BESCK104C_Module 1.pptx presentation ppt

NATIONAL EDUCATION SOCEITY OF KARNATAKA
Dr. H N National College of Engineering, Bengaluru – 70.
1
Course Title: Introduction to Electronics & Communication
Course Code: ESCK104C
Course Teacher: Dr. D Jayadevappa
Contact Details:
Mob : 9986134424
Email : principal@dr.hnnce.org.in

Course Objectives
Preparation: To prepare students with fundamental knowledge/overview in
the field of Electronics and Communication Engineering.
Core Competence: To equip students with a basic foundation in electronic
engineering fundamentals required for comprehending the operation and
application of electronic circuits, logic design, embedded systems and
communication systems.
Professionalism & Learning Environment: To inculcate in first year engineering
students an ethical and a professional attitude by providing an academic
environment inclusive of effective communication, teamwork, ability to relate
engineering issues to a broader social context and life-long learning needed
for a successful professional career.
2

Syllabus
Module-1 (8 hours)
Power Supplies –Block diagram, Half-wave rectifier, Full-wave rectifiers and filters,
Voltage regulators, Output resistance and voltage regulation, Voltage multipliers.
Amplifiers – Types of amplifiers, Gain, Input and output resistance, Frequency response,
Bandwidth, Phase shift, Negative feedback, multi-stage amplifiers (Text 1)
Module-2 (8 hours)
Oscillators – Barkhausen criterion, sinusoidal and non-sinusoidal oscillators, Ladder
network oscillator, Wein bridge oscillator, Multivibrators, Single-stage astable oscillator,
Crystal controlled oscillators (Only Concepts, working, and waveforms. No mathematical
derivations)
Operational amplifiers -Operational amplifier parameters, Operational amplifier
characteristics, Operational amplifier configurations, Operational amplifier circuits (Text 1)
3

Syllabus
Module-3 (8 hours)
Boolean Algebra and Logic Circuits: Binary numbers, Number Base Conversion, octal &
Hexa Decimal Numbers, Complements, Basic definitions, Axiomatic Definition of Boolean
Algebra, Basic Theorems and Properties of Boolean Algebra, Boolean Functions,
Canonical and Standard Forms, Other Logic Operations, Digital Logic Gates (Text 2: 1.2,
1.3, 1.4, 1.5,2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7)
Combinational logic: Introduction, Design procedure, Adders- Half adder, Full adder
(Text 2:4.1, 4.2, 4.3).
Module-4 (8 hours)
Embedded Systems – Definition, Embedded systems vs general computing systems,
Classification of Embedded Systems, Major application areas of Embedded Systems,
Elements of an Embedded System, Core of the Embedded System, Microprocessor vs
Microcontroller, RISC vs CISC
4

Syllabus
Sensors and Interfacing – Instrumentation and control systems, Transducers, Sensors,
Actuators, LED, 7-Segment LED Display. (Text 3).
Module-5 (8 hours)
Analog Communication Schemes – Modern communication system scheme, Information
source, and input transducer, Transmitter, Channel or Medium – Hardwired and Soft
wired, Noise, Receiver, Multiplexing, Types of communication systems. Types of
modulation (only concepts) – AM , FM, Concept of Radio wave propagation (Ground,
space, sky)
Digital Modulation Schemes: Advantages of digital communication over analog
communication, ASK, FSK, PSK, Radio signal transmission Multiple access techniques.
(Text 4)
5

Text Books
Suggested Learning Resources:
1.Mike Tooley, ‘Electronic Circuits, Fundamentals & Applications’,4th
Edition,
Elsevier, 2015.
DOI https://doi.org/10.4324/9781315737980. eBook ISBN9781315737980
2. Digital Logic and Computer Design, M. Morris Mano, PHI Learning, 2008
ISBN-978-81-203-0417-84.
3. K V Shibu, ‘Introduction to Embedded Systems’, 2nd Edition, McGraw Hill
Education (India), Private Limited, 2016
4. S L Kakani and Priyanka Punglia, ‘Communication Systems’, New Age
International Publisher, 2017.
6

