Transcript Name of the participant: D .Padma Subhashini

Objectives:
On completion of this period you would be able to :
•
•
•
•
•
Define feedback.
Need for feedback.
Types of feedback.
Define negative and positive feedback.
Advantages and disadvantages of negative and positive
feedback.
• Compare negative and positive feed back.
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Topics to be discussed:
working of amplifiers and their parameters.
Topics under discussion:
what is Feed back?
Comparison of different types of feed back.
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What is Feed back?
• The process by which a fraction of output energy is
injected back to the input is called feed back.
• The amplifiers using this technique are called feed back
amplifiers.
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Characteristics of basic amplifiers
Important characteristics of an amplifier are its :
• Voltage gain.
• Input impedance.
• Output impedance.
• Band width.
These parameters are more or less fixed for a
basic amplifier.
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Need for feed back
• The parameters of a basic amplifier are required to
changed as per the need.
• These changes can be brought out efficiently by
introducing feed back in the amplifier.
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Types of feed back
Depending upon the effect of feed back signal there
are two types of feed back.
• Negative feed back.
• Positive feed back.
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What is negative feed back?
When the feed back signal decreases the
net input signal i.e., the feedback signal is
1800out of phase with respect to input
signal, it is called negative or
Degenerative feed back.
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What is positive feed back?
When the feed back signal increases
the net input signal i.e., the feedback
signal is in phase with the input signal,
it is called Positive or Regenerative
feed back.
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Advantages of Negative Feed back:
• Improves stability in gain.
• Reduces distortion.
• Reduces the noise level at the output.
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Disadvantages of negative feed back
• decreases the gain.
• decreases the distortion.
• decreases the noise.
Due to these features:
it is used in amplifiers.
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Disadvantages of positive feed back
• Increases the distortion.
• Increases the noise.
• Poor stability.
Due to these features:
• it is seldom used in amplifiers.
• it is used in oscillators.
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Comparison of positive and negative feed back
Negative feed back
1800out of phase with the
input signal.
Positive feed back
In phase with the input
signal
Net input
signal
Decreases
Increases
Gain
Decreases
Increases
Noise
Decreases
Increases
Stability
Improved
Poor
Uses
Amplifiers
oscillators
Feed back
signal
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Principle of feed back.
+
Rs
vs
-
+
Ii
Vi
Mixing
net
work
+
Vf
Basic
amplifi
er
A
I
V
Sampli
ng
networ
k
Io
Vo
RL
-
Feed
back
network
. b
Block Diagram of Feed Back Amplifier
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• Feed back amplifier is one in which a part of the
output of an amplifier is fed back to the input.
• It consists of
basic amplifier with a gain A.
feed back network.
sampling network.
mixing network.
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Basic amplifier
• This stage simply amplifies the signal that is
present at its input .
• The voltage gain of this amplifier is “A.”
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Feed back network
It is usually a passive two port network which
may contain resistors , capacitors & inductors.
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Sampling Network
•
Using this network, we sample the output voltage or
current. The sampled energy is fed to the feedback
network linearly in series or shunt with the output network.
Sampled Signal is Voltage
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Sampled Signal is Current
17
Mixing network
• This network mixes the feedback energy with the
applied input. The feedback signal is connected in
series or in shunt with the input signal.
SHUNT FEEDBACK
SERIES FEEDBACK
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Gain of negative feed back amplifier
mixer
A
b
A
gain of amplifier without feedback
difference signal
Xi
Xs
input signal.
feed back amplifier
Af
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Xo
output signal
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Negative feed back is applied by feeding the
fraction of the output voltage bXo back to the
amplifier input.
Xi = Xs-Xf
Xf = ß.Xo
Xo = A.Xi
Substituting in Xi
Xo =( Xs- ß.Xo)A
Xo(1+A. ß) = A.Xs
Xo/Xs = A/(1+A. ß)
Xo/Xs is called the gain of the feed back amplifier.
Af =A/(1+A.b)
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Advantages of negative feed back amplifier.
• Increased stability.
