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ELECTRONICDEVICES
&
CIRCUITS
Prepared By-
Prof. Pramod R. Bokde
Assistant Professor,
Persuing Ph.D. (Biomedical Engg.), M.Tech(VLSI),
AMIE, LMISTE
2011
SMT .BHAGWATI CHATURVEDICOLLEGE OF ENGINEERING, NAGPUR
DEPARTMENT OF ELECTRONICS
ENGINEERING
[[[[# FEEDBACK# FEEDBACK# FEEDBACK# FEEDBACK
AMPLIFIERSAMPLIFIERSAMPLIFIERSAMPLIFIERS]]]]
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FEEDBACK AMPLIFIERS UNIT - 5
2 Prepared By Prof. P.R. Bokde, Assistant Professor,
Smt. Bhagwati Chaturvedi College of Engineering, Nagpur
QUE (1) : What is feedback? Define Negative feedback and Positive feedback.
Draw Block Schematic of amplifier with negative feedback. What type of
feedbacks are used in amplifier and oscillator?
Ans : Feedback :
When part of the output is sampled and fed back to the input of the amplifier,
then it is calledfeedback amplifier. Therefore, at the input of amplifier we have two
signals : input signal and part of the output signal which is fed back to the input. Both
these signals may be in phase or out of phase.
Positive Feedback : When input signal and part of output signal are in phase,
the feedback signal is calledPositive Feedback.
Negative Feedback: When input signal and part of output signal are out of
phase , the feedback is calledNegative Feedback.
Fig : Block Schematic of Negative Feedback Amplifier
Amplifiers Use Negative Feedback whereas Oscillators use Positive Feedback.
QUE(2) : Obtain the equation for overall voltage gain with negative feedback of
negative feedback amplifier.
Ans : In a negative feedback amplifier a small portion of the output voltage is feedback
to the input. When the feedback voltage is applied in series with the source signal,
then the process is called series voltage feedback.
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Smt. Bhagwati Chaturvedi College of Engineering, Nagpur
The gain of the closed loop amplifier is given by,..(1)
From fig, the input voltage Vin to the amplifier will be difference between Vs andVf. Hence this type of feedback is known as negative feedback.
i.e
(2)
The gain of the feedback network is ,
..(3)
Substituting equation (3) in equation (2), we get
(4)
Substituting equation (4) in equation (1) , We get,
..(5)
Dividing equation (5) numerator and denominator by Vi, we get ,
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Smt. Bhagwati Chaturvedi College of Engineering, Nagpur
Where, AV = Vo/ Vi (open loop gain)
Thus negative feedback reduces the gain of the amplifier.
QUE(3) : Show Block schematically the different feedback connections in an
amplifier. Explain the effect of each type of feedback on input and output
impedance.
Ans : There are four different topologies for feedback network amplifiers.
(1)Voltage Series feedback(2)Voltage Shunt Feedback(3)Current Series feedback(4) Current Shunt feedback
VOLTAGE SERIES FEEDBACK (VOLTAGE AMPLIFIER):
It is also called as shunt derived series fed feedback. The amplifier and feedback circuit
are connected series parallel. Here the fraction of the output voltage is applied in series
with the input voltage via the feedback. The input to the feedback network is in
parallel with the output of the amplifier. Therefore output resistance of the amplifier is
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FEEDBACK AMPLIFIERS UNIT - 5
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Smt. Bhagwati Chaturvedi College of Engineering, Nagpur
reduced by shunting effect of the input to the feedback network.
Similarly, Vi sees two circuit elements in series , hence the input resistance of theamplifier as a whole is increased due to feedback. Therefore,
Series impedance always increases the input impedance by a factor of (1+ A)
VOLTAGE SHUNT FEEDBACK (TRANS-RESISTANCE AMPLIFIER):
It is also known as shunt derived shunt fed feedback. It is parallel-parallel
prototype. Here a small portion of output voltage is coupled back to the input voltagein parallel (Shunt).
