This document provides an overview of feedback amplifiers, including positive and negative feedback, advantages of negative feedback, basic feedback concepts, and classifications of amplifiers based on input and output parameters. It discusses four main feedback configurations - series-shunt, shunt-series, series-series, and shunt-shunt. For each configuration, it describes the basic circuit, transfer functions, and examples such as op-amps and common emitter/base circuits.

1. CHAPTER 3
FEEDBACK
AMPLIFIERS
Dr.Saju Subramanian
Associate Professor

2. Outline
1. Introduction to Feedback
2. Feedback Amplifier – Positive & Negative
3. Advantages/Disadvantages of Negative
Feedback
4. Basic Feedback Concept
5. Classification of Amplifiers
6. Series – Shunt Configuration
7. Shunt – Series Configuration
8. Series - Series Configuration
9. Shunt – Shunt Configuration

3. Introduction to Feedback
 Feedback is used in virtually all amplifier system.
 Invented in 1928 by Harold Black – engineer in
Western Electric Company
 methods to stabilize the gain of amplifier for use
in telephone repeaters.
 In feedback system, a signal that is proportional to
the output is fed back to the input and combined
with the input signal to produce a desired system
response.
 However, unintentional and undesired system
response may be produced.

4. Feedback Amplifier
 Fe e dback is a technique where a proportion of
the output of a system (amplifier) is fed back and
recombined with input
 There are 2 types of feedback amplifier:
 Positive feedback
 Negative feedback
A
β
input output

5. Positive Feedback
 Positive feedback is the process when the
output is adde d to the input, amplified again,
and this process continues.
 Positive feedback is used in the design of
oscillator and other application.
A
β
input output
+

6. Positive Feedback - Example
 In a PA system
get feedback when you put the microphone in
front of a speaker and the sound gets
uncontrollably loud (you have probably heard
this unpleasant effect).

7. Negative Feedback
 Negative feedback is when the output is
subtracted from the input.
 The use of negative feedback reduces the gain.
Part of the output signal is taken back to the input
with a negative sign.
A
β
input output

8. Negative Feedback - Example
 Speed control
If the car starts to speed up above the desired
set-point speed, negative feedback causes the
throttle to close, thereby reducing speed;
similarly, if the car slows, negative feedback
acts to open the throttle

9. Feedback Amplifier - Concept
Basic structure of a single - loop feedback amplifier

10. Advantages of Negative
Feedback
1. Gain Sensitivity – variations in gain is
reduced.
2. Bandwidth Extension – larger than that of
basic amplified.
3. Noise Sensitivity – may increase S-N ratio.
4. Reduction of Nonlinear Distortion
5. Control of Impedance Levels – input and
output impedances can be increased or
decreased.

11. Disadvantages of Negative
Feedback
1. Circuit Gain – overall amplifier gain is
reduced compared to that of basic amplifier.
2. Stability – possibility that feedback circuit will
become unstable and oscillate at high
frequencies.

12. Basic Feedback Concept
Basic configuration of a feedback amplifier

13. Basic Feedback Concept
 The output signal is:
where A is the amplification factor
 Feedback signal is
where ß is the feedback transfer function
 At summing node:
 Closed-loop transfer function or gain is
if
εAS=oS
oSβ=fbS
fbi SS −=εS
A
A
S
S
i
o
β+
==
1
fA
ββ
β
1
1 =≅>>
A
A
then fAA

14. Classification of Amplifiers
Classify amplifiers into 4 basic categories
based on their input (parameter to be
amplified; voltage or current) & output signal
relationships:
 Voltage amplifier (series-shunt)
 Current amplifier (shunt-series)
 Transconductance amplifier (series-series)
 Transresistance amplifier (shunt-shunt)

15. Feedback Configuration
Series:
connecting theconnecting the
feedback signalfeedback signal
in series within series with
thethe
input signalinput signal
voltage.voltage.
Shunt:
connecting
the
feedback
signal
in shunt
(parallel) with
an input

16. Series - Shunt Configuration
vv
v
vf
A
A
A
β+
=
1

17. Series - Shunt Configuration
if Lo RR <<
then the output of feedback network is an open
circuit;
Output voltage is:
εVAV vo =
feedback voltage is:
ovVVfb β=
By neglecting Rs due to ; error voltage
is:
si RR >>
fbi V−=VVε
vv
v
i
o
vf
A
A
V
V
A
β+
==∴
1
where ßv is closed-loop voltage transfer function

