Statement of Problem:
The purpose of this lab is to understand the concept behind compensating an amplifier using
phase and gain margins. For this lab, Cadence software will be used to design for phase margins
of 45 and 60 degrees. Once the amplifier macro-model is compensated, it will then be used to
create a non-inverting amplifier with series voltage feedback.
Procedure & Analysis:
1) Design for phase margin of 450:
Determining fPM from the uncompensated amplifier phase plot:
Uncompensated Amplifier Phase Plot
Determining 20Log|A(fPM)| from the uncompensated gain plot:
Uncompensated Amplifier Gain Plot
Calculating Compensation Frequency:
 = 0.1
fPM = 761 KHz
From gain vs. frequency plot,
20 Log|A(fPM )| = 78 dB
Since,
20 Log (0.1) = -20 dB, 20 Log |AC (fPM )| = 20 dB
Now,
Difference between compensated and uncompensated gain is:
78 dB – 20 Log |AC (fPM )| = 58 dB
Then,
 f
 10 * Log 1   PM
  f C
 f
Log 1   PM
  f C
 f
1   PM
  f C



2



2




  10 5.8

2
2
fC 
f PM
10 5.8  1
fC = 958 Hz
So,

  58


  5.8

f C  f PM  105.8 f C
2
2
2
C
1
2 *  * f C * RC
C
1
2 *  * 958 * 100
C = 1.65 uF
Compensating the amplifier macro-model such that the phase margin is 450:
Compensated Phase Plot
Compensated Gain Plot
After Compensation,
Phase at fPM = -1350
20Log|AC(fPM)| = 20 dB
2) Design of non-inverting amplifier using macro-model design with phase margin of 450:
Frequency Response
Frequency Peaking at 808 KHz
3dB bandwidth = 1.31 MHz
Transient Response
Rise time = 102 us – 101 us = 1 us
Overshoot = 11.7 mV – 9.99 mV = 1.71 mV
3) Design for phase margin of 600:
Determining fPM from the uncompensated amplifier phase plot:
Uncompensated Amplifier Phase Plot
Determining 20Log|A(fPM)| from the uncompensated gain plot:
Uncompensated Amplifier Gain Plot
Calculating Compensation Frequency:
 = 0.1
fPM = 468 KHz
From gain vs. frequency plot,
20 Log|A(fPM )| = 79.1 dB
Since,
20 Log (0.1) = -20 dB, 20 Log |AC (fPM )| = 20 dB
Now,
Difference between compensated and uncompensated gain is:
79.1 dB – 20 Log |AC (fPM )| = 59.1 dB
Then,
 f
 10 * Log 1   PM
  f C
 f
Log 1   PM
  f C
 f
1   PM
  f C



2



2




  10 5.91

2
2
fC 
f PM
10 5.91  1
fC = 519 Hz
So,

  59.1


  5.91

f C  f PM  105.91 f C
2
2
2
C
1
2 *  * f C * RC
C
1
2 *  * 519 *100
C = 3.066 uF
Compensating the amplifier macro-model such that the phase margin is 600:
Compensated Phase Plot
Compensated Gain Plot
After Compensation,
Phase at fPM = -1200
20Log|AC(fPM)| = 20 dB
4) Design of non-inverting amplifier using macro-model design with phase margin of 600:
Frequency Response
Frequency Peaking at 344 KHz
3dB bandwidth = 790 KHz
Transient Response
Rise time = 101 us – 100 us = 1 us
Overshoot = 10.6 mV – 9.99 mV = 0.61 mV
Conclusion:
From the 2 designs it can be seen that the 600 phase margin design has significantly less
overshoot than the design of 450 phase margin. The rise times for the two designs were similar.
However, the 3dB bandwidth decreased when the phase margin was increased. This is the design
trade-off.