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TABLE OF CONTENTS • Syllabus • Section-A
TABLE OF CONTENTS
• Syllabus
• Section-A
SYLLABUS
• High Frequency Analysis Of Bjt And Multistage Amplifier: Hybrid Pi Model, CE Short Circuit Gain, Frequency Response, Alpha Cut off Frequency, Gain Bandwidth Product, Emitter Follower at High Frequencies. RC Coupled Transistor Amplifier, Lower & Upper Cut off Frequency, Frequency Response curve & Bandwidth, Transformer Coupled Amplifier, Direct Coupled Amplifier, Cascode Amplifier, Darlington Pair Amplifier, Distortion In Amplifiers. Feedback Amplifiers: Feedback concept , Transfer Gain with Feedback, General Characteristics of Negative Feedback, Advantages & disadvantages, Input And Output Resistance, Voltage Series Feedback topology, Voltage Shunt, Current Series & Current Shunt topology ,Equivalent circuit for each topology, Effects of Negative Feedback.
FEEDBACK CIRCUITS
CLASSIFICATION OF AMPLIFIER
CONCEPT OF FEEDBACK
TYPES OF FEEDBACK
FEEDBACK TOPOLOGIES
Classification of Amplifiers
• Before proceeding with the concept of feedback it is useful to classify amplifiers into 4 basic categories based on their input & output signal relationships.
– Voltage amplifier
– Current amplifier
– Transconductance amplifier
– Transresistance amplifier
1. Voltage amplifier if
si RR
then si VV
and if Lo RR
then,
svivo VAVAV
hence i
ov
V
VA
with LR
represent the open circuit voltage gain.
iVoV
Classification of amplifiers
2. Current amplifier if
si RR
then si II
and if Lo RR
then,
siiio IAIAI
hence
i
oi
I
IA
with 0LR
represent the short circuit current gain.
iIoI
Classification of amplifiers
3.Transconductance amplifier
if si RR
then si VV
and if Lo RR
then,
smimo VGVGI
hence
i
om
V
IG
with 0LR
represent the short circuit mutual or transfer conductance
iVoI
Classification of amplifiers
4. Transresistance amplifier if
si RR
then si II
and if Lo RR
then ,
smimo iRIRV
hence
i
om
I
VR
with LR
represent the open circuit mutual or transfer resistance.
iIoV
Classification of amplifiers
Valve example
• As the water nears the specified level, the valve is closed.
• Negative feedback is most commonly used to control systems.
Concept of feedback
• Feedback is a technique where a proportion of the output of a system (amplifier) is fed back and recombined with input.
There are two types of feedback amplifier.
Positive feedback
Negative feedback
input outputA
What is feedback?
Types of feedback
• Positive feedback is the process when the output is added to the input, amplified again, and this process continues.
Example: In a PA system, you 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).
1.Positive Feedback
input outputA
Types of feedback
• Negative feedback is when the output is subtracted from the input.
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.
The use of negative feedback reduces the gain. Part of the output signal is taken back to the input with a negative sign.
2. Negative Feedback
input outputA
Types of feedback
Negative Feedback Gain
The gain with feedback (or closed-loop gain) Af as follows:
io XAX . of XX .fsi XXX
A
A
X
XA
s
of
1
The quantity A is called the loop gain, and the quantity (1+A) is called the amount of feedback.
Types of feedback
oXiX
fX
sX A
Advantages of Negative Feedback
1. Stabilization of gain
– make the gain less sensitive to changes in circuit components e.g. due to changes in temperature.
2. Reduce non-linear distortion
– make the output proportional to the input, keeping the gain constant, independent of signal level.
3. Reduce the effect of noise
– minimize the contribution to the output of unwanted signals generated in circuit components or extraneous interference.
Advantages of Negative Feedback (cont.)
4. Extend the bandwidth of the amplifier
– Reduce the gain and increase the bandwidth
5. Modification the input and output impedances
– raise or lower the input and output impedances by selection of the appropriate feedback topology.
Disadvantages of Negative Feedback
1. Circuit gain - Reduce
2. Stability – Tend to be oscillate
Basic structure of feedback amplifier
A : open-loop gain
B : feedback factor
B
LoadSource xs xi
xf
xo +
-
A
of Bxx io Axx
fsi xxx
AB
A
x
xA
s
of
1The gain of the feedback amplifier
Error signal
If AB is very large, the overall gain becomes a function of the feedback network only.
