Feedback in amplifier

An amplifier is one of the most important applications of transistor. Generally, transistor in CE configuration was used for faithful amplification of signal due to high gain, high input impedance and high power gain. But it has been observed that feedback in an amplifier introduces significant improvement in gain and gives amplified output in required form.

1. Physics
BSc III Sem V
Shri Shivaji Science College, Amravati
Dr. Vaishali V. Deshmukh

2. FEEDBACK IN AMPLIFIERS
Oscillators and Multivibrators
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3. Concept of Feedback
What is feedback in an amplifier
Topologies of Feedback
Voltage and current feedback
Types of Feedback
Positive and negative feedback
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What is an amplifier
Basic Amplifier

4. What is an Amplifier?
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▰Adevicewhichacceptaninputsignalandproducesan
outputsignalproportionaltotheinput,iscalledanamplifier.
V
S : is the signal voltage,
R
S : is the internal resistance of the source,
V
1: is the actual input voltage to the amplifier,
I
1: is the input current to the amplifier,
V
2: is the output voltage across the load R
L,
I
2: is the output current flowing through the load R
L.

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✓Theimportantfunctionofanamplifieristheamplification.
✓Circuitthatincreasestheamplitudeoftheweaksignals.
✓Theimportantparametersofanamplifierareinputimpedance,output
impedance,currentgainandvoltagegain.
✓Agooddesignofanamplifiercircuitmustpossesshighinputimpedance,
lowoutputimpedanceandhighcurrentgain.
✓Constituteanessentialpartofradio,televisionandcommunicationdevices.
✓Indiscretecircuits,bipolarJFET,FETisusedasamplifyingelement.
✓Wefindtheuseofamplifiersinmedicaldevices,scientificequipment,
automation,militarytools,communicationdevices,andeveninhousehold
equipment.

6. Limitations of basic amplifier
▰Theinstabilityofa.c.gainisduetothechangein
supply,changeinh-parameters,ageofthedevice.
▰Theinputandoutputimpedancearenotaccording
totherequirementsofthebasicamplifier.
▰Distortionislarge.
▰Noiseismore.
▰Bandwidthisnotsufficientlylarge.
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7. 7
FEEDBACK AMPLIFIER
Feedback is the process of taking a
proportion of an amplifier´s output
signal and feeding it back into the
input. Feedback can be arranged to
increase or decrease the input
signal.
An amplifier that has a feedback
connection established between the
output to the input is known as the
feedback amplifier.

8. Positive
Thepositivefeedbackcanbedefined
aswhenthefeedbackcurrent
otherwisevoltageisappliedfor
increasingthei/pvoltage,thenitis
namedaspositivefeedback.Direct
feedbackisanothernameofthis
positivefeedback.Becausepositive
feedbackgeneratesunnecessary
distortion;itisnotoftenusedin
amplifiers.But,itamplifiestheoriginal
signalpowerandcanbeusedin
oscillatorcircuits.
Types of feedback
Negative
Thenegativefeedbackcanbedefinedas
ifthefeedbackcurrentotherwisevoltage
canbeappliedforreducingtheamplifier
i/p,thenitiscalledasnegativefeedback.
Inversefeedbackisanothernameofthis
negativefeedback.Thiskindoffeedback
isregularlyusedinamplifiercircuits.
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9. Fig1: Positive feedback 9

10. Fig2: Negative feedback 10

11. Advantages of negative feedback
▰Stability of gain is improved
▰Reduction in distortion
▰Reduction in noise
▰Increase in input impedance
▰Decrease in output impedance
▰Increase in the range of uniform application
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Though thegainof negative feedback amplifier isreduced, there are many
advantages of negative feedback such as,
It is because of these advantages negative feedback is frequently
employed in amplifiers.

12. Comparison between Negative & Positive feedback
▰Whenthefeedbackvoltageorcurrentisin
phasewiththeinputsignalandthusaids
it,itiscalledpositivefeedback.
▰Positivefeedbackincreasesthegainof
theamplifier.
▰Noiseintheoutputsignalincreases
▰Distortionincreases
▰Bandwidthdecreases
▰Itisusedinoscillators
▰Whenthefeedbackvoltageorcurrentis
outofphasewiththeinputsignalandthus
opposesit,itiscalledpositivefeedback.
▰Negativefeedbackreducesthegainofthe
amplifier.
▰Noiseintheoutputsignaldecreases
▰Distortiondecreases
▰Itisusedinamplifiers
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Positive Feedback Negative Feedback

13. Principle of Feedback Amplifier
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Afeedbackamplifiergenerallyconsistsoftwoparts.Theyaretheamplifierandthefeedbackcircuit.Afeedback
networkisalineartwo-portnetworkthatcontainsresistors,inductors,capacitors.Itsfunctionistofedbacksome
portionofoutputtotheinput.
From the above figure, the gain of the amplifier is represented as A. the gain of the amplifier is the ratio of output
voltage V
oto the input voltage V
i. the feedback network extracts a voltage V
f= β V
ofrom the output V
oof the amplifier.

