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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
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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
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Input/Output Resistance (Series-Series)
Input Resistance:
i
i
i
i
rTI
VTIVR
)1(
)1(
in
Output Resistance (Closed loop output resistance with zero input voltage)
oo
o
o
oo
o
oo
of
o
oV
rTIVR
rVIT
rVAVI
IVV
IVR
i
)1(
port,output from
port,input from
|
out
0out
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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+
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Input/Output Resistance (Shunt-Shunt)
Input Resistance:
)1(
)1(
in
Tr
TIrI
IVR
i
i
i
i
Output Resistance (Closed loop output resistance with zero input voltage)
)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
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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
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Input/Output Resistance (Shunt-Series)Input Resistance:
)1(
)1(
in
Tr
I
rT
IIrI
IVR
i
i
ii
i
i
i
i
Output Resistance (Closed loop output resistance with zero input voltage)
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