18. Series - Shunt Configuration
Or
 Input current
 Rif with feedback
Assume Vi=0 and Vx
applied to output
terminal.
Or
 Input current
 Rof with feedback
Input Resistance, Rif Output Resistance, Rof
)( εεε β VAVVV vvfbi +=+=V
)1( vv
i
A
V
V
β
ε
+
=
)1( vvi
i
i
i
AR
V
R
V
I
β
ε
+
==
)1( vvi
i
i
if AR
I
V
R β+==
0=+=+ xvfb VVVV βεε
xvVV βε −=
o
vvx
o
vx
i
R
AV
R
VAV
I
)1( βε +
=
−
=
)1( vv
o
x
x
of
A
R
I
V
R
β+
==

19.  Series input connection increase input resistance –
avoid loading effects on the input signal source.
 Shunt output connection decrease the output resistance
- avoid loading effects on the output signal when output
load is connected.
Equivalent circuit of shunt - series feedback circuit or
voltage amplifier
Series - Shunt Configuration

20. For ideal non-inverting op-
amp amplifier
Feedback transfer function;
Series - Shunt Configuration
 Non-inverting op-amp is an example of the
series-shunt configuration.






+==
1
2
1
R
R
V
V
A
i
o
vf






+
=
1
2
1
1
R
R
β

21. Series - Shunt Configuration
Equivalent circuit )1(
/
1
11
1
221
1
21
1
21
1
vi
i
i
i
i
if
v
oi
v
v
v
v
i
o
vf
ofb
fbi
AR
RV
V
I
V
R
R
R
VA
VV
RR
R
VV
A
A
RR
R
A
A
V
V
A
V
RR
R
V
VV
β
β
ε
ε
εε
ε
ε
+===






+
+=





+
+=
+
=






+
+
==






+
≅
−=
=
V
VAV vo

22. Series - Shunt Configuration
Example:
Calculate the feedback amplifier gain of the
circuit below for op-amp gain, A=100,000;
R1=200 Ω and R2=1.8 kΩ.
Solution: Avf = 9.999 or 10

23. Series - Shunt Configuration
 Basic emitter-follower and source-follower circuit
are examples of discrete-circuit series-shunt
feedback topologies.
• vi is the input signal
• error signal is base-
emitter/gate-source
voltage.
• feedback voltage =
output voltage 
feedback transfer
function, ß v = 1

24. Series - Shunt Configuration
 Small-signal voltage gain:
 Open-loop voltage gain:
 Closed-loop input resistance:
 Output resistance:
e
E
e
E
Em
Em
i
o
vf
r
R
r
R
Rg
r
Rg
r
V
V
A
+
=






++






+
==
11
1
1
π
π
e
E
Emv
r
R
Rg
r
A =





+=
π
1












++=++= EmEmif Rg
r
rRrgrR
π
πππ
1
1)1(
Em
E
m
Eof
Rg
r
R
rg
r
RR






++
=
+
=
π
π
π
1
1
)1(

25. Shunt – Series Configuration
ii
i
if
A
A
A
β+
=
1

26. Shunt – Series Configuration
 Basic current amplifier with input resistance, Ri
and an open-loop current gain, Ai.
 Current IE is the difference between input signal
current and feedback current.
 Feedback circuit samples the output current –
provide feedback signal in shunt with signal
current.
 Increase in output current – increase feedback
current – decrease error current.
 Smaller error current – small output current –
stabilize output signal.

27. Shunt – Series Configuration
if si RR <<
then the output is a short circuit; output current is:
εIAI io =
feedback current is:
oi II fb β=
Input signal current:
fbi II += εI
ii
i
i
o
if
A
A
I
I
A
β+
==∴
1
then εIIi ≈
where ßi is closed-loop current transfer function

28. Shunt – Series Configuration
Or
 Input current
 Rif with feedback
Input Resistance, Rif
)( εεε β IAIII iifbi +=+=I
)1( ii
i
A
I
I
β
ε
+
=
)1( ii
ii
ii
A
RI
RIV
β
ε
+
==
)1( ii
i
i
i
if
A
R
I
V
R
β+
==
Assume Ii=0 and Ix applied
to output terminal.
 Rof with feedback
Output Resistance, Rof
[ ]
oiixx
oxiixx
oixx
xi
xifb
RAIV
RIAIV
RIAIV
II
IIII
)1(
)(
)(
0
β
β
β
β
ε
ε
εε
+=
−−=
−=
−=
=+=+
( )iio
x
x
of AR
I
V
R β+== 1