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Thisvoltageisaddedforpositivefeedbackandsubtractedfornegative
feedback,fromthesignalvoltageV
s.Now,
Vi = Vs + Vf = Vs + βVo
Vi = Vs –Vf = Vs − βVo
Thequantityβ=V
f/V
oiscalledasfeedbackratioorfeedbackfraction.
Letusconsiderthecaseofnegativefeedback.TheoutputV
omustbeequal
totheinputvoltage(V
s-βV
o)multipliedbythegainAoftheamplifier.
Hence,
(Vs−βVo) A=Vo
Or
AVs−AβVo = Vo
Or
AVs = Vo (1+Aβ)
Therefore,
Vo/Vs=A/ (1+Aβ)

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Let A
fbe the overall gain (gain with the feedback) of the amplifier. This is defined as the ratio
of output voltage V
oto the applied signal voltage V
s, i.e.
Af = Output voltage /Input signal voltage = Vo/Vs
So,fromtheabovetwoequations,wecanunderstandthat,Theequationofgainofthefeedback
amplifier,withnegativefeedbackisgivenby
Af = A/ (1+Aβ)
The equation of gain of the feedback amplifier, with positive feedback is given by
Af = A/ (1−Aβ)
Case1: when β is positive i.e. when feedback voltage is in phase with input voltage( +VeFB)
then |1-Aβ|<1, A
f >A. Therefore, positive feedback increases the amplifier gain.
Case2: when β is negative i.e. when feedback voltage is out of phase with input voltage( -VeFB)
then |1-Aβ|>1, A
f < A.Therefore, negative feedback reduces the amplifier gain.
The term Aβis called loop gain.

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Voltage-Series Feedback
Negative Voltage Feedback: In this method, the voltage feedback
to the input of amplifier is proportional to the output voltage.
Inthevoltageseriesfeedbackcircuit,afractionofthe
outputvoltageisappliedinserieswiththeinputvoltage
throughthefeedbackcircuit.Thisisalsoknownasshunt-
drivenseries-fedfeedback,i.e.,aparallel-seriescircuit.
Thefigureshowstheblockdiagramofvoltageseries
feedback,bywhichitisevidentthatthefeedbackcircuit
isplacedinshuntwiththeoutputbutinserieswiththe
input.Asthefeedbackcircuitisconnectedinshuntwith
theoutputandtheinputaswell,boththeoutput
impedanceandtheinputimpedancearedecreased.

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Voltage-Shunt Feedback
Negative Voltage Feedback: In this method, the voltage feedback
to the input of amplifier is proportional to the output voltage.
Inthevoltageshuntfeedbackcircuit,afractionofthe
outputvoltageisappliedinparallelwiththeinput
voltagethroughthefeedbacknetwork.Thisisalso
knownasshunt-drivenshunt-fedfeedbacki.e.,a
parallel-parallelprototype.Thefigureshowstheblock
diagramofvoltageshuntfeedback,bywhichitis
evidentthatthefeedbackcircuitisplacedinshuntwith
theoutputandalsowiththeinput.Asthefeedback
circuitisconnectedinshuntwiththeoutputandthe
inputaswell,boththeoutputimpedanceandtheinput
impedancearedecreased.

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Current-Series Feedback
Negative Current Feedback: In this method, the voltage feedback
to the input of amplifier is proportional to the output current.
Inthecurrentseriesfeedbackcircuit,afractionofthe
outputvoltageisappliedinserieswiththeinputvoltage
throughthefeedbackcircuit.Thisisalsoknownasseries-
drivenseries-fedfeedbacki.e.,aseries-seriescircuit.The
figureshowstheblockdiagramofcurrentseriesfeedback,
bywhichitisevidentthatthefeedbackcircuitisplacedin
serieswiththeoutputandalsowiththeinput.Asthe
feedbackcircuitisconnectedinserieswiththeoutputand
theinputaswell,boththeoutputimpedanceandtheinput
impedanceareincreased.

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Current-Shunt Feedback
Negative Current Feedback: In this method, the voltage feedback
to the input of amplifier is proportional to the output current.
Inthecurrentshuntfeedbackcircuit,afractionofthe
outputvoltageisappliedinserieswiththeinputvoltage
throughthefeedbackcircuit.Thisisalsoknownasseries
drivenshunt-fedfeedbacki.e.,aseries-parallelcircuit.
Thefigureshowstheblockdiagramofcurrentshunt
feedback,bywhichitisevidentthatthefeedbackcircuit
isplacedinserieswiththeoutputbutinparallelwiththe
input.Asthefeedbackcircuitisconnectedinserieswith
theoutput,theoutputimpedanceisincreasedanddueto
theparallelconnectionwiththeinput,theinput
impedanceisdecreased.

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Characteristics
Types of Feedback
Voltage-SeriesVoltage-ShuntCurrent-SeriesCurrent-Shunt
Voltage GainDecreases Decreases Decreases Decreases
Bandwidth Increases Increases Increases Increases
Input resistanceIncreases Decreases Increases Decreases
Output resistance
Decreases Decreases Increases Increases
Harmonic
distortion
Decreases Decreases Decreases Decreases
Noise Decreases Decreases Decreases Decreases

21. Reference Books
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1.Principle of Electronics by V.K. Mehata
2.Basic Electronics (Solid State) by B.L. Theraja
3. Handbook of Electronics by Gupta Kumar

22. “
Once you stop learning,
you start dying
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Albert Einstein

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