Assessment Details (both CIE and SEE)
The weightage of Continuous Internal Evaluation (CIE) is 50% and for Semester
End Exam (SEE) is 50%. The minimum passing mark for the CIE is 40% of the
maximum marks (20 marks out of 50).
The minimum passing mark for the SEE is 35% of the maximum marks (18 marks
out of 50). A student shall be deemed to have satisfied the academic
requirements and earned the credits allotted to each subject/ course if the
student secures not less than 35% (18 Marks out of 50) in the semester-end
examination (SEE), and a minimum of 40% (40 marks out of 100) in the sum total
of the CIE (Continuous Internal Evaluation) and SEE (Semester End Examination)
taken together.
7

Course Outcomes (COs)
8
COs Statement Module
CO1
Explain the concepts of electronic circuits encompassing power supplies,
and amplifiers
1
CO2
Describe the concepts of electronic circuits encompassing oscillators and
operational amplifiers
2
CO3
Present the basics of digital logic engineering including Boolean
Algebra and Logic Circuits
3
CO4 Discuss the characteristics of embedded systems and sensors interfacing. 4
CO5 Explain the analog and digital communication schemes including
modulation techniques.
5

Range of Conduciveness
The semiconductors fall somewhere midway between conductors
and insulators.
10

Range of Conduciveness
Semiconductors have special electronic properties which allow
them to be insulating or conducting depending on their
composition.
11

1874
Ferdinand Braun
The first semiconductor device was born.
12

Scientific Principle of Conduction
13

Devices made by Semiconductor
Microprocessors LED
Transistors
Capacitors
14

Silicon is a very common element, the main element
in sand & quartz.
Silicon
15

Current Flow
If a voltage is applied, then both the electron and the hole can
contribute to a small current flow.
16

P-N Junction
We create a p-n junction by joining together two pieces of
semiconductor, one doped n-type, the other p-type.
17

Diode
A diode is the simplest possible semiconductor device.
18

This voltage is needed to start the hole-electron combination process at the
junction.
When forward-biased, there is a small amount of voltage necessary to get the diode
going. In silicon, this voltage is about 0.7 volts.
Cut-in Voltage
19

Diode Characteristic
When reverse-biased, an ideal diode would block all current. A real diode
lets perhaps 10 microamps through -- not a lot, but still not perfect.
20

Module 1: Power Supplies
Introduction
 All electronic circuits require a source of well-regulated d.c. at
voltages of typically between 5 V and 30 V.
 In some cases this supply can be derived directly from batteries (e.g.
6 V, 9 V, 12 V) but in many others it is desirable to make use of a
standard a.c. mains outlet.
 This topic explains how rectifier and smoothing circuits operate and
how power supply output voltages can be closely regulated.
 This topic is also provides a brief description of some practical power
supply circuits
21

Block diagram of power supply
22
Fig. 1:
Fig. 2:
Fig. 3:

Working
 Since the mains input is at a relatively high voltage, a step-down
transformer of appropriate turns ratio is used to convert this to a low
voltage.
 The A.C output from the transformer secondary is then rectified using
conventional silicon rectifier diodes to produce an unsmoothed
(sometimes referred to as pulsating D.C) output.
 This is then smoothed and filtered before being applied to a circuit
which will regulate (or stabilize) the output voltage so that it remains
relatively constant in spite of variations in both load current and
incoming mains voltage.
23

Contd.,
 Figure 2 shows how some of the electronic components used in the power
supply.
 The iron-cored step-down transformer feeds a rectifier arrangement
(often based on a bridge circuit).
 The output of the rectifier is then applied to a high-value reservoir capacitor.
 This capacitor stores a considerable amount of charge and is being
constantly topped-up by the rectifier arrangement.
 The capacitor also helps to smooth out the voltage pulses produced by the
rectifier.
 Finally, a stabilizing circuit (often based on a series transistor regulator and a
zener diode voltage reference) provides a constant output voltage.
24