Since the feedback network shunts both the input and output of the amplifier, it
decreases both its output and input impedance by a factor 1 / (1+ A). A shunt
feedback always decreases input impedance.
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Smt. Bhagwati Chaturvedi College of Engineering, Nagpur
CURRENT SERIES FEEDBACK(TRANSCONDUCTANCE AMPLIFIER)
It is also known as series derived series fed feedback. It is a series-series prototype.
Here a part of the output current is made to feedback a proportional current in series
with the input since it is a series pick-up and series feedback, both the input and
output impedances of the amplifier are increased due to feedback.
CURRENT SHUNT FEEDBACK(CURRENT AMPLIFIER
It is also called as series-derived shunt fed feedback. It is a parallel-series
prototype. Here the feedback network picks up a part of the output current and
develops a feedback voltage in parallel (shunt) with the input voltage . Feedback
network shunts the input , but it is in series with the output , hence the output
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Smt. Bhagwati Chaturvedi College of Engineering, Nagpur
resistance of the amplifier is increased , whereas its input resistance is decreased by a
factor of loop gain.
QUE(4) : What are the advantages of negative feedback? Assess analytically the
effect of negative feedback on Ri and Ro of voltage series and current shunt
feedback. (R.T.M.N. U. Winter 2005)
Ans : For advantages of negative feedback, please refer question no. 6.
Effect of negative feedback on Ri and Ro of voltage series feedback.
Input Resistance :
The voltage series feedback topology is shown in fig. With amplifier is replaced
by Thevenins model. Here, Av represents the open circuit voltage gain taking Rs into
account .
The input resistance with feedback is given as-
(1)
Applying KVL to the input side, we get,
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Smt. Bhagwati Chaturvedi College of Engineering, Nagpur
.(2)
The output voltage Vo is given as,
(3)
Where
Substituting value of Vo from equation (3) in equation (2) we get,
Therefore, the input impedance with negative feedback increases.
Output Resistance :
In this topology, the output resistance can be measured by shorting the input source Vs
= 0 and looking into the output terminals with RL disconnected, as shown in fig.
Applying KVL to the output side, we get,
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9 Prepared By Prof. P.R. Bokde, Assistant Professor,
Smt. Bhagwati Chaturvedi College of Engineering, Nagpur
..(1)
The input voltage is given as , (2)
Substituting equation (2) in equation (1) we get,
Thereore, the output impedance with negative feedback decreases.
Effect of negative feedback on Ri and Ro of Current Shunt feedback.
Input Resistance :
The current shunt feedback topology is shown in fig, with amplifier input and output
circuit replaced by Nortons equivalent circuit.Applying KCL to the input node we get,
(1)
The output current Io is given as,
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10 Prepared By Prof. P.R. Bokde, Assistant Professor,
Smt. Bhagwati Chaturvedi College of Engineering, Nagpur
..(2)
Where ,
Substituting equation (2) in equation (1), we get,
The input resistance with feedback is given as,
Therefore , input resistance of current shunt feedback with negative feedback
decreases.
Output Resistance :
In this topology, the output resistance can be measured by open circuiting the input
source Is = 0 and looking into output terminals with RL disconnected, as shown in fig.
Applying the KCL to the output node we get,
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Smt. Bhagwati Chaturvedi College of Engineering, Nagpur
.(1)
The input current is given as,
.(2)
Substituting equation (2) in equation (1) we get,
Therefore the output resistance of current shunt feedback amplifier increases with
negative feedback/
QUE(5) : Compare all types of negative feedback.
Ans : The following table gives the comparison of all types of negative feedback.
Parameter Voltage Series Current Series Current Shunt Voltage Shunt
Gain with
feedbackDecreases Decreases Decreases Decreases
Stability Improves Improves Improves Improves
Frequency
ResponseImproves Improves Improves Improves
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Smt. Bhagwati Chaturvedi College of Engineering, Nagpur
Frequency
distortionReduces Reduces Reduces Reduces
Noise and
non-linear
Distortion
Reduces Reduces Reduces Reduces
Input
resistance
Increases Increases
Decreases Decreases
Output
Resistance
Decreases Increases Increases Decreases
Bandwidth Increases Increases Increases Increases
QUE (6): Discuss the advantages and Disadvantage of negative feedback amplifier
ANS : It is possible to improve important characteristics of four basic amplifier types
by the proper use of negative feedback.