Basic structure of a single - loop feedback amplifier
Basic structure of feedback amplifier
Feedback Network
• This block is usually a passive two-port network. • contain resistors, capacitors, and inductors. • Usually it is simply a resistive network.
Sampling Network
• The output voltage is sampled by connecting the feedback network in shunt across the output. • Type of connection is referred to as voltage or shunt or node sampling.
• The output current is sampled by connecting the feedback network in series with the output • Type of connection is referred to as current or series or loop sampling.
Sampling Network (cont.)
Comparator or Mixer Network
• voltage - applied feedback . • identified as voltage or series or loop mixing.
Comparator or Mixer Network (Cont.)
• current - applied feedback • identified as current or shunt or node mixing.
Feedback Topologies
• Four basic feedback topologies based on the parameter to be amplified (voltage or current) and the output parameter (voltage or current).
• The four feedback circuit can be described by the types of connections at the input and output of circuit.
1. Series-shunt topology
Voltage-mixing voltage-sampling
Feedback Topologies
2.Shunt-series topology
Current-mixing current-sampling
Feedback Topologies
3.Series-series topology
Voltage-mixing current-sampling
Feedback Topologies
4.Shunt-shunt topology
Current-mixing voltage-sampling
Feedback Topologies
Ideal series-shunt feedback
Rof Rif Ri
Ii
Vi Vs
Vf
Vo
Vo AVi
Ro
Vo
Feedback Topologies
ARV
AVVR
V
VR
RV
V
I
VR i
i
iii
i
si
ii
s
i
sif
1
A
RR o
of
1
1. Input resistance with feedback
2.Output resistance with feedback
Ideal series-shunt feedback (cont.)
Feedback Topologies
Practical series-shunt feedback
Lof
out
RR
R11
1
sifin RRR
Rs
Vs
Rif Rin Rout Rof
Vo
Feedback Topologies
Ideal shunt-series feedback
Feedback Topologies
If
Is
Io
Io
Rif Rof
Vi
Ii
Ri Ro
AIi
Io
A
A
I
IA
s
of
1
A
RR i
if
1
ARR oof 1
Ideal shunt-series feedback (cont.)
Feedback Topologies
2. Input resistance with feedback
3.Output resistance with feedback
1. Gain of feedback amplifier
Practical shunt-series feedback
Lofout RRR
RL
Io
Io
Rout Rin
Is
Feedback Topologies
sif
in
RR
R11
1
Ideal series-series feedback
Feedback Topologies
Rof
Rif
- Vf +
Vs Io
Io
Io Io
Vi Ri AVi Ro
Ii
A
A
V
IA
s
of
1
ARR iif 1
ARR oof 1
Ideal series-series feedback (cont.)
2. Input resistance with feedback
3.Output resistance with feedback
1. Gain of feedback amplifier
Feedback Topologies
Practical series-series feedback
sifin RRR Lofout RRR '
RL Vs
Rs
Rout Rin
Io
Io
Feedback Topologies
Ideal shunt-shunt feedback
Ii
Is
If
Vi Ri
Ro
Rof Rif
Vo AIi
Vo
Feedback Topologies
s
of
I
VA
A
AAf
1
A
RR o
of
1A
RR i
if
1
Ideal shunt-shunt feedback
Feedback Topologies
Practical shunt-shunt feedback Rs
Is
Rout Rin
RL Vo
Feedback Topologies
sif
in
RR
R11
1
Lof
out
RR
R11
1
Feedback relationship
A
AA f
1
ARR iif 1
A
RR o
of
1
A
AA f
1
ARR iif 1
ARR oof 1
A
AA f
1
A
RR i
if
1
A
RR o
of
1
A
AA f
1
A
RR i
if
1
ARR oof 1
Gain Input resistance Output
resistance
Without feedback A Ri Ro
Series-shunt
Series-series
Shunt-shunt
Shunt-series
Feedback Topologies
Feedback Amplifier Topologies
Exercise
• Consider the non-inverting op-amp circuit with parameters R1 = 10 k, R2 = 30 k, and A = 104. Assume Ri = . Determine the closed-loop voltage gain. If the open-loop gain increases by a factor of 10, what is the percent change in the closed-loop gain?
(Ans : 3.9984, 0.036%)
FEEDBACK CIRCUITS