29. Shunt - Series Configuration
 Shunt input connection decrease input resistance –
avoid loading effects on the input signal current source.
 Series output connection increase the output resistance
- avoid loading effects on the output signal due to load
connected to the amplifier output.
Equivalent circuit of shunt - series feedback circuit or
voltage amplifier

30. Shunt - Series Configuration
 Op-amp current amplifier – shunt-series
configuration.
 Ii’ from equivalent source of Ii and Rs.• Iε is negligible and
Rs>>Ri;
• assume V1 virtually
ground;
• Current I1:
• Output current:
• Ideal current gain:
fbii II == 'I
FiFfbo RIRI −=−=V
1/ RVo−=1I






+=+=
1
1 1
R
R
III F
ifboI






+==
1
1
R
R
I
A F
i
i
oI

31. Shunt - Series Configuration
 Ai is open-loop current
gain
and
 Assume V1 is virtually
ground:
 I1 current:
 Output current
fbifbi IIII −≅−= 'εI
)( fbii IIA −== εIAI io
Ffb RI−=oV
Closed-loop current gain:






=−=
11
1
R
R
I
R
V F
fb
o
I






+=+=
1
1
R
R
IIII F
fbfbfboI






+
+
==
1
1
1
R
R
A
A
I
I
F
i
i
i
o
ifA

32. Shunt - Series Configuration
 Common-base circuit is example of discrete
shunt-series configuration.
 Amplifier gain: Closed-loop current gain:
RLIo
Ii
Iε
RLIo
Ii
Iε
Ifb
βε == iAI/oI
i
i
i
o
if
A
A
I
I
A
+
=
+
==
11 β
β

33. Shunt - Series Configuration
 Common-base circuit with RE and RC
RCIoRE
Ii
V-
V+
RCIoRE
Ii
i
E
i
m
E
m
i
o
if
A
R
r
A
rg
R
r
rg
I
I
A
+





+
=
+





+
==
π
π
π
π
11

34. Series – Series Configuration
gg
g
gf
A
A
A
β+
=
1

35. Series – Series Configuration
 The feedback samples a portion of the output
current and converts it to a voltage – voltage-
to-current amplifier.
 The circuit consist of a basic amplifier that
converts the error voltage to an output current
with a gain factor, Ag and that has an input
resistance, Ri.
 The feedback circuit samples the output
current and produces a feedback voltage, Vfb,
which is in series with the input voltage, Vi.

36. Series – Series Configuration
Assume the output is a short circuit, the output
current:
εVAI go =
feedback voltage is:
oz IVfb β=
Input signal voltage (neglect Rs=∞ ):
fbi VV += εV
gz
g
i
o
gf
A
A
V
I
A
β+
==∴
1
where ßz is a resistance feedback transfer function

37. Series – Series Configuration
Assume Ii=0 and Ix applied
to output terminal.
 Rof with feedback
Output Resistance, Rof
[ ]
ogzxx
oxzgxx
ogxx
xz
xzfb
RAIV
RIAIV
RIAIV
II
IIII
)1(
)(
)(
0
β
β
β
β
ε
ε
εε
+=
−−=
−=
−=
=+=+
( )gzo
x
x
of AR
I
V
R β+== 1
Or
 Input current
 Rif with feedback
Input Resistance, Rif
)( εεε β VAVVV gzfbi +=+=V
)1( gz
i
A
V
V
β
ε
+
=
)1( gzi
i
i
i
AR
V
R
V
I
β
ε
+
==
)1( gzi
i
i
if AR
I
V
R β+==

38. Series – Series Configuration
 Series input connection increase input
resistance
 Series output connection increase the output
resistance
Equivalent circuit of series - series feedback

39. Series – Series Configuration
 The series output
connection samples the
output current 
feedback voltage is a
function of output current.
 Assume ideal op-amp
circuit and neglect
transistor base-current:
Ei
o
gf
Eofbi
RV
I
A
RIV
1
==
==V