• Increased band width.
• Less distortion .
• Reduces non linear distortion and noise.
• Input and output impedance can be modified as
desired.
• High fidelity i.e., more linear operation.
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Disadvantages of negative feedback
Reduces the gain of an amplifier.
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Basis of classification
Classification of negative feed back
amplifiers is based on the method
of mixing and sampling employed.
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Classification
Negative feed back amplifiers are classified into:•
•
•
•
Voltage series feedback amplifier.
Current series feedback amplifier.
Current shunt feedback amplifier.
Voltage shunt feedback amplifier.
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Voltage series feedback amplifier
Basic
V amplifie
r
Vs
i
Vo R
L
A
Vf
Feed
back
b
This connection increases the input resistance and
decreases the output resistance .
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Current series feedback amplifier
Vs
Vi
V
f
Basic
amplifie
r
A
Io
R
L
Feed
back
b
This connection increases both input resistance
and output resistance
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Current shunt feedback amplifier
Basic
I amplifie
r
A
Is
i
I
f
Io
R
L
Feed
back
b
This connection decreases input resistance and
increases the output resistance.
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Voltage shunt feedback amplifier
Is
I
I
i
f
Basic
amplifie
r
A
Vo
R
L
Feed
back
b
This connection decreases both input resistance
and output resistance.
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Signals in feedback amplifiers
signal
Voltage
series
Output signal
Type of
feedback
Current
series
Current
shunt
Voltage
shunt
Voltage
Current
Current
Voltage
Voltage
Voltage
Current
Current
Feedback
signal
Voltage
Voltage
Current
Current
Different
signal
Voltage
Voltage
Current
Current
Voltage
gain Av
Trans
conductan
ce Gm
Resistan
ce Vf/Io
Current
gain Ai
Trans
resistanc
e Rm
Conducta
nce If/Vo
Input signal
Gain A
Feedback
factor b
Voltage
ratio
Vf/Vo
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Current
ratio
If/Io
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Voltage shunt feedback amplifier
• A resistor is connected between the collector and
base of the transistor.
• The output voltage Vo is much greater than the input
voltage Vi and is 180 degrees out of phase with Vi.
• The portion of output is connected through the
feedback resistor Rf to the base.
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Voltage shunt feedback amplifier
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Hence:
• The feedback current If=(Vi-V0)/Rf ~V0/Rf = β V0.
• Feedback factor β = -1/Rf.
• The feedback current is proportional to the output
voltage, this circuit is an example of voltage-shunt
feedback amplifier.
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Current series feedback
• The feedback signal is the voltage Vf across RE and the
sampled
signal is the load current Io.
• The feedback signal Vf= Io RE.
• The feedback voltage is directly proportional to the output
current.
• The feedback factor b = Vf/Io= -Io.RE/Io= -RE.
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Current series feedback
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Current shunt feedback
• A two transistor CE amplifier with feedback from the
second emitter to the base of first through the resistor Rf.
• The input current is the difference of the current at the
base of transistor due to Vs and the current If.
• This is smaller than the magnitude of current without
feedback.
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Current shunt feedback
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• Feedback signal If =RE Io/ (Rf + RE.)
• Feedback factor β = If/Io = RE/(Rf +RE.)
If,
Rf >>RE
b = RE/Rf
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Voltage series feedback
• Common collector amplifier is also known as emitter
follower.
• The output voltage Vo is developed across the emitter
resistor RE and is fedback to the input in series.
• The feedback factor b = Vf/Vo = 1.
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Voltage series feedback
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Comparison of different feedback amplifiers
S .No Characteristics
Current
series
Current
shunt
Stability of transfer Improves Av Improves
gain
Rm
Improves
Improves Ai
2
Frequency & phase decreases
distortion
decreases
decreases
decreases
3
Nonlinear
distortion
decreases
decreases
decreases
decreases
4
Noise
decreases
decreases
decreases
decreases
5
Bandwidth
increases
increases
increases
increases
6
Output resistance
decreases
decreases
increases
increases
7
Input resistance
increases
decreases
increases
decreases
1
Voltage
series
Voltage
shunt
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Voltage amplifiers
• The primary function of a voltage amplifier is to raise the
voltage level of the signal.
