Analog & Digital ElectronicsCourse No: PH-218

Lec-14: Feedback Amplifier

Course Instructors:

� Dr. A. P. VAJPEYI

Department of Physics,

Indian Institute of Technology Guwahati, India 1

Marks distribution: Quiz-2

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5

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Marks

Students

2

0

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Highest: 07 ( 1 student)

Lowest: 00 ( 5 students)

Feedback Amplifier

� Feedback amplifier contains two component namely feedback circuitand amplifier circuit.

� Feedback circuit is essentially a potential divider consisting ofresistances R1 & R2

� The purpose of feedback circuit is to return a fraction of the outputvoltage to the input of the amplifier circuit.

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Feedback circuit Feedback amplifier

21

2

1

2

RR

R

V

V

+==β

feedback fraction β

� Both negative feedback and positive feedback are used in amplifiercircuits.

� Negative feedback returns part of the output to oppose theinput, whereas in positive feedback the feedback signal aids the inputsignal.

Feedback

If Vf = 0 (there is no feedback)

i

o

S

o

V

V

V

VA ==

44

Feedback amplifier

iS VV

)( fsio VVAAVV −== of VV β=

If feedback signal Vf is connected in

series with the input, then Vi = VS- Vf

But

)( oso VVAV β−=

)1( A

A

V

VA

s

of

β+==

so AVAV =+ )1( β

Af : closed-loop gain of the amplifierA: Open-loop gain of the amplifier gain

� Negative feedback can reduce the gain of the amplifier, but it has many

advantages, such as gain stabilization, reduction of nonlinear distortion

and noise, control of input and output impedances, and extension of

bandwidth.

Advantages of Negative feedback

For negative feedback: βA > 0 ; For positive feedback: βA < 0

Feedback

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bandwidth.

� Gain stabilization

)1( A

AAf

β+=

2)1(

1

AA

dAf

β+=

A

dA

AA

dA

f

f

)1(

1

β+=

Therefore percentage change in Af (due to variations in some circuit

parameter) is reduced by (1+βA) times compared to without feedback.

Advantages of Negative feedback

� Reduction of nonlinear distortion

(a) Open loop gain

(b) Closed loop gain

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� Bandwidth extension

Basic Feedback Topologies

� Depending on the input signal (voltage or current) to be amplified

and form of the output (voltage or current), amplifiers can be classified into four categories. Depending on the amplifier category, one of four types of feedback structures should be used.

� Voltage series feedback (Af = Vo/Vs) – Voltage amplifier

� Voltage shunt feedback (Af = Vo/Is) – Trans-resistance amplifier

� Current series feedback (Af = Io/Vs) - Trans-conductance amplifier

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f o s

� Current shunt feedback (Af = Io/Is) - Current amplifier

� Here voltage refers to connecting the output voltage as input to thefeedback network. Similarly current refers to connecting the output current

as input to the feedback network.

�Series refers to connecting the feedback signal in series with the input

voltage; Shunt refers to connecting the feedback signal in shunt (parallel)with an input current source.

Feedback topologies

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Feedback topologies: Voltage Shunt Feedback

� Output resistance of the

amplifier and output resistance

of the feedback circuit are inparallel hence effective outputresistance of the feedback

amplifier will reduce.

�Similarly overall inputresistance of the feedback

amplifier will reduce due toparallel connection of amplifier

� Since effective input resistance is small hence input should be a current. �(for ideal voltage source – input resistance is very high compare to internal source resistance, if not then, lot of voltage will be dropped at internal

source resistance and voltage source won’t be a ideal voltage source )

� Effective output resistance is also small compare to the resistance of

amplifier without feedback hence less voltage will drop at Ro-eff and most of the voltage occurs at RL. Hence output ckt will behave like a voltage source. Thus voltage shunt feedback ckt behave like a current controlled voltage source.

parallel connection of amplifierand feedback resistor.

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Feedback topologies: Voltage Shunt Feedback

Voltage Gain :

)1

(

)1(

)(

)(

A

A

I

VA

VAAI

VVIA

VI

IIAiIAV

of

oS

ooS

of

fSo

β

β

β

β

+==

+=

=−

⋅=

−=⋅=

10

1 AIS

fβ+

Input Impedance :

)1(

)

in

in

A

rZ

AII

rI

VI

rI

II

V

I

VZ

i

ii

ii

oi

ii

fi

i

S

i

β

ββ

+=

+

⋅=

+

⋅=

+==

Output Impedance :

o

oo

o

ioo

ofi

o

oV

r

AVV

r

AIVI

VII

I

VZ

S

β

β

+=

−=

−=−=

==

port,output from

port,input from

| 0out

)1(out

A

r

I

VZ o

o

o

β+==

Feedback topologies: Voltage Series Feedback

� Output resistance of the amplifier and output resistance of the feedbackcircuit are in parallel hence effective output resistance of the feedback

amplifier will reduce.

�Input resistance of the amplifier and feedback network are in series

hence effective input resistance will increase.

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Feedback topologies: Voltage Series Feedback

Voltage Gain :

)1

(

)1(

)(

)(

A

A

V

VA

VAAV

VVVA

VV

VVAiVAV

of

oS

ooS

of

fSo

β

β

β

β

+==

+=

=−

⋅=

−=⋅=

Input Impedance :

)1()1(

in

in

ArI

AVZ

I

AVV

I

VV

I

VV

I

VZ

i

S

i

S

ii

S

oi

S

fi

S

S

ββ

ββ

+=+

=

+=

+=

+==

Output Impedance :

β

β

β

⋅⋅+

⋅−=

==⋅+

⋅−=

==

VAV

VV

VVV

r

VAVI

I

VZ

oi

Soi

o

ioo

o

oVS

0

| 0out

1212

1 AVS

fβ+

IS

β

β

⋅+==

⋅⋅+=

A

r

I

VZ

r

VAVI

o

o

o

o

ooo

1out

Feedback topologies: Current Series Feedback

� Output resistance of the amplifier and output resistance of the feedback

circuit are in series hence effective output resistance of the feedback amplifierwill increase.

�Input resistance of the amplifier and feedback network are in series henceeffective input resistance will increase.

� Thus Current series feedback circuit behave like a voltage controlled currentsource. 13

+

Basic amplifierIS

+

−Vi ri

Vf=βIo

ro

AVi

Iο

VS

Vο+−

)1

(

)1(

)(

)(

A

A

V

IA

IAAV

IIVA

IV

VVAiVAI

S

of

oS

ooS

of

fSo

β

β

β

β

+==

+=

=−

⋅=

−=⋅=

Voltage Gain :

Feedback topologies: Current Series Feedback

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+−

Feedback network

Vf=βIo 1 AVS β+

Input Impedance :

)1()1(

in

in

ArI

AVZ

I

AVV

I

VV

I

VV

I

VZ

i

S

i

S

ii

S

oi

S

fi

S

S

ββ

ββ

+=+

=

+=

+=

+==

Output Impedance :

o

ioo

o

oV

r

VAVI

I

VZ

S

⋅−=== ;| 0out

β⋅+==

A

r

I

VZ o

o

o

1out

o

ooo

Sfi

r

IAVI

VVV

⋅⋅+=

==+

β

0

Feedback topologies: Current Shunt Feedback

�Effective output resistance of the feedback amplifier will increase.

�Effective input resistance will decrease.

� Thus current shunt feedback circuit behave like a current controlledcurrent source.

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