9AEI303.33 1 1 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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9AEI303.33 1 1
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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• decreases the gain.• decreases the distortion.• decreases the noise.
Disadvantages of negative feed back
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 back1800out of phase with the input signal.
Decreases
Decreases
Decreases
Improved
Amplifiers
Positive feed backIn phase with the input signal
Increases
Increases
Increases
Poor
oscillators
Feed back signal
Net input signal
Gain
Noise
Stability
Uses
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Principle of feed back.
Basic amplifier A
Mixing net work
Sampling network
Feed back network.
Rs
vs Vi V VoIoI
RL
Vf
+
-
++
--
Ii
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 Sampled Signal is Current
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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.
SERIES FEEDBACK SHUNT FEEDBACK
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Gain of negative feed back amplifier
A
mixer
A gain of amplifier without feedback Xi difference signal
Xs input signal. Af feed back amplifier
Xo output signal gain.
Xf feed back signal
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Negative feed back is applied by feeding the fraction of the output voltage Xo 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.)
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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
• Voltage series feedback amplifier.• Current series feedback amplifier.• Current shunt feedback amplifier.• Voltage shunt feedback amplifier.
Negative feed back amplifiers are classified into:-
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Voltage series feedback amplifier
Basic amplifi
er A
Vs
Feed back
Vf
Vi Vo RL Vo
This connection increases the input resistance and decreases the output resistance .
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Current series feedback amplifier
This connection increases both input resistance and output resistance
Basic amplifi
er A
VsVi RL
Vf
Feed back
Io Io
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Current shunt feedback amplifier
This connection decreases input resistance and increases the output resistance.
Basic amplifi
er A
Feed back
Io RLIi
If
IoIs
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Voltage shunt feedback amplifier
This connection decreases both input resistance and output resistance.
Feed back
RL
Basic amplifi
er A
Vo Vo
Is Ii
If
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Signals in feedback amplifierssignal Type of feedback
Voltage series
Current series
Current shunt
Voltage shunt
Output signal
Input signal
Feedback signalDifferent signalGain A
Feedback factor
Voltage Voltage
Voltage
Voltage
Voltage
Voltage
Voltage
Voltage
Current Current
Current
Current Current
Current
Current
Current
Voltage gain Av
Trans conductance Gm
Trans resistance Rm
Current gain Ai
Voltage ratio Vf/Vo
Current ratio If/Io
Resistance Vf/Io
Conductance If/Vo
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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 = 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
= 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.