• The voltage amplifiers are designed to achieve
maximum voltage amplification.
• Small signal amplifiers give large output voltage taking
small input signal amplitudes.
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Voltage amplifiers
• The first few stages in the multistage amplifier are used
to achieve only voltage amplification.
• Only very little power can be drawn from its output.
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Power amplifiers
• A transistor amplifier which raises the power level of the
signals is known as transistor power amplifier.
• This amplifier can feed large amount of power to the
load.
• Power amplifiers are also called large signal amplifiers.
• In general the last stage of multi stage amplifier is the
power amplifier.
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Differences between power and
voltage amplifiers
S.No Voltage amplifiers
Power amplifiers
1
These are small signal
amplifiers
These are large signal
amplifiers
2
Power output is small
Voltage output is small
3
The transistor with high β
is used
The transistor with low ß is
used
4
Collector current is less
Collector current is high
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5
Input signal voltages are very
low in magnitude
Input signal voltages are
high in magnitude
6
R-C coupling is used
Transformer coupling is
used
7
The devices used are low
current devices with large
output resistance
Cascading and cascoding is
used
The devices have to supply
large currents and are of
low output resistance
Push pull complimentary
symmetry and Darlington
circuits are used
Considerable heat is
generated. Cooling
arrangements are required
8
9
Heat generated is very less. No
cooling arrangements are
required
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Differences between power
and voltage amplifiers
S.No Voltage amplifiers
Power amplifiers
10
Output must be free from
distortion
Reasonable distortion is
permitted.
11
Their operation is restricted Non linear operation is also
to linear portion.
allowed
12
Used as preamplifier
Used in output stage
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The characteristics that make an amplifier as voltage
amplifier are:
• Higher value of β (>100) – thin base.
• Low input resistance.
• High collector load resistance.
•
Low collector current.
• R-C coupling.
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The characteristics that make an amplifier as power
amplifier are:
• Thick base i.e., smaller value of ß.
• Collector current is high.
• Low collector load resistance.
• Transformer coupling.
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Power Amplifiers
•
Converts DC power from supply into AC power.
• They are large signal amplifiers.
• Larger portion of load line is used during signal operation
than small signal amplifiers.
• Classification is based on percentage of input cycle for
which amplifier operates in its linear region.
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Classification of Amplifiers based on
Frequency
• Audio frequency or AF power amplifiers used in the
audio range i.e. 20 Hz to 20 KHz.
Example : Public Address System etc.
• Radio frequency or RF power amplifiers used in the
radio frequency range from 30KHz to 3MHz.
Example : Radio receiver etc.
• Video frequency or VF power amplifiers used in the
video frequency range > 3MHz.
Example : TV receiver etc.
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Classification according to Period
of Conduction
• Class A power amplifier.
Conduction angle θ = 360o i.e. full cycle
• Class B power amplifier.
Conduction angle θ = 180o i.e. half cycle
• Class C power amplifier.
Conduction angle θ <180o i.e. less than half cycle
• Class AB power amplifier.
Conduction angle 360o< θ >180o i.e. more than half cycle
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Class A Power Amplifier
The period of conduction is 3600
(Full Cycle)
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Class B Power Amplifier
• Period of conduction is 1800 (Half Cycle)
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Class C Power Amplifier
• Period of Conduction is less than 1800
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Class AB Power Amplifier
• Period of Conduction is greater than 180, less than
3600.
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Classification based on Configuration
• CE Amplifier.
Uses CE configuration
• CB Amplifier.
Uses CB configuration
• CC Amplifier.
Uses CC configuration
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Class b power amp
• An Amplifier is biased at cut-off (class-B)
• We can get more output power for a given amount of
input power
• It is more difficult to implement the circuit in order to get
a linear reproduction of the input waveform.
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Class-B push-pull Amplifier
Fig.27.1
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Construction
• Two Transistors are connected in push-pull
arrangement.
