+ Post New Thread
Results 1 to 16 of 16
  1. #1
    Advanced Member level 1
    Points: 4,408, Level: 15

    Join Date
    Jul 2006
    Location
    USA
    Posts
    412
    Helped
    8 / 8
    Points
    4,408
    Level
    15

    Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB?

    The phase margin is 72 degree when the gain is 0dB. However, the phase margin is close to 0 before the frequency reaches the unity-gain bandwidth.

    Is the op-amp stable? Why? Thank you.
    Click image for larger version. 

Name:	stability.jpg 
Views:	29 
Size:	185.1 KB 
ID:	157193

    •   AltAdvertisement

        
       

  2. #2
    Advanced Member level 3
    Points: 10,316, Level: 24

    Join Date
    Nov 2001
    Location
    US
    Posts
    896
    Helped
    312 / 312
    Points
    10,316
    Level
    24

    Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB

    It may not be. Two ways to know.
    1. generate a Nyquist plot.
    2. Run a transient analysis to check the step response for oscillations.

    Also, you may try to move the zero towards lower frequencies and thus not let the phse drop so much.


    1 members found this post helpful.

  3. #3
    Advanced Member level 5
    Points: 31,084, Level: 43
    BigBoss's Avatar
    Join Date
    Nov 2001
    Location
    Turkey
    Posts
    4,564
    Helped
    1375 / 1375
    Points
    31,084
    Level
    43

    Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB

    Why Phase starts from 180 degree ? Is it an Inverter Amplifier ??
    If it's so, the danger is much behind from where the Gain drops down to 0.
    While the Gain is higher while the Phase is close to 0, so it will close to oscillate..


    1 members found this post helpful.

  4. #4
    Super Moderator
    Points: 264,284, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    46,183
    Helped
    14044 / 14044
    Points
    264,284
    Level
    100

    Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB

    The bode plot is simple enough (monotonous gain, only one crossing of the 0 dB line) to see that the circuit is unconditional stable. Nyquist criterion can be applied, but it's not necessary to assess stability in this case.

    The phase margin is 72 degree when the gain is 0dB. However, the phase margin is close to 0 before the frequency reaches the unity-gain bandwidth.
    The correct statement is: "The phase margin is 72 degree". Closed loop phase approaches 0 degree, not phase margin which is only a single value.

    Although the amplifier is stable, the "sagging" phase will probably show in the transient response.

    Why Phase starts from 180 degree ? Is it an Inverter Amplifier ??
    The phase of a negative feedback loop always starts at 180 degree.


    2 members found this post helpful.

    •   AltAdvertisement

        
       

  5. #5
    Advanced Member level 5
    Points: 12,968, Level: 27

    Join Date
    Aug 2015
    Location
    Melbourne
    Posts
    2,130
    Helped
    775 / 775
    Points
    12,968
    Level
    27

    Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB

    On the face of it - it is quite stable - adding other delays to bring the total delay to 360 deg is where it will start to oscillate - then the feedback reinforces the input - at 180deg the feedback still subtracts from the input ...


    1 members found this post helpful.

  6. #6
    Super Moderator
    Points: 264,284, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    46,183
    Helped
    14044 / 14044
    Points
    264,284
    Level
    100

    Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB

    The effect of the 2-pole/1-zero phase on closed loop transient response (red) can be seen in comparison with a regular 2-pole amplifier (cyan), both circuits have similar phase margin and gain transient frequency, also shown a 1-pole amplifier (green).

    Click image for larger version. 

Name:	loopgain.PNG 
Views:	23 
Size:	65.7 KB 
ID:	157201 Click image for larger version. 

Name:	closedloop.PNG 
Views:	22 
Size:	49.9 KB 
ID:	157202


    1 members found this post helpful.

    •   AltAdvertisement

        
       

  7. #7
    Advanced Member level 1
    Points: 4,408, Level: 15

    Join Date
    Jul 2006
    Location
    USA
    Posts
    412
    Helped
    8 / 8
    Points
    4,408
    Level
    15

    Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB

    Thank you all of you. Yes, it is an inverting amplifier. Thank FvM to give the simulation comparison.



  8. #8
    Advanced Member level 3
    Points: 6,866, Level: 19

    Join Date
    Feb 2014
    Posts
    984
    Helped
    336 / 336
    Points
    6,866
    Level
    19

    Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB

    Is this a fixed frequency inverter (50 or 60hz for example)? If so PR (Proportional Resonant) control is another good option.

    PR control adds a tuned resonant element to the feedback that works like an AC integrator. Any error signal at the tuned frequency is amplified creating very high gain at a that single frequency.


    1 members found this post helpful.

  9. #9
    Advanced Member level 1
    Points: 4,408, Level: 15

    Join Date
    Jul 2006
    Location
    USA
    Posts
    412
    Helped
    8 / 8
    Points
    4,408
    Level
    15

    Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB

    It is an opamp, not a fixed frequency inverter. Thank you for the information.