Rectifiers
 Semiconductor diodes are commonly used to convert A.C into D.C in
which case they are referred to as rectifiers.
 The simplest form of rectifier circuit makes use of a single diode and,
since it operates on only either positive or negative half-cycles of the
supply, it is known as a half-wave rectifier.
 Figure 4 shows a simple half-wave rectifier circuit. Mains voltage
223V is applied to the primary of a step-down transformer (T1). The
secondary of T1 steps down the 240 V r.m.s. to 12 V r.m.s. (the turns
ratio of T1 will thus be 240/12 or 20:1).
 Diode D1 will only allow the current to flow in the direction shown (i.e.
from cathode to anode).
25

Contd.,
26
 D1 will be forward biased during each positive half-cycle (relative to
common) and will effectively behave like a closed switch.
 When the circuit current tries to flow in the opposite direction, the
voltage bias across the diode will be reversed, causing the diode to
act like an open switch [see Fig. (a) and (b), respectively]

Contd.,
 The switching action of D1 results in a pulsating output voltage which is developed across
the load resistor (RL ).
 Since the mains supply is at 50 Hz, the pulses of voltage developed across RL will also be
at 50 Hz even if only half the a.c. cycle is present.
 During the positive half-cycle, the diode will drop the 0.6 V to 0.7 V forward threshold
voltage normally associated with silicon diodes.
 However, during the negative half-cycle the peak a.c. voltage will be dropped across D1
when it is reverse biased. This is an important consideration when selecting a diode for a
particular application.
 Assuming that the secondary of T1 provides 12 V r.m.s., the peak voltage output from the
transformer’s secondary winding will be given by: Vpk = 1.414xVrms
=1.414 x12 V= 16.97 V.
27

Contd.,
 The peak voltage applied to D1 will thus be approximately 17 V. The
negative half-cycles are blocked by D1 and thus only the positive half
cycles appear across RL.
 However, the actual peak voltage across RL will be the 17 V positive
peak being supplied from the secondary on T1, minus the 0.7 V
forward threshold voltage dropped by D1.
 In other words, positive half-cycle pulses having a peak amplitude of
16.3 V will appear across RL.
Problems:
28

Reservoir and smoothing circuits
 Figure shows a rectifier circuit with filter. The capacitor, C1, has been
added to ensure that the output voltage remains at, or near, the peak
voltage even when the diode is not conducting.
 When the primary voltage is first applied to T1, the first positive half-
cycle output from the secondary will charge C1 to the peak value seen
across RL .
 Hence C1 charges to 16.3 V at the peak of the positive half-cycle.
 Because C1 and RL are in parallel, the voltage across RL will be the
same as that across C1.
29

Half-wave rectifier circuit with reservoir capacitor
 The time required for C1 to charge to the maximum (peak) level is
determined by the charging circuit time constant (the series resistance
multiplied by the capacitance value).
 In this circuit, the series resistance comprises the secondary winding
resistance together with the forward resistance of the diode and the
(minimal) resistance of the wiring and connections. Hence C1 charges very
rapidly as soon as D1 starts to conduct.
30

Output waveforms of the Half-wave rectifier
31
C1 is referred to as a reservoir
capacitor. It stores charge during the
positive half-cycles of secondary
voltage and releases it during
the negative half-cycles. The circuit
is thus able to maintain a reasonably
constant output voltage across RL. Even
so, C1 will discharge by a small amount
during the negative half-cycle periods
from the transformer secondary

Contd.,
 In the waveform diagram, the secondary voltage waveform together
with the voltage developed across RL with and without C1 is presented.
 This gives rise to a small variation in the d.c. output voltage (known as
ripple).
 Since ripple is undesirable we must take additional precautions to
reduce it.
 One obvious method of reducing the amplitude of the ripple is that of
simply increasing the discharge time constant.
 This can be achieved either by increasing the value of C1 or by
increasing the resistance value of RL.
32

Half-wave rectifier circuit with R–C smoothing filter
33
 Figure below shows a further refinement of the simple power supply
circuit. This circuit employs two additional components, R1 and C1, which
act as a filter to remove the ripple.
 The value of C1 is chosen so that the component exhibits a negligible
reactance at the ripple frequency (50 Hz for a half-wave rectifier or
100 Hz for a full-wave R1 and C1 act like a potential divider. The amount
of ripple is reduced by an approximate factor equal to

