+ Post New Thread
Results 1 to 18 of 18
  1. #1
    Advanced Member level 4
    Points: 6,552, Level: 19
    Achievements:
    7 years registered

    Join Date
    Jan 2012
    Location
    Italy
    Posts
    1,169
    Helped
    44 / 44
    Points
    6,552
    Level
    19

    GBW id different from -3dB frequency

    Hello

    My operational amplifier Open loop frequency responce showing first order performance with GBW=48.66 MHz and phase margin about 58 Deg as you can see below

    Click image for larger version. 

Name:	openloopAC.png 
Views:	13 
Size:	50.3 KB 
ID:	158098


    After running the closed loop AC response with G= 1 I was expecting to get f-dB = GBW, but it is not the case, kindly see the below image

    Click image for larger version. 

Name:	closedAC.png 
Views:	9 
Size:	52.6 KB 
ID:	158099

    What does this mean ?

    Thank you

  2. #2
    Advanced Member level 5
    Points: 25,130, Level: 38
    barry's Avatar
    Join Date
    Mar 2005
    Location
    California, USA
    Posts
    4,812
    Helped
    1065 / 1065
    Points
    25,130
    Level
    38

    Re: GBW id different from -3dB frequency

    GBW is gain times bandwidth. At 48MHz you've got A=1, and that's where it starts to rolloff. (Well, not exactly, since you've got some peaking). GBW has nothing to do with the -3dB point. It simply identifies the frequency point where, if the opamp where open loop, the gain drops to 1.


    1 members found this post helpful.

  3. #3
    Advanced Member level 5
    Points: 41,401, Level: 49

    Join Date
    Mar 2008
    Location
    USA
    Posts
    6,667
    Helped
    1971 / 1971
    Points
    41,401
    Level
    49

    Re: GBW id different from -3dB frequency

    Your gain peaking is hiding the true rolloff you'd see in
    A=10, A=100, AVOL gain vs frequency plots. You might
    look at whether more or less peaking gives the best
    bandwidth and stability margin.


    1 members found this post helpful.

  4. #4
    Super Moderator
    Points: 266,359, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    46,544
    Helped
    14165 / 14165
    Points
    266,359
    Level
    100

    Re: GBW id different from -3dB frequency

    Open loop frequency response showing first order performance
    No it's not first order. First order means phase margin of 90 degree. There's considerable phase lag from additional poles, it causes gain peaking and the increased -3dB frequency in closed loop.


    1 members found this post helpful.

    •   AltAdvertisement

        
       

  5. #5
    Advanced Member level 4
    Points: 6,552, Level: 19
    Achievements:
    7 years registered

    Join Date
    Jan 2012
    Location
    Italy
    Posts
    1,169
    Helped
    44 / 44
    Points
    6,552
    Level
    19

    Re: GBW id different from -3dB frequency

    Thank you friends,

    I just want to say that gain bandwidth product is not equal to unity gain frequeny,

    in my amplifier the GBW equal to 43 MHz and as I presented before the unity gain frequency is 48.668 MHz. However, most of the designers they treat it the same which is confusing for me and might be the answer of my problem



    •   AltAdvertisement

        
       

  6. #6
    Advanced Member level 4
    Points: 6,552, Level: 19
    Achievements:
    7 years registered

    Join Date
    Jan 2012
    Location
    Italy
    Posts
    1,169
    Helped
    44 / 44
    Points
    6,552
    Level
    19

    Re: GBW id different from -3dB frequency

    Hi again,

    Below you find please in swquence the closed loop ac response with gain of 2, 4, 8, 16 and G=128

    Four your further discussion

    thank you

    Click image for larger version. 

Name:	G2.png 
Views:	3 
Size:	24.3 KB 
ID:	158114

    Click image for larger version. 

Name:	G4.png 
Views:	3 
Size:	24.2 KB 
ID:	158115

    Click image for larger version. 

