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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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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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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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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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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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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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.(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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..(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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..(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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.(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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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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(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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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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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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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

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