feedback amplifier -...

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Feedback Amplifier

• General Feedback structure• Negative Feedback• Feedback Topology• Analysis of feedback applications

– Close-Loop Gain– Input/Output resistances

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General Feedback Structure

A

Source Load+

-

Vs

Vf

V V A : Open Loop GainA = Vo / V

: feedback factor = Vf / Vo

VAVVVV

VVVVV

o

oS

of

fs

1 :Note

1 :feedback ofAmount :Gain Loop

)1

(11

:gain loop Close

ACL

s

oCL

A

AAT

TT

AA

VVA

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Negative Feedback Properties• Negative feedback takes a sample of the output signal and applies

it to the input to get several desirable properties. In amplifiers, negative feedback can be applied to get the following properties

– Desensitized gain : gain less sensitive to circuit component variations

– Reduce nonlinear distortion : output proportional to input (constant gain independent of signal level)

– Reduce effect of noise

– Control input and output impedances by applying appropriate feedback topologies

– Extend bandwidth of amplifier

• All of these properties can be achieved by trading off gain

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Gain De-sensitivity• Feedback can be used to desensitize the closed-loop gain to variations in the

basic amplifiler. • Assume is constant. Take differentials of the closed loop gain equation

gives,

• Divided by Av, the close loop gain sensitivity is equal to,

• This result shows the effects of variations in A on ACL is mitigated by the feedback amount.

• (1+A) is also called the desensitivity amount.

22 )1(or

)1(1

1 AdAdA

AdAdA

AAA CL

CLCL

Differential respected with A

AdA

AAA

AdA

AdA

CL

CL

1

1)1()1( 2

Types of feed back• Negative feedback: If the signal fed back is of opposite polarity or out of phase by

180◦ (or odd integer multiple of 180◦) with respect to input signal, feedback iscalled negative feedback.

• –ve feedback is also known as degenerative feedback because when used itdegenerates (reduces)the output voltage amplitude and in turn reduces thevoltage gain.

Uses:

• When used in amplifier,-ve feedback stabilizes the gain, increases the bandwidthand changes the input and output resistances, reduced voltage gain, decrease innon linear distortion and reduces the effect of variations in temperature andsupply voltages on the output of op-amp.

• Positive feedback: If the signal fed back is of the same polarity or in phase withthe input signal, the feedback is called positive feedback.

• In + ve feedback the feedback signal aids the input signal, so referred asregenerative feedback. +ve feedback is used in oscillator circuits.

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A op amp that uses feedback is called feedback amplifier.Feedback forms a close loop between input and output so referred as closed loop amplifier also.Feedback amplifier consists of two parts: op-amp and feedback circuit (made up of either passive ,active or combination of both components)There are four ways to connect these 2 blocks according to whether the voltage or current is fed back to the input in series or in parallel:-

Voltage series feedback The voltage across RL is input voltage to feedback circuit. Voltage shunt feedback Feedback quantity is the output of feedback circuit and

esazx proportional to output voltage.

Current series feedback Load current flows into feedback circuit . Output of feedback circuit Current shunt feedback (either current or voltage) is proportional to load current.

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Basic Feedback TopologiesDepending 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.

(Type of Feedback) (Type of Sensing)

(1) Series (Voltage) Shunt (Voltage)

(2) Series (Voltage) Series (Current)

(3) Shunt (Current) Shunt (Voltage)

(4) Shunt (Current) Series (Current)

RL

Fig. A voltage-series Fig. B voltage-shunt

Fig. C current-series Fig. D current-shunt

RL

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Feedback Structure (Series-Shunt) OR VOLTAGE SERIES

Voltage amplifier voltage-controlled voltage source

Requires high input impedance, low output impedance

Voltage-voltage feedback

Voltage Gain Calculation:

)1(1

get weAnd, where

)1

(1Gain) Voltage Loop (Close

AVVA

AVV

ATT

TVVA

VA

VVVV

VVVAV

i

io

i

oCL

oo

fi

of

o

+

+

+

Basic amplifier

Feedback network

Ii +

+

Vori

ro

AV

Vf=Vo

Vi

V

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Input/Output Resistance (Series-Shunt)

