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Electrical Engineering BA (B), Analog
Electronics,
ET064G 7.5 Credits
ET065G 6 Credits
Muhammad Amir Yousaf
1
Exam question
• An amplifier have the following open-loop gain:
AV 0  1  10
1
5
1
1
f
f
f




1  j
 1  j
 1  j

100
kHz
300
kHz
1
MHz




•
Draw a detailed asymptotic bode-diagram for the open-loop gain.
Both the amplitude and phase should be clearly visualized. (2p)
• What is the gain-bandwidth product for the amplifier? (1p)
• What is the lowest amplification the amplifier can be used (AVmin),
without any stabilization network assuming 45˚ phase margin, (ΦM).
(2p)
• Stabilize the amplifier with a dominant pole compensation so that it
can be useful as a 20 dB amplifier with ΦM>45˚.
Muhammad Amir Yousaf
2
100db
80
60
40
20
10
Muhammad
Amir Yousaf 1K
100
10K
100K
1M
10M
3
0
-45
-90
-135
-180
-225
-270
10
Muhammad
Amir Yousaf 1K
100
10K
100K
1M
10M
4
0
100db
-45
80
-90
60
-135
40
A feedback factor of 80db or Acl of 80 db
can make this circuit
stable.
-180
-225
20
-270
10
Muhammad
Amir Yousaf 1K
100
10K
100K
1M
10M
5
A feedback factor of 80db or Acl of 80 db
can make this circuit
stable.
100db
0
-45
-90
80
(Acl)db
60
-135
40
-180
(AolB)db
-225
20
-270
10
Muhammad
Amir Yousaf 1K
100
10K
100K
1M
10M
6
b) Highest bandwidth 10K
without stabilization
100db
0
-45
-90
80
(Acl)db
60
-135
40
-180
20
-225
-270
10
Muhammad
Amir Yousaf 1K
100
10K
100K
1M
10M
7
A feedback factor of 80db or Acl of 80 db
can make this circuit
stable.
100db
0
-45
80
-90
60
-135
40
-180
20
-225
-270
10
Muhammad
Amir Yousaf 1K
100
10K
100K
1M
10M
8
Lead Compensation
Muhammad Amir Yousaf
9
Lead Compensation
Muhammad Amir Yousaf
10
A Zero is placed at
10KHz to eliminate the
pole at this frequency
Aol
100db
0
-45
-90
80
Phase
Response
60
-135
40
-180
20
Gain Response ’Lead
Compensation Circuit’
-225
-270
10
Muhammad
Amir Yousaf 1K
100
10K
100K
1M
10M
11
A Zero gives +45
degree phase shift at
10KHz
Aol
100db
0
-45
-90
80
Phase
Response
-135
60
40
20
-180
90
45
Phase Response ’Lead
Compensation Circuit’
-225
0
10
-270
Muhammad
Amir Yousaf 1K
100
10K
100K
1M
10M
12
Adding Aol and Lead
compensation circuit’s
Gain.
Aol
100db
0
-45
-90
80
Resultant Aol after
compensation
60
-135
40
-180
20
Gain Response ’Lead
Compensation Circuit’
-225
-270
10
Muhammad
Amir Yousaf 1K
100
10K
100K
1M
10M
13
A Zero gives +45
degree phase shift at
10KHz
Amplifier
Phase
Response
-135
-180
90
Phase Response ’Lead
Compensation Circuit’
-225
0
10
-45
-90
Resultant
phase response
45
0
-270
Muhammad
Amir Yousaf 1K
100
10K
100K
1M
10M
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Aol and phase response
after compensation
Aol
100db
After compensation, we get 135degree phase shift at
1MHz
-135
-180
40
20
-45
-90
80
60
0
Gain at 1MHz is 40dB. This means that the
feedback factor must be atleast -40dB to get
Loop Gain(AolB) = 0db a@ 1MHz
-225
So amplifier is stable for Acl >= 40dB
-270
10
Muhammad
Amir Yousaf 1K
100
10K
100K
1M
10M
15
 But our requirement was to make it stable for 20dB
amplification.
 Still not achieved……
Muhammad Amir Yousaf
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Lead Compensation
Here is the solution.
This factor can be
adjusted for lower
gain in Lead
compensation ciruits
Muhammad Amir Yousaf
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100db
Amplifier should be stabilized
with 20dB gain.
80
0
-45
Aol
60
-90
40
-135
20
-180
0
Compensation circuit
Gain Response is
adjusted by selecting
proper values of R1,R2
-225
-20
10
-270
Muhammad
Amir Yousaf 1K
100
10K
100K
1M
10M
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100db
Resultant Gian Response
80
0
-45
Aol
60
-90
40
-135
20
-180
0
Compensation circuit
Gain Response is
adjusted by selecting
proper values of R1,R2
-225
-20
10
-270
Muhammad
Amir Yousaf 1K
100
10K
100K
1M
10M
19
100db
Resultant Gian Response
80
0
-45
Aol
60
-90
40
-135
20
0
-20
10
Now we have 20 dB at
1MHz @ -135 degree
phaseshift
-180
Feedback factor should
be -20 and Acl =
20dB….
Achieved
Muhammad
Amir Yousaf 1K
100
-225
-270
10K
100K
1M
10M
20