Electrical Engineering BA (B), Analog Electronics, ET064G 7

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Presentation transcript:

Electrical Engineering BA (B), Analog Electronics, ET064G 7 Electrical Engineering BA (B), Analog Electronics, ET064G 7.5 Credits ET065G 6 Credits Muhammad Amir Yousaf

Exam question An amplifier have the following open-loop gain: 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

100db 80 60 40 20 Muhammad Amir Yousaf 10 100 1K 10K 100K 1M 10M

-45 -90 -135 -180 -225 -270 Muhammad Amir Yousaf 10 100 1K 10K 100K 1M -45 -90 -135 -180 -225 -270 Muhammad Amir Yousaf 10 100 1K 10K 100K 1M 10M

100db -45 80 -90 -135 60 A feedback factor of -80db or Acl of 80 db can make this circuit stable. 40 -180 20 -225 -270 Muhammad Amir Yousaf 10 100 1K 10K 100K 1M 1M 10M

A feedback factor of -80db or Acl of 80 db can make this circuit stable. 100db -45 80 -90 (Acl)db -135 60 40 -180 (AolB)db 20 -225 -270 Muhammad Amir Yousaf 10 100 1K 10K 100K 1M 1M 10M

b) Highest bandwidth 10K without stabilization b) Highest bandwidth 10K without stabilization 100db -45 80 -90 (Acl)db -135 60 40 -180 20 -225 -270 Muhammad Amir Yousaf 10 100 1K 10K 100K 1M 1M 10M

A feedback factor of -80db or Acl of 80 db can make this circuit stable. 100db -45 80 -90 -135 60 40 -180 20 -225 -270 Muhammad Amir Yousaf 10 100 1K 10K 100K 1M 1M 10M

Lead Compensation Muhammad Amir Yousaf

Lead Compensation Muhammad Amir Yousaf

Aol A Zero is placed at 10KHz to eliminate the pole at this frequency A Zero is placed at 10KHz to eliminate the pole at this frequency Aol 100db -45 80 -90 Phase Response -135 60 40 -180 Gain Response ’Lead Compensation Circuit’ 20 -225 -270 Muhammad Amir Yousaf 10 100 1K 10K 100K 1M 1M 10M

Aol A Zero gives +45 degree phase shift at 10KHz Phase Response A Zero gives +45 degree phase shift at 10KHz Aol 100db -45 80 -90 Phase Response -135 60 40 -180 90 Phase Response ’Lead Compensation Circuit’ 45 20 -225 -270 Muhammad Amir Yousaf 10 100 1K 10K 100K 1M 1M 10M

Aol Adding Aol and Lead compensation circuit’s Gain. Adding Aol and Lead compensation circuit’s Gain. Aol 100db -45 80 -90 Resultant Aol after compensation -135 60 40 -180 Gain Response ’Lead Compensation Circuit’ 20 -225 -270 Muhammad Amir Yousaf 10 100 1K 10K 100K 1M 1M 10M

A Zero gives +45 degree phase shift at 10KHz A Zero gives +45 degree phase shift at 10KHz -45 Amplifier Phase Response -90 Resultant phase response -135 -180 90 Phase Response ’Lead Compensation Circuit’ 45 -225 -270 Muhammad Amir Yousaf 10 100 1K 10K 100K 1M 1M 10M

So amplifier is stable for Acl >= 40dB Aol and phase response after compensation Aol 100db -45 80 -90 After compensation, we get -135degree phase shift at 1MHz -135 60 40 -180 Gain at 1MHz is 40dB. This means that the feedback factor must be atleast -40dB to get Loop Gain(AolB) = 0db a@ 1MHz 20 -225 So amplifier is stable for Acl >= 40dB -270 Muhammad Amir Yousaf 10 100 1K 10K 100K 1M 1M 10M

But our requirement was to make it stable for 20dB amplification. Still not achieved…… Muhammad Amir Yousaf

Lead Compensation Here is the solution. This factor can be adjusted for lower gain in Lead compensation ciruits Muhammad Amir Yousaf

Amplifier should be stabilized with 20dB gain. Amplifier should be stabilized with 20dB gain. 80 -45 Aol 60 -90 40 -135 -180 20 Compensation circuit Gain Response is adjusted by selecting proper values of R1,R2 -225 -20 -270 Muhammad Amir Yousaf 10 100 1K 10K 100K 1M 1M 10M

Resultant Gian Response 100db Resultant Gian Response 80 -45 Aol 60 -90 40 -135 -180 20 Compensation circuit Gain Response is adjusted by selecting proper values of R1,R2 -225 -20 -270 Muhammad Amir Yousaf 10 100 1K 10K 100K 1M 1M 10M

Aol Resultant Gian Response 100db Resultant Gian Response 80 -45 Aol 60 -90 40 -135 Now we have 20 dB at 1MHz @ -135 degree phaseshift -180 20 Feedback factor should be -20 and Acl = 20dB…. Achieved -225 -20 -270 Muhammad Amir Yousaf 10 100 1K 10K 100K 1M 1M 10M