  10. #10
    Advanced Member level 3
    Points: 6,866, Level: 19

    Join Date
    Feb 2014
    Posts
    984
    Helped
    336 / 336
    Points
    6,866
    Level
    19

    Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB

    Can I ask what the application is? I’ve experimented with this compensation scheme but have only recently found other examples of it in custom audio opamps.

    I speculate that putting a limit on the “extra” pole will limit the large signal impact while maintaining the small signal benefit for locking in output accuracy at mid range frequencies.


    1 members found this post helpful.

  11. #11
    Advanced Member level 1
    Points: 4,408, Level: 15

    Join Date
    Jul 2006
    Location
    USA
    Posts
    412
    Helped
    8 / 8
    Points
    4,408
    Level
    15

    Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB

    It is a tranimpednace amplifier (TIA) used for converting the input current into output voltage.

    There are two poles, P1 and P2, and a zero, Z1. P1 is the dominant pole, and Z1 samller than P2. Z1 compensates P2.

    I am not sure about the influence of the poles and zero to middle range frequency signal.



  12. #12
    Super Moderator
    Points: 264,284, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    46,183
    Helped
    14044 / 14044
    Points
    264,284
    Level
    100

    Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB

    I am not sure about the influence of the poles and zero to middle range frequency signal.
    Run an .AC simulation in closed loop to see the effect. You get about 1.6 dB gain peaking in the 1 - 3 kHz range, corresponding to the overshoot in pulse response.


    1 members found this post helpful.

  13. #13
    Advanced Member level 3
    Points: 6,866, Level: 19

    Join Date
    Feb 2014
    Posts
    984
    Helped
    336 / 336
    Points
    6,866
    Level
    19

    Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB

    I was implying that the added open loop gain in the 1-100hz range would result in lower closed loop error. Looking at these closed loop simulation that's not true however due to the early onset of the peaking. The peaking means the phase delay stays lower at higher frequencies in case that matters.

    I expect that if the extra pole is moved left away from crossover the peaking will be reduced.


    1 members found this post helpful.

  14. #14
    Advanced Member level 1
    Points: 4,408, Level: 15

    Join Date
    Jul 2006
    Location
    USA
    Posts
    412
    Helped
    8 / 8
    Points
    4,408
    Level
    15

    Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB

    Oh, I see. Yes, the higher gain leads to an accurate settling, and the seperate pole improves the phase margin.

    The plots from FvM are used to illustrate that it is still stable even if the phase drops close to zero before the unit gain bandwidth given that the phase margin is large enough at the unit gain bandwidth.

    Thank you.


    1 members found this post helpful.

    •   AltAdvertisement

        
       

  15. #15
    Full Member level 4
    Points: 1,593, Level: 9

    Join Date
    Jan 2019
    Location
    Ireland
    Posts
    220
    Helped
    91 / 91
    Points
    1,593
    Level
    9

    Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB

    Quote Originally Posted by shanmei View Post
    The phase margin is 72 degree when the gain is 0dB. However, the phase margin is close to 0 before the frequency reaches the unity-gain bandwidth.

    Is the op-amp stable? Why? Thank you.
    Click image for larger version. 

Name:	stability.jpg 
Views:	29 
Size:	185.1 KB 
ID:	157193
    When the phase shift is close to 180 degrees, you still have a lot of open loop gain. This means that the denominator of G/1+GH can still be written as ~|GH| which would mean that the transfer function can be simplified as ~ 1/H. Of course there can be a lot of exceptions. But in most cases, it means that the closed loop transfer function does not become infinity at a frequency and therefore it is stable.

    Only where the loop gain comes close to 1 (or 0dB) and the phase shift is close to 180 degrees, will there be a chance that the closed loop TF goes to infinity and therefore you will have oscillations.


    1 members found this post helpful.

  16. #16
    Advanced Member level 1
    Points: 4,408, Level: 15

    Join Date
    Jul 2006
    Location
    USA
    Posts
    412
    Helped
    8 / 8
    Points
    4,408
    Level
    15

    Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB

    Quote Originally Posted by vivekroy View Post
    When the phase shift is close to 180 degrees, you still have a lot of open loop gain. This means that the denominator of G/1+GH can still be written as ~|GH| which would mean that the transfer function can be simplified as ~ 1/H. Of course there can be a lot of exceptions. But in most cases, it means that the closed loop transfer function does not become infinity at a frequency and therefore it is stable.

    Only where the loop gain comes close to 1 (or 0dB) and the phase shift is close to 180 degrees, will there be a chance that the closed loop TF goes to infinity and therefore you will have oscillations.
    Wow, that is a great explanation for this frequency response. Thank you.

    You are right, the key is whether the denominator of the loop gain becomes to be zero.

    And the gain is not the opamp gain, it is the loop gain.



--[[ ]]--