Half-wave rectifier circuit with L–C smoothing filter
 A further improvement can be achieved
by using an inductor, L1, instead of a resistor
in the smoothing circuit.
 Figure shows the circuit of a half-wave power
supply with an L–C smoothing circuit.
 At the ripple frequency, L1 exhibits a high value
of inductive reactance while C1 exhibits a low
value of capacitive reactance. The combined
effect is that of an attenuator which greatly
reduces the amplitude of the ripple while having
a negligible effect on the direct voltage.
34
Improved ripple filter

Full-wave rectifiers
 Unfortunately, the half-wave rectifier circuit is relatively inefficient as
conduction takes place only on alternate half-cycles.
 A better rectifier arrangement would make use of both positive and
negative half-cycles. These full-wave rectifier circuits offer a
considerable improvement over their half-wave counterparts. They are
not only more efficient but are significantly less demanding in terms of
the reservoir and smoothing components.
 There are two basic forms of full-wave rectifier;
bi-phase type
bridge rectifier type
35

Bi-phase rectifier circuit
36
• Figure shows a simple bi-phase rectifier circuit.
Mains voltage (240 V) is applied to the primary
of the step-down transformer (T1) which has two
identical secondary windings, each providing
12 V r.m.s. (the turns ratio of T1 will thus be
240/12 or 20:1 for each secondary winding).
• On positive half-cycles, point A will be positive
with respect to point B. Similarly, point B will be
positive with respect to point C.
• In this condition D1 will allow conduction (its
anode will be positive with respect to its
cathode) while D2 will not allow conduction (its
anode will be negative with respect to its
cathode). Thus D1 alone conducts on positive
half-cycles.

Working
37
Bi-phase rectifier with D1 conducting and D2
non-conducting
Bi-phase rectifier with D2 conducting and D1
non-conducting

Bi-phase rectifier with reservoir capacitor
38

Waveforms for the bi-phase rectifier (without C1 & with C1)
39

Bridge rectifier circuits
 An alternative to the use of the bi-phase circuit is that of using a four-
diode bridge rectifier shown in figure in which opposite pairs of
diodes conduct on alternate half-cycles.
 This arrangement avoids the need to have two separate secondary
windings
40
Full-wave bridge rectifier circuit
Four diodes connected as a bridge

Working of Bridge Rectifier
 full-wave bridge rectifier arrangement is shown in Figure.
 Mains voltage (240 V) is applied to the primary of a step-down transformer (T1).
 The secondary winding provides 12 V r.m.s. (approximately 17 V peak) and has a
turns ratio of 20:1, as before.
 On positive half-cycles, point A will be positive with respect to point B.
 In this condition D1 and D2 will allow conduction while D3 and D4 will not allow
conduction.
 Conversely, on negative half-cycles, point B will be positive with respect to point A.
 In this condition D3 and D4 will allow conduction while D1 and D2 will not allow
conduction.
 Figure shows the bridge rectifier circuit with the diodes replaced by four switches.
41

Bridge rectifier with D1 and D2 conducting,
D3 and D4 non-conducting
42

Bridge rectifier with D1 and D2 non-conducting, D3
and D4 conducting
43

Contd.,
 In Figure (a) D1 and D2 are conducting on a positive half-cycle while
in Figure (b) D3 andD4 are conducting.
 Once again, the result is that current is routed through the load in the
same direction on successive half-cycles.
 As with the bi-phase rectifier, the switching action of the two diodes
results in a pulsating output voltage being developed across the load
resistor (RL ).
 Once again, the peak output voltage is approximately 16.3 V (i.e.
17 V less the 0.7 V forward threshold voltage).
44

Waveforms for the bridge rectifier
45

Bridge rectifier with reservoir capacitor
46

Smoothing circuit using reservoir capacitor
47

Contd.,
 Figure shows how a reservoir capacitor (C1) can be added to maintain the
output voltage when the diodes are not conducting.
 This component operates in exactly the same way as for the bi-phase circuit,
i.e. it charges to approximately 16.3 V at the peak of the positive half-cycle
and holds the voltage at this level when the diodes are in their non-conducting
states.
 This component operates in exactly the same way as for the bi-phase circuit
and the secondary and rectified output waveforms. Once again note that the
ripple frequency is twice that of the incoming a.c. supply.
 Finally, R–C and L–C ripple filters can be added to bi-phase and bridge
rectifier circuits.
48