Input Resistance:

ii

i

i

i

ii

i

i

i

rTIVR

rTV

rVI

VTVIVR

)1(

)1(

)1(

in

in

Output Resistance (Closed loop output resistance with zero input voltage)

+AV

ro

Vo

Io

+

Tr

Ar

IVR

rVAVI

VVVVV

rVAVI

IVR

oo

o

o

o

ooo

o

io

o

oo

o

oVi

11

0

|

out

0out

11

Series-Shunt Example

+

V

Vi

Vf

R1

R2

Vo

Equivalent circuit

+

AV

V

Vi

r r+

Vo

Vf Vo

R1 R2

Amplifier

Feedback

It is observed that:(1) Series connection in input ports(2) Shunt connection in output ports Series-Shunt connection

h-parameter should be used.

40 and M10 ,9 ,1 ,10 :Given 215

orrkRkRA

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Feedback Structure (Series-Series) OR current series

+

Basic amplifier

Feedback network

Ii

+

V ri

Vf=Io

ro

AV

I

Vi

V+

)1(1

get weAnd, where

)1

(1Gain) tanceTransadmit Loop (Close

:nCalculatioGain

AVVA

AVI

ATT

TVIA

IAIVVV

IVVAI

i

io

i

oCL

oo

fi

of

o

13

Input/Output Resistance (Series-Series)

Input Resistance:

i

i

i

i

rTI

VTIVR

)1(

)1(

in


oo

o

o

oo

o

oo

of

o

oV

rTIVR

rVIT

rVAVI

IVV

IVR

i

)1(

port,output from

port,input from

|

out

0out

14

Feedback Structure (Shunt-Shunt) or voltage shunt

)1(1

get weAnd, where

)1

(1Gain) anceTransimped Loop (Close

)1()(

)(:nCalculatioGain

AIIA

AIV

ATT

TIVA

VTAIVVIA

VIIIAIAV

i

io

i

oCL

oi

ooi

of

fio

Feedback network

Ii ri

If= Vo

Vi

+

I

Basic amplifier

+

Vo

ro

AI+

15

Input/Output Resistance (Shunt-Shunt)

Input Resistance:

)1(

)1(

in

Tr

TIrI

IVR

i

i

i

i


)1(

port,output from

port,input from

|

out

0out

Tr

IVR

rTVV

rAIVI

VII

IVR

o

o

o

o

oo

o

oo

of

o

oVi

16

Feedback Structure (Shunt-Series)

)1(1

get weAnd, where

)1

(1Gain)Current Loop (Close

)1()(

)(:nCalculatioGain

AIIA

AII

ATT

TIIA

ITAIIIIA

IIIIAIAI

i

io

i

oCL

oi

ooi

of

fio

Feedback network

If= Io

+

IAI

IBasic amplifier

Ii Vi ri ro

17

Input/Output Resistance (Shunt-Series)Input Resistance:

)1(

)1(

in

Tr

I

rT

IIrI

IVR

i

i

ii

i

i

i

i


oo

o

oooo

ooo

ooo

of

o

oV

rTIVR

rITIVrAIIV

AIrVIIII

IVR

i

)1(

)()(

/ port,output from

port,input from

|

out

0out

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SummaryFeedback Structure

Close loop gain

Input impedance

Output impedance

Parameter used

Series-Shunt h-parameter

Series-Series

z-parameter

Shunt-Shun

y-parameter

Shunt-Series

g-parameter

)1

(1T

TVV

i

o

)1

(1T

TVI

i

o

)1

(1T

TIV

i

o

)1

(1T

TII

i

o

irTR )1(in

irTR )1(in

TrR i

1in

TrR i

1in

TrR o

1out

TrR o

1out

orTR )1(out

orTR )1(out

feedback amplifier -...

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