amplifier feedback

8/17/2019 Amplifier Feedback

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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


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Marks distribution: Quiz-2

3

4

5

6

7

8

Marks

Students

2

0

1

2

1 2 3 4 5 6 7 8 9 10 11 12

Highest: 07 ( 1 student)

Lowest: 00 ( 5 students)


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Feedback Amplifier

Feedback amplifier contains two component namely feedback circuitand amplifier circuit.

Feedback circuit is essentially a potential divider consisting of

resistances R1 & R2

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

3

Feedback circuitFeedback amplifier

21

2

1

2

R R

R

V

V

+

== β

feedback fraction β


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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)

oo V V A ==

44

Feedback amplifier

iS

)( f sio V V A AV V −== o f V V β =

If feedback signal Vf is connected inseries with the input, then Vi = VS- Vf

But

)( oso V V AV β −=

)1( A

A

V

V A

s

o f

β +==

so AV AV =+ )1( β

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


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

advantages, such as gain stabilization, reduction of nonlinear distortionand noise, control of input and output impedances, and extension of

Advantages of Negative feedback

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

Feedback

5

Gain stabilization

)1( A

A A f

β +

=2

)1(

1

A A

dA f

β +

= A

dA

A A

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.


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Advantages of Negative feedback

Reduction of nonlinear distortion

(a) Open loop gain

(b) Closed loop gain

6

Bandwidth extension


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Basic Feedback Topologies

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

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

Voltage series feedback (Af = V o /V s ) – Voltage amplifier

Voltage shunt feedback (Af = V o /I s ) – Trans-resistance amplifier

Current series feedback (Af = I o /V s ) - Trans-conductance amplifier

7

Current shunt feedback (Af = I o /I s ) - 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 inputvoltage; Shunt refers to connecting the feedback signal in shunt (parallel)with an input current source.


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Feedback topologies

8


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Feedback topologies: Voltage Shunt Feedback

Output resistance of theamplifier and output resistance

of the feedback circuit are inparallel hence effective outputresistance of the feedbackamplifier will reduce.Similarly overall inputresistance of the feedback

amplifier will reduce due to

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 internalsource 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 ofthe 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.

para e connec on o amp erand feedback resistor.

9


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Feedback topologies: Voltage Shunt Feedback

Voltage Gain :

)(

)1(

)(

)(

AV A

V A AI

V V I A

V I

I I Ai I AV

o

oS

ooS

o f

f S o

β β

β

==

+=

=−

⋅=

−=⋅=

10

S

Input Impedance :

)1(

)

in

in

A

r Z

AI I

r I

V I

r I

I I

V

I

V Z

i

ii

ii

oi

ii

f i

i

S

i

β

β β

+

=

+

⋅=

+

⋅=

+

==

Output Impedance :

o

oo

o

io

o

o f i

o

oV

r

AV V

r

AI V

I

V I I

I

V Z

S

β

β

+=

−=

−=−=

==

port,outputfrom

port,inputfrom

| 0out

)1(out

Ar

I V Z o

o

o

β +==


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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 serieshence effective input resistance will increase.

11


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Feedback topologies: Voltage Series Feedback

Voltage Gain :

)(

)1(

)(

)(

AV A

V A AV

V V V A

V V

V V AiV AV

o

oS

ooS

o f

f S o

β β

β

==

+=

=−

⋅=

−=⋅=

Input Impedance :

)1()1(

in

in

Ar I

AV Z

I AV V

I V V

I

V V

I

V Z

ii

S

ii

S

oi

S

f i

S

S

β β

β β

+=+

=

+

=

+

=

+

==

Output Impedance :

β

β

⋅−=

==⋅+

⋅−=

==

V V

V V V

r

V AV I

I

V Z

oi

S oi

o

io

o

o

oV S

0

| 0out

1212

S

β ⋅+==

⋅⋅+

=

A

r

I

V Z

r I

o

o

o

o

ooo

1out


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Feedback topologies: Current Series Feedback

Output resistance of the amplifier and output resistance of the feedbackcircuit 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 current

source. 13


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Basic amplifier I S

+

−

V i r i

= o

r o

AV i

I ο

V S

V ο+−

)(

)1()(

)(

A I A

I A AV I I V A

I V

V V AiV A I

o f

oS

ooS

o f

f S o

β β

β

==

+=

=−

⋅=

−=⋅=

Voltage Gain :

Feedback topologies: Current Series Feedback

14

+−

Feedback network

S

Input Impedance :

)1()1(

in

in

Ar

I

AV Z

I

AV V

I

V V

I

V V

I

V Z

i

S

i

S

ii

S

oi

S

f i

S

S

β β

β β

+=+

=

+=

+=

+

==

Output Impedance :

o

ioo

o

oV

r

V AV I

I

V Z

S

⋅−==

=;| 0out

β ⋅+==

A

r

I

V Z o

o

o

1out

o

ooo

S f i

r

I AV I

V V V

⋅⋅+=

==+

β

0


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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 controlled

current source.

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amplifier feedback

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