The following are the advantages of negative feedback :
(1)Increased Input impedance : Normally high input resistance of voltageamplifier can be made higher by a factor (1 + A).
(2)Reduced Output Impedance : Normally low output resistance of a voltageamplifier can be lowered by a factor (1 + A).
(3)Gain Stability : The transfer gain Afof the amplifier with feedback can bestabilized.
(4) Increased Bandwidth : The proper use of negative feedback improvesfrequency response of the amplifier by a factor (1 + A).
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Smt. Bhagwati Chaturvedi College of Engineering, Nagpur
(5)Reduced Non-linear distortion : There is a significant improvement in thelinearity of operation of the feedback amplifier compared with that of the
amplifier without feedback.
(6)Reduced Noise : Noise voltage is reduced by factor (1 + A).(7)Reduced Amplitude distortion(8)Reduced Frequency distortion
(8)Reduced phase distortion.Disadvantage :
(1)The transfer gain Af of the amplifier with feedback is reduced in comparisonwith the transfer gain A of an amplifier without feedback by a factor (1 + A).
(2)The negative feedback amplifier designed for a particular frequency range maybreak out into oscillation at some high or low frequency.
QUE(7) : Explain the statement : Negative feedback increases the stability.
ANS : We know that , for negative feedback the gain of the feedback amplifier is givenby,
Where, Afis the gain of feedback amplifier and A is the gain of amplifier without
feedback i.e. open loop gain. Now , if we select, A >> 1, then,
This means that Af(gain of amplifier with feedback) depends only on thefeedback network i.e. depends only on feedback factor , so it is stable.
However A voltage gain without feedback normally depends on hfe, transistor to
transistor replacement, aging etc, whereas feedback network is usually made of stable
elements such as resistors, capacitors, inductors etc, hence is stable.
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Smt. Bhagwati Chaturvedi College of Engineering, Nagpur
QUE(8) : Explain the effect of Negative feedback on Bandwidth.
ANS : Bandwidth is the range of frequency over which the amplifier gives faithful
amplification. Let B be the bandwidth of an amplifier without feedback and let fL and fH
be lower and upper cut-off frequencies of an amplifier without negative feedback.Therefore the 3 dB bandwidth without feedback is given by-
..(1)
Let (BW)fbe the bandwidth of an amplifier with negative feedback. Also let fLfand fHf
be the lower and higher cut-off frequencies with negative feedback. Therefore the 3 dB
bandwidth with negative feedback is-
..(2)
It can be proved that with negative feedback, the lower cut-off and upper cut-off
frequencies of an amplifier are :
.(3)
This shows that the lower cut-off frequency with feedback is smaller by a factor
1 / (1 + A)
And ) ..(4)
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Smt. Bhagwati Chaturvedi College of Engineering, Nagpur
This shows that the upper cut-off frequency is greater than fH by a factor 1 / (1 + A).
The gain bandwidth product is constant for an amplifier with or without feedback.
Let A and Afbe the voltage gain of an amplifier without and with feedback, then wecan write,
.(5)
But ,
..(6)
This shows that the negative feedback in amplifier increases the bandwidth. This
improves the frequency response of an amplifier.
QUE(9) : Explain in brief the comparison between negative feedback voltage and
current amplifiers as far as the impedance level and gain is concerned. Draw the
block schematic for above mentioned amplifiers.
ANS : For figures , refer question no.(2)
S.N. Negative feedback voltage amplifier Negative feedback current amplifier
1
Input impedance increases by a factor
(1+ A) if feedback is voltage series
type.
Input impedance increases by a factor
(1+ A) if feedback is current series
type.
2
Input impedance decreases by a factor
1 /(1+ A) if feedback is voltage shunttype.
Input impedance decreases by a factor
1 /(1+ A) if feedback is current shunttype.