Zener Diode basics
Zener Diode Characteristics:
Zener diode
 Operates at voltages that exceed breakdown voltage
 Manufactured with a specific breakdown voltage (EZ)
 Packaged like PN junction diodes
 Power dissipation
 Based on temperature and lead lengths
49

Zener Diode package & Symbol
Zener diode packages.
Schematic symbol for a zener diode.

Zener breakdown phenomenon
51
Zener Breakdown:
When reverse biased
voltage applied to the zener diode
reaches zener voltage, it starts
allowing large amount of electric
current.
At this point, a small increase in
reverse voltage will rapidly
increases the electric current.
Because of this sudden rise in electric
current, breakdown occurs called
zener breakdown

Zener diode basic operation
52

Zener Diode Ratings
 Positive zener voltage-temperature coefficient
 Breakdown voltage increases as temperature increases
 Negative zener voltage-temperature coefficient
 Breakdown voltage decreases as temperature increases
53

Voltage Regulation with Zener Diodes
Typical zener diode regulator circuit.
54

Voltage Regulation with Zener Diodes (cont’d.)
Zener diode voltage regulator with load.
55

Testing Zener Diodes
Setup for testing zener diode regulation.
56

Summary
 Zener diodes are designed to operate at voltages greater than the
breakdown voltage (peak reverse voltage)
 The breakdown voltage of a zener diode is determined by the
resistivity of the diode
 Zener diodes are used to stabilize or regulate voltage
 Zener diode regulators provide a constant output voltage despite
changes in the input voltage or output current
 To determine whether a zener diode is regulating at the proper
voltage, a regulation test must be performed
57

Voltage regulators
 A simple voltage regulator is shown in Figure.
 RS is included to limit the zener current to a safe value when the load is disconnected.
 When a load (RL ) is connected, the zener current (IZ ) will fall as current is diverted
into the load resistance (it is usual to allow a minimum current of 2 mA to 5 mA in
order to ensure that the diode regulates).
58

Contd.,
59
 The output voltage (VZ ) will remain at the zener voltage until regulation fails at the
point at which the potential divider formed by RS and RL produces a lower output
voltage that is less than VZ .
 The ratio of RS to RL is thus important. At the point at which the circuit just begins to fail
to regulate:
Where, VIN is the unregulated input voltage. Thus the maximum value for RS can be
calculated from:

Contd.,
 The power dissipated in the zener diode will be given by
PZ = I Z × VZ ,
hence the minimum value for RS can be determined from the off-load
condition when:
60

Output resistance
 In a perfect power supply, the output voltage would remain constant
regardless of the current taken by the load. In practice, however, the
output voltage falls as the load current increases. To account for this fact,
we say that the power supply has internal resistance (ideally this should
be zero). This internal resistance appears at the output of the supply and
is defined as the change in output voltage divided by the corresponding
change in output current. Hence:
61

Regulation
 The regulation of a power supply is given by the relationship:
 Ideally, the value of regulation should be very small. Simple shunt zener
diode regulators of the type shown in Figure are capable of producing
values of regulation of 5% to 10%.
 More sophisticated circuits based on discrete components produce values of
between 1% and 5% and integrated circuit regulators often provide values
of 1% or less.
62

Voltage multipliers
 By adding a second diode and capacitor, we
can increase the output of the simple half-wave
rectifier that we have discussed earlier. A
voltage doubler using this technique is shown in
figure.
 In this arrangement C1 will charge to the
positive peak secondary voltage while C2 will
charge to the negative peak secondary
voltage.
 Since the output is taken from C1 and C2
connected in series the resulting output voltage
is twice that produced by one diode alone.
63

