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Analog & Digital Electronics
Course No: PH-218
Lec-14: Feedback Amplifier
Course Instructors:
Dr
...
P
...
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
...
Negative feedback returns part of the output to oppose the
input, whereas in positive feedback the feedback signal aids the input
signal
...
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
...
Depending on the amplifier
category, one of four types of feedback structures should be used
...
Similarly current refers to connecting the output current
as input to the feedback network
...
7
Feedback topologies
8
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
...
Since effective input resistance is small hence input should be a current
...
Hence output ckt will behave like a voltage source
...
9
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
10
Feedback topologies: Voltage Series Feedback
Input resistance of the amplifier and feedback network are in series
hence effective input resistance will increase
...
11
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
12
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
...
Thus Current series feedback circuit behave like a voltage controlled current
13
source
...
Effective input resistance will decrease
...
behave like a current controlled
15