• It consists of two centre-tapped transformers T1&T2 and
two identical transistors Q1&Q2.
• Transformer T1 is secondary centre tapped input
transformer and is a phase splitter
• It is required to produce two signal voltages that are 1800
out of phase.
• T2 is an output primary center tapped transformer.
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Construction
• T2 is required to couple the a.c. output signal from
the collector to the load.
• Transistor Q1 and Q2 are biased at cutoff.
• To get a balanced circuit, two emitters are
connected to the centre tap of T1 and Vcc supply to
the centre tap of T2.
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Operation
• With no input signal transistors Q1 & Q2 are cut off.
• No current is drawn from the Vcc supply.
• Input signal is applied through center tapped
transformer.
• The inputs supplied to the bases of two transistors are
out of phase.
• Thus the two transistors conduct on alternate half cycles
of the input signal.
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Operation
• Current for each transistor flows opposite directions
through Secondary of T2 transformer.
• The magnetic flux set up by each currents result in
opposite flux.
• Net flux is Zero.
• Transformer is not required to handle a large flux due to
d.c. currents.
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Operation
• During positive half cycle of the input signal, base of Q1 is
positive and Q2 is negative.
• Q1 conducts and Q2 is off.
• During negative half cycle of the input signal, base of Q1 is
negative and Q2 is positive.
• Q1 is off and Q2 conducts.
• Each transistor handles one half of the input signal.
• Output transformer joins these two halves and produces full
sine wave.
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Advantages of Class B push – pull power
amplifier
• Possible to obtain greater power output.
• Efficiency is higher (78.5%)
• Negligible power loss at no signal.
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Disadvantages
• Higher harmonic distortion.
• Self – bias cannot be used.
• Supply voltages must have good regulation.
• Transformers are bulky and expensive.
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Class-B Operation
Vi
n
t0
t1
Av
t2
Vout
t0
t1
Fig 11.1
In class-B operation collector current flows only 1800.(Half cycle).
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Class-B operation
• More output power is obtained than class-A amplifier.
• The transistor dissipates no power with zero input signal.
• Overall efficiency is 75%.
• Power dissipated by the transistor is less in class-B
Amplifier.
• Average current drawn by the circuit is smaller than that
of class-A amplifier.
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Q-point location for class-B operation
Fig 11.2
The Q-point is at cut off.
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Q-point location for class-B operation
• Class-B amplifier is biased at the cut off point, Icq=0 and
Vceq=Vcc
• It is brought out of cutoff and operates in its linear region
when input signal drives the transistor into conduction.
• The output is not a replica of the input.
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Max value of A.C. o/p power in class-B Amplifier
• Location of a Q-point for class-B amplifier along with
maximum variation of Vce and Ic with largest possible signal.
• In a class-B amplifier power is developed only during one half
cycle of the input signal.
• PO(ac)=1/2[VCP/√2].[ICP/√2]=1/(4*VCP.ICP).
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Advantages of push-Pull amplifier
• Circuit efficiency of a class-B push-pull amplifier is 78.5%.
• No power is drawn from the d.c. supply under no signal
condition.
• Eliminates even order harmonics in a.c. output signal.
• Due to the absence of even harmonics the circuit gives
more output per device, for a given amount of distortion.
• No d.c. component in the out put signal.
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Applications of Power Amp
•
•
•
•
•
•
The AF power amplifiers are used as output
stages in the audio equipment such as
Radio receiver.
Tape recorder.
TV receiver.
Public address system.
Stereo system.
Sound system in cinema theatre.
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Applications:
Audio frequency power amplifiers are used in
• Telephone circuits.
• Repeater circuits.
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Applications:
Radio frequency power amplifiers are used at
the output of broadcast transmitters for feeding
large power to the antenna.
•
•
•
•
Radio transmitter.
TV transmitter.
Radar transmitter.
Satellite transmitter.
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Applications:
Power amplifiers are also used in
• Industrial electronic equipment.
• Medical electronic equipment.
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