A Voltage Tripler
 The voltage doubler can be extended to
produce higher voltages using the
cascade arrangement shown in figure.
 Here C1 charges to the positive peak
secondary voltage, while C2 and C3
charge to twice the positive peak
secondary voltage.
 The result is that the output voltage is the
sum of the voltages across C1and C3
which is three times the voltage that
would be produced by a single diode.
64

Amplifiers
Introduction
An amplifier is an electronic device or circuit which is used to increase
the magnitude of the signal applied to its input.
65

Types of amplifier
Many different types of amplifier are found in electronic circuits. Before
we explain the operation of transistor amplifiers in detail, we shall
briefly describe the main types of amplifier.
(1) a.c. coupled amplifiers: In a.c. coupled amplifiers, stages are coupled
together in such a way that d.c. levels are isolated and only the a.c.
components of a signal are transferred from stage to stage.
(2) d.c. coupled amplifiers: In d.c. (or direct) coupled amplifiers, stages
are coupled together in such a way that stages are not isolated to d.c.
potentials. Both a.c. and d.c. signal components are transferred from
stage to stage.
66

Contd.,
(3) Large-signal amplifiers: Large-signal amplifiers are designed to
cater for appreciable voltage and/or current levels (typically from 1 V
to 100 V or more).
(4) Small-signal amplifiers: Small-signal amplifiers are designed to cater
for low-level signals (normally less than 1 V and often much smaller).
Small-signal amplifiers have to be specially designed to combat the
effects of noise.
(5) Audio frequency amplifiers: Audio frequency amplifiers operate in
the band of frequencies that is normally associated with audio signals
(e.g. 20 Hz to 20 kHz)
67

Contd.,
(6) Wideband amplifiers: Wideband amplifiers are capable of amplifying a
very wide range of frequencies, typically from a few tens of hertz to several
megahertz.
(7) Radio frequency amplifiers: Radio frequency (RF) amplifiers operate in the
band of frequencies that is normally associated with radio signals (e.g. from 100
kHz to over 1 GHz). Note that it is desirable for amplifiers of this type to be
frequency selective and thus their frequency response may be restricted to a
relatively narrow band of frequencies.
(8) Low-noise amplifiers Low: noise amplifiers are designed so that they contribute
negligible noise (signal disturbance) to the signal being amplified. These
amplifiers are usually designed for use with very small signal levels (usually less
than 10 mV or so)
68

Gain
One of the most important
parameters of an amplifier is the
amount of amplification or gain
that it provides. Gain is simply the
ratio of output voltage to input
voltage, output current to input
current, or output power to input
power as shown in figure. These
three ratios give, respectively,
the voltage gain, current gain and
power gain.
69

Contd.,
Since power is the product of current and voltage (P = IV ), we can infer
that:
70

Input & output resistance
Input resistance: It is the ratio of input voltage to input current and it is expressed in
ohms. The input of an amplifier is normally purely resistive (i.e. any reactive component
is negligible) in the middle of its working frequency range (i.e. the mid-band). In some
cases, the reactance of the input may become appreciable (e.g. if a large value of
stray capacitance appears in parallel with the input resistance). In such cases we would
refer to input impedance rather than input resistance.
Output resistance: It is the ratio of open-circuit output voltage to short-circuit output
current and is measured in ohms. Note that this resistance is internal to the amplifier
and should not be confused with the resistance of a load connected externally. As with
input resistance, the output of an amplifier is normally purely resistive and we
can safely ignore any reactive component. If this is not the case, we would once again
need to refer to output impedance rather than output resistance.
71

Contd.,
72
Figure shows how the input and output resistances
are ‘seen’ looking into the input and output
terminals, respectively.
Finally, it’s important to note that, although these
resistances are meaningful in terms of the signals
present, they cannot be measured using a
conventional meter!