40. Series – Series Configuration
 Assume IE≅IC and Ri≈∞
( )
( ) Egm
gm
i
o
gf
Eoigmo
Eoifbi
gmbm
E
fb
o
RArg
Arg
V
I
A
RIVArgI
RIVVVV
VArgIrg
R
V
I
π
π
π
ε
εππ
+
==
−=
−=−=
===
1

41. Series – Series Configuration

42. Series – Series Configuration
Em
LC
C
m
i
o
gf
Emfbi
Emfb
LC
C
mo
Rg
r
RR
R
g
V
I
A
Rg
r
VVVV
RVg
r
V
V
RR
R
VgI






++






+
−
==














++=+=






+=






+
−=
π
π
ππ
π
π
π
π
1
1
1
1
)(

43. Shunt – Shunt Configuration
zz
z
zf
A
A
A
β+
=
1

44. Shunt – Shunt Configuration
 The feedback samples a portion of the output
voltage and converts it to a current – current-
to-voltage amplifier.
 The circuit consist of a basic amplifier that
converts the error current to an output voltage
with a gain factor, Az and that has an input
resistance, Ri.
 The feedback circuit samples the output
voltage and produces a feedback current, Ifb,
which is in shunt with the input current, Ii.

45. Shunt – Shunt Configuration
Assume the output is a open circuit, the output
voltage:
εIAV zo =
feedback voltage is:
ogVI fb β=
Input signal voltage (neglect Rs=∞ ):
fbi II += εI
zg
z
i
o
zf
A
A
I
V
A
β+
==∴
1
where ßg is a conductance feedback transfer function

46. Shunt – Shunt Configuration
Or
 Input current
 Rif with feedback
Input Resistance, Rif
)( εεε β IAIII zgfbi +=+=I
)1( zg
i
A
I
I
β
ε
+
=
)1( zg
ii
ii
A
RI
RIV
β
ε
+
==
)1( zg
i
i
i
if
A
R
I
V
R
β+
==
Assume Vi=0 and Vx
applied to output
terminal.
Or
 Input current
 Rof with feedback
Output Resistance, Rof
0=+=+ xgfb VVVV βεε
xgVV βε −=
o
zgx
o
zx
i
R
AV
R
VAV
I
)1( βε
+
=
−
=
)1( zg
o
x
x
of
A
R
I
V
R
β+
==

47. Shunt – Shunt Configuration
Equivalent circuit of shunt - shunt feedback circuit
or
voltage amplifier

48. Shunt – Shunt Configuration
 Basic inverting op-amp circuit is an example of
shunt-shunt configuration.
 Input current splits between feedback current
and error current.
 Shunt output connection samples the output
voltage  feedback current is function of output
voltage.
2
2
R
I
V
A
IIwhere
RIV
i
o
zf
ifb
fbo
−==
=
−=

49. Shunt – Shunt Configuration
 Az is open-loop
transresistance gain
factor (-ve value)
( )
2
2
1
/
R
A
A
I
V
A
RVIwhere
IIAIAV
z
z
i
o
zf
ofb
fbizzo
+
−
==
−=
−−== ε

50. Shunt – Shunt Configuration

51. Shunt – Shunt Configuration






−





+





+





+






−−
==
=





+





−+





+





+
−
+=
=
−
++
F
m
FFFC
F
m
i
o
zf
F
o
i
F
m
FFC
o
F
o
i
F
o
m
C
o
R
g
RRrRR
R
g
I
V
A
R
V
I
R
g
RrRR
V
R
VV
r
V
I
R
VV
Vg
R
V
111111
1
0
11111
0
π
π
π
π
π
π
π

52. ( )
( )





 −
+
≅=






+





−





+





+






++
==
−=












−
=
F
z
z
i
o
zf
F
C
z
FFF
C
F
C
z
i
o
zf
Cm
C
m
z
R
A
A
I
V
A
R
Rr
A
RR
r
R
R
R
Rr
A
I
V
A
Rrg
rR
g
A
1
1
1
11
11
ππ
π
π
π
Shunt – Shunt Configuration
 Open-loop transresistance gain factor Az is
found by setting RF=∞
 Multiply by (rπRC)
 Assume RC <<RF
& rπ<< RF

53. Feedback Amplifier
Input and output Impedances
 Summary
1. For a series connection at input or output,
the resistance is increased by (1+βA).
2. For a shunt connection at input or output,
the resistance is lowered by (1+βA).

54. Feedback Amplifier