which will completely distort the signal.
fig 7 : A square signal affected by a noise of S/N=10. Plotted using MatLab®
In Figure 7 we can see an example of a square signal of frequency 10 kHz with a
noise of 10 % of the signal : S/N=10.
CLASSES OF ELECTRONIC AMPLIFIERS
According to the electronic architecture of an amplifier, to the way that the
transistors are interconnected, the purpose of amplifiers and their specifications are
different. We can however commonly distinguish four working families and
classify them according to the list below. Next to the class name is given the
proportion of the input signal that the active components use to realize the
amplification process. This concept will be more detailed on the next articles of the
amplifier tutorial.

Class A : 100 % of the input signal is used: The power stage always delivers
current (the transistors are always in passing state). The response of the
amplifier is thus very fast since there is no delay to activate the transistors.
The distortion is very limited but the efficiency is usually low, always less
than the theoretical maximum of +50 %. The efficiency is the ratio
Pused/Psupply and represents the losses between the power effectively used to
amplify and the power supplied to the amplifier. Thanks to these advantages,
class A amplifiers are usually more expensive, but very appreciated in the
music industry because they can amplify sound without modifying the
content.

Class B : 50 % of the input signal is used: in that architecture, the
amplification stage does not always deliver current (the transistors are on
stand-by mode). This type of amplifier is thus slower than the class A and
presents more distortion but has a better efficiency up to 70 %. Class B
amplifiers are cheaper to manufacture since they do not need high quality
power supplies to operate.

Class AB : >50 % and <100 % of the input signal is used: Such as the name
refers to, this class is a mix between classes A and B. The amplifier has the
capacity to first work has a class A when no or small inputs are applied and
can switch to a class B when the inputs increase. Most of the affordable
amplifiers for TVs and headphones for example are class AB amplifiers
since they can deliver a good output on a wide range of power.

Class C : <50 % of the input signal is used: This type of amplifiers is used
for high frequency application such as in kitchen microwaves for example.
Their efficiency is very high (above 80 %) but they generate a lot of
distortion.
The class D is known better under the names chopper and inverter. In these
architectures, active components (transistors) are used as commutators : they act as
a short or open circuit. They are mostly used to control electric engines and they
present a very high efficiency around 80 to 90 %. In this class, 0 % of the input
signal is used to realise the amplification. More classes that are not mentioned here
are available for very specific applications and properties.
In the next tutorial, we will look much closer to the internal architecture of
electronic amplifier and more specifically to the role bipolar transistors play. A
serie of three tutorial will detail the three elementary configurations of bipolar
transistors to amplify signals : thecommon emitter, collector or base amplifiers.

Amplifier Classes
Class AB Amplifiers
Common Collector Amplifier
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HomeArticlesAmplifiers
AMPLIFIERS

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
Class AB Amplifiers
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Class B Amplifiers
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
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Page:of 4
Half
Wave Rectifier
Lab#03
A
Basic
Electronics
Page
19
LAB
#03‐A
Objectives
To
calculate
and
draw
the
DC
output
voltages
of
half
wave
rectifiers.
Theory
Rectifiers
A
rectifier
is
an
electrical
device
that
converts
alternating
current
(AC)
to
direct
current
(DC),
a
process
known
as
rectification
.
Rectifiers
have
many
uses
including
as
components
of
power
supplies
and
as
detectors
of
radio
signals.
Rectifiers
may
be
made
of
solid
state
diodes,
vacuum
tube
diodes,
mercury
arc
valves,
and
other
components.
Half
wave
rectification
In
half
wave
rectification,
either
the
positive
or
negative
half
of
the
AC
wave
is
passed,
while
the
other
half
is
blocked.
Because
only
one
half
of
the
input
waveform
reaches
the
output,
it
is
very
inefficient
if
used
for
power
transfer.
Half
wave
rectification
can
be
achieved
with
a
single
diode
in
a
one
phase
supply,
or
with
three
diodes
in
a
three
phase
supply.
The
output
DC
voltage
of
a
half
wave
rectifier
can
be
calculated
with
the
following
two
ideal
equations.
Preparatory
Exercise
Q1)
What
is
half
wave
rectifier?
Half
Wave Rectifier
Lab#03
A
Basic
Electronics
Page
20
Requirement
Instruments
1.
Transformer/
Function
Generator
2.
Digital
Multi
meter
(DMM)
3.
Oscilloscope
Components
1.
Diode
:
Silicon
(D1N4002)
2.
Resistors:
2.2k
Ω
,
3.3k
Ω
Procedure
Half
Wave Rectification
1.
Construct
the
circuit
of
Fig.
3.1
.
Set
the
supply to
6
V
p
p
sinusoidal
wave
with
the
frequency
of
600
Hz.
Put
the
oscilloscope
probes
at
function
generator
and
sketch
the
input
waveform
obtained.
2.
Put
the
oscilloscope
probes
across
the
resistor
and
sketch
the
output
waveform
obtained.
Measure
and
record
the
DC
level
of
the
output
voltage
using
the
DMM.
Function
Generator
Fig.
3.1
3.
Reverse
the
diode
of
circuit
of
Fig.
3.1
.
Sketch
the
output
waveform
across
the
resistor.
Measure
and
record
the
DC
level
of
the
output
voltage.
4.
Comment
on
the
results
obtained
from
step
2
and
3.
Half
Wave Rectifier
Lab#03
A
Basic
Electronics
Page
21
Observation
Results and Calculations
1.
Input
waveform,
V
i
V(volt)
Time
(s)
2.
Output
waveform,
V
o
V(volt)
Time
(s)
DC
level
of
V
o
(measured)
=
Reversed
Bias
Diode
3.
Output
waveform,
V
o
V(volt)
Time
(s)
Half
Wave Rectifier
Lab#03
A
Basic
Electronics
Page
22
DC
level
of
V
o
(measured)
=
Calculation
Result