Frequency response
 The frequency response characteristics for various types of amplifier
are shown in figure.
 Note that, for response curves of this type, frequency is almost
invariably plotted on a logarithmic scale.
 The frequency response of an amplifier is usually specified in terms of
the upper and lower cut-off frequencies of the amplifier.
 These frequencies are those at which the output power has dropped to
50% (otherwise known as the 3 dB points) or where the voltage gain
−
has dropped to 70.7% of its mid-band value.
73

Frequency response characteristics for various types of amplifiers
74

Contd.,
Figures (a) and (b) respectively, show how the bandwidth can be
expressed in terms of either power or voltage (the cut-off frequencies,
f1 and f2 , and bandwidth are identical).
75
Figure (a): Frequency response and bandwidth (output power
plotted against frequency)
Figure (b): Frequency response and bandwidth (outpu
voltage plotted against frequency)

Bandwidth
 The bandwidth of an amplifier is usually taken as the difference between
the upper and lower cut-off frequencies [i.e. f2 f1 shown in figures (a) &
−
(b)].
 The bandwidth of an amplifier must be sufficient to accommodate the range
of frequencies present within the signals that it is to be presented with.
 Many signals contain harmonic components (i.e. signals at 2f, 3f, 4f, etc.
where f is the frequency of the fundamental signal).
 To reproduce a square wave, for example, requires an amplifier with a very
wide bandwidth (note that a square wave comprises an infinite series of
harmonics).
76

Phase shift
 Phase shift is the phase angle between the input and output signal
voltages measured in degrees.
 The measurement is usually carried out in the mid-band where, for
most amplifiers, the phase shift remains relatively constant.
 Note also that conventional single-stage transistor amplifiers provide
phase shifts of either 180° or 360°.
77

Negative feedback
 Many practical amplifiers use negative feedback in order to precisely
control the gain, reduce distortion and improve bandwidth.
 The gain can be reduced to a manageable value by feeding back a
small proportion of the output.
 The amount of feedback determines the overall (or closed-loop) gain.
Because this form of feedback has the effect of reducing the overall
gain of the circuit, this form of feedback is known as negative feedback.
 An alternative form of feedback, where the output is fed back in such a
way as to reinforce the input (rather than to subtract from it) is known as
positive feedback. This form of feedback is used in oscillator circuits.
78

Amplifier with negative feedback applied
79

Contd.,
 Figure shows the block diagram of an amplifier stage with negative feedback
applied.
 In this circuit, the proportion of the output voltage fed back to the input is given
by and the overall voltage gain will be given by
β
 Now Vin' = Vin Vout (by applying Kirchhoff’s Voltage Law) (note that the
− β
amplifier’s input voltage has been reduced by applying negative feedback)
thus:
Vin =Vin' + Vout and
β
Vout = Av × Vin (note that Av is the internal gain of the amplifier)
80

Contd.,
 Hence, the overall gain with negative feedback applied will be less than
the gain without feedback.
 Furthermore, if Av is very large the overall gain with negative feedback
applied will be given by: G = 1/ (when Av is very large).
β
 Note, also, that the loop gain of a feedback amplifier is defined as the
product of and Av
β
81

Multistage amplifiers
 In order to provide sufficiently large values of gain, it is frequently necessary
to use a number of interconnected stages within an amplifier.
 The overall gain of an amplifier with several stages (i.e. a multi-stage
amplifier) is simply the product of the individual voltage gains.
 Hence: AV = AV1 × AV2 × AV3, etc.
 Note, however, that the bandwidth of a multistage amplifier will be less than
the bandwidth of each individual stage.
 In other words, an increase in gain can only be achieved at the expense of a
reduction in bandwidth.
 Signals can be coupled between the individual stages of a multi-stage
amplifier using one of a number of different methods shown in figure.
82

Different methods used for inter-stage coupling
R–C coupling:
 The most commonly used method is
that of R–C coupling as shown in
figure.
 In this coupling method, the stages
are coupled together using
capacitors having a low reactance at
the signal frequency and resistors
(which also provide a means of
connecting the supply). Figure (a)
shows a practical example of this
83

A typical two-stage high-gain R–C coupled common-emitter amplifier
84

L-C Coupling
 A similar coupling method,
known as L–C coupling, is
shown in figure.
 In this method, the inductors
have a high reactance at the
signal frequency.
 This type of coupling is
generally only used in RF and
high-frequency amplifiers.
85

Transformer coupling
 Two further methods,
transformer coupling
and direct coupling,
are shown in figures.
 The latter method is
used where d.c. levels
present on signals must
be preserved.
86

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