Analog & Digital Electronics
Course No: PH-218
Lec-14: Feedback Amplifier
Course Instructors:
Dr. A. P. VAJPEYI
Department of Physics,
Indian Institute of Technology Guwahati, India
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Marks distribution: Quiz-2
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7
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5
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Marks
Students
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2
1
0
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Highest: 07 ( 1 student)
Lowest: 00 ( 5 students)
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Feedback Amplifier
Feedback amplifier contains two component namely feedback circuit
and 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 output
voltage to the input of the amplifier circuit.
feedback fraction β
V2
R2
β= =
V1 R1 + R2
Feedback amplifier
Feedback circuit
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Feedback
Both negative feedback and positive feedback are used in amplifier
circuits.
Negative feedback returns part of the output to oppose the
input, whereas in positive feedback the feedback signal aids the input
signal.
If Vf = 0 (there is no feedback)
A=
Vo Vo
=
VS Vi
If feedback signal Vf is connected in
series with the input, then Vi = VS- Vf
Vo = AVi = A(Vs − V f )
Feedback amplifier
Vo = A(Vs − βVo )
Af : closed-loop gain of the amplifier
A: Open-loop gain of the amplifier gain
But
V f = βVo
Vo (1 + βA) = AVs
Vo
A
Af =
=
Vs (1 + βA)
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Feedback
For negative feedback: βA > 0 ; For positive feedback: βA < 0
Advantages of Negative feedback
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.
Gain stabilization
A
Af =
(1 + β A)
dA f
A
=
1
(1 + β A) 2
dA f
1
dA
=
Af
(1 + βA) A
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
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 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
Current shunt feedback (Af = Io/Is) - Current amplifier
Here voltage refers to connecting the output voltage as input to the
feedback 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.
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Feedback topologies
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Feedback topologies: Voltage Shunt Feedback
Output resistance of the
amplifier and output resistance
of the feedback circuit are in
parallel hence effective output
resistance of the feedback
amplifier will reduce.
Similarly
overall
input
resistance of the feedback
amplifier will reduce due to
parallel connection of amplifier
and feedback resistor.
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.
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Feedback topologies: Voltage Shunt Feedback
Voltage Gain :
Vo = A ⋅ I i = A( I S − I f )
I f = β ⋅ Vo
A( I S − βVo ) = Vo
AI S = (1 + β A)Vo
Af =
Vo
A
=(
)
IS
1 + βA
Input Impedance :
V
Vi
Z in = i =
I S Ii + I f
I i ⋅ ri
I i ⋅ ri
=
=
I i + βVo ) I i + βAI i
Z in =
ri
(1 + βA)
Output Impedance :
Z out |VS =0 =
Vo
Io
from input port,
I i = − I f = − β Vo
Z out =
Vo
ro
=
I o (1 + β A)
from output port,
Io =
Vo − AI i Vo + βAVo
=
ro
ro
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Feedback topologies: Voltage Series Feedback
Input resistance of the amplifier and feedback network are in series
hence effective input resistance will increase.
Output resistance of the amplifier and output resistance of the feedback
circuit are in parallel hence effective output resistance of the feedback
amplifier will reduce.
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Feedback topologies: Voltage Series Feedback
Voltage Gain :
Vo = A ⋅Vi = A(VS − V f )
V f = β ⋅Vo
A(VS − β Vo ) = Vo
AVS = (1 + βA)Vo
V
A
Af = o = (
)
VS
1 + βA
Input Impedance :
Z in =
=
Output Impedance :
VS Vi + V f
=
IS
IS
Z out |VS =0 =
Vi + βVo Vi + βAVi
=
IS
IS
Io =
Z in =
Vi (1 + βA)
= ri (1 + βA)
IS
Vo
Io
Vo − A ⋅ Vi
ro
Vi + β ⋅ Vo = VS = 0
Vi = − β ⋅ Vo
Io =
Vo + A ⋅ β ⋅ Vo
ro
Z out =
Vo
ro
=
Io 1+ A ⋅ β
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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 amplifier
will increase.
Input resistance of the amplifier and feedback network are in series hence
effective input resistance will increase.
Thus Current series feedback circuit behave like a voltage controlled current
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source.
Feedback topologies: Current Series Feedback
Basic amplifier
IS
I o = A ⋅ Vi = A(VS − V f )
V f = β ⋅ Io
AV i
+
Vi
−
VS
Voltage Gain :
Iο
ri
ro
Vο
+
−
A(VS − β I o ) = I o
AVS = (1 + β A) I o
Af =
Vf=βIo +
−
Io
A
=(
)
VS
1 + βA
Input Impedance :
VS Vi + V f
Z in =
=
IS
IS
Feedback network
Output Impedance :
Vi + V f = VS = 0
V + A ⋅ β ⋅ Io
Io = o
ro
Z out |VS =0 =
Z out
Vo
V − A ⋅ Vi
; Io = o
Io
ro
V
ro
= o =
Io 1 + A ⋅ β
=
Vi + βVo Vi + βAVi
=
IS
IS
Z in =
Vi (1 + βA)
= ri (1 + βA)
IS
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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
current source.
behave like a current controlled
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