3
Output impedance decreases by a
factor 1 /(1+ A) if feed back is voltage
series type.
Output impedance increases by a
factor (1+ A) if feed back is current
series type.
4 Output impedance decreases by a
factor 1 /(1+ A) if feed back is voltage
Output impedance increases by a
factor (1+ A) if feed back is current
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shunt type. Shunt type.
5
Voltage gain decreases by a factor
1 /(1+ A), if the feedback is series
voltage type.
Voltage gain decreases by a factor
1 /(1+ A), if the feedback is series type.
6Voltage gain is not changed if the
feedback is shunt type.
Voltage gain is not changed if the
feedback is shunt type.
7Current gain is not changed if feedback
is series type.
Current gain is not changed if feedback
is series type.
8Current gain decreases by a factor
1 /(1+ A)
Current gain decreases by a factor
1 /(1+ A)
QUE(10) : Explain the effect of negative feedback on frequency distortion and
Noise & Non-linear distortion .
ANS :
(1) Effect on Frequency Distortion : We know that if the feedback networkdoes not contain reactive elements, the overall gain is not a function of
frequency. Under such conditions frequency and phase distortion is
substantially reduced.
If is made up of reactive components, the reactances of these
components will change with frequency, changing the . As a result, gain
will also change with frequency.
(2) Effect on Noise & Non-Linear Distortion : Signal feedback reduces theamount of noise signal and non-linear distortion. The factor (1+A) reduces
both input noise and resulting non-linear distortion for considerable
improvement. Thus noise and non-linear distortion also reduced by same
factor of gain.
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Smt. Bhagwati Chaturvedi College of Engineering, Nagpur
QUE(11) : Give the classification of feedback amplifiers .
ANS :
CLASSIFICATION OF AMPLIFIERS :The amplifiers can be classified into four broad categories :
(1)Voltage Amplifier(2)Current Amplifier(3)Trans-conductance Amplifier(4) Trans-resistance Amplifier
VOTLAGE AMPLIFIERS :
Fig shows a Thevinins equivalent circuit of an amplifier.
If the amplifier input resistance Ri is large compared with the source resistance
Rs then Vi = Vs. If the external load resistance RL is large compared with the output
resistance Ro of the amplifier then Vo = Av Vi = Av Vs. Such amplifier circuit provides a
voltage output proportional to the voltage input and the proportionality factor does
not depend on the magnitudes of the source and load resistances. Hence, this amplifieris called voltage amplifier. An ideal voltage amplifier must have infinite input
resistance Ri and zero output resistance Ro. For practical voltage amplifier we must
have Ri >> Ro.
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CURRENT AMPLIFIER :
Current shows Nortons equivalent circuit of a current amplifier.
If amplifier input resistance Ri = 0, then Ii = Is. If amplifier output resistance Ro = ,then IL = Ai Ii. Such amplifier provides a current output proportional to the signal
current, and the proportionality factor is independent of source and load resistances.
This amplifier is called current amplifier. An ideal current amplifier must have zero
input resistance Ri and infinite output resistance Ro. For practical current amplifier we
must have Ri > RL.
TRANSCONDUCTANCE AMPLIFIER :
Fig .shows a transconductance amplifier with a Thevinins equivalent in its input
circuit and Nortons equivalent in its output circuit.
In this amplifier an output current is proportional to the input signal voltage and
the proportionality factor is independent of the magnitudes of the source and load
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resistances. Ideally , this amplifier must have an infinite input resistance Ri and infinite
output resistance Ro. For practical transconductance amplifier we must have Ri >> Rs
and Ro >> RL.
TRANSRESISTANCE AMPLIFIER:
Fig shows a trans-resistance amplifier with a Nortons equivalent in its input
circuit and a Thevinins equivalent in its output circuit.
In this amplifier an output voltage is proportional to the input signal current and
the proportionality factor is independent on the source and load resistances. Ideally,
this amplifier must have zero input resistance Ri and zero output resistance Ro. For
practical trans-resistance amplifier we must have Ri