Name:	G8.png 
Views:	3 
Size:	24.0 KB 
ID:	158116

    Click image for larger version. 

Name:	G16.png 
Views:	3 
Size:	25.0 KB 
ID:	158117

    Click image for larger version. 

Name:	G128.png 
Views:	2 
Size:	25.8 KB 
ID:	158118



    •   AltAdvertisement

        
       

  7. #7
    Super Moderator
    Points: 266,359, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    46,544
    Helped
    14165 / 14165
    Points
    266,359
    Level
    100

    Re: GBW id different from -3dB frequency

    GBW has nothing to do with the -3dB point. It simply identifies the frequency point where, if the opamp where open loop, the gain drops to 1.
    Don't want to decide about right or wrong, but there are at least other commonly used definitions. According to Analog Devices and TI application notes and datasheets, GBW is the product of (non-inverting) gain and small signal bandwidth. It's usually measured at higher gains (e.g. 10 or 100). Particularly for decompensated OPs, the GBW is considerably different from the open loop unity gain frequency, but smaller differences occur as soon as the phase margin is different from 90 degree.


    1 members found this post helpful.

  8. #8
    Advanced Member level 4
    Points: 6,552, Level: 19
    Achievements:
    7 years registered

    Join Date
    Jan 2012
    Location
    Italy
    Posts
    1,169
    Helped
    44 / 44
    Points
    6,552
    Level
    19

    Re: GBW id different from -3dB frequency

    Quote Originally Posted by FvM View Post
    Don't want to decide about right or wrong, but there are at least other commonly used definitions. According to Analog Devices and TI application notes and datasheets, GBW is the product of (non-inverting) gain and small signal bandwidth. It's usually measured at higher gains (e.g. 10 or 100). Particularly for decompensated OPs, the GBW is considerably different from the open loop unity gain frequency, but smaller differences occur as soon as the phase margin is different from 90 degree.
    Dear FvM. I have also read this from TI, they stated as you said that for uncompensated GBW and the unity gain frequency are different because there are different slope region when the gain is rolling down, mean transition from -20 dB/decade to -40 dB/decade and so on... I would expect that if I have in my open loop response such different gain slop in the region up to unity gain frequency, you saw in my first post how my amplifier is rolling with one slope only,

    Can you please give me your comment on the last results I uploaded,

    Thank you once again



  9. #9
    Advanced Member level 3
    Points: 11,130, Level: 25

    Join Date
    Nov 2001
    Location
    US
    Posts
    992
    Helped
    377 / 377
    Points
    11,130
    Level
    25

    Re: GBW id different from -3dB frequency

    I assume you are simulating your loop gain with already compensated amplifier and if it is compensated well, meaning the non-dominant pole is about 3x farther in frequency than the UGBW, then closing the loop should have a -3dB frequency pretty close to the GBW.
    However, from your original plot of the loop gain I see that right about 0dB crossing there seems to be some slight flattening of the magnitude response and at the same time the phase drops. This suggest for RHP zero. If your amplifier is Miller compensated, did you take care for mitigating the effects of the RHP zero that usually appears in that kind of compensation? Also, it will be better if you simulated your loop gain to higher frequency, this way you can see what happens after it crosses 0dB and where the non-dominant poles are.
    Last edited by sutapanaki; 10th March 2020 at 19:57.


    1 members found this post helpful.

  10. #10
    Advanced Member level 4
    Points: 6,552, Level: 19
    Achievements:
    7 years registered

    Join Date
    Jan 2012
    Location
    Italy
    Posts
    1,169
    Helped
    44 / 44
    Points
    6,552
    Level
    19

    Re: GBW id different from -3dB frequency

    Quote Originally Posted by sutapanaki View Post
    I assume you are simulating your loop gain with already compensated amplifier and if it is compensated well, meaning the non-dominant pole is about 3x farther in frequency than the UGBW, then closing the loop should have a -3dB frequency pretty close to the GBW.
    However, from your original plot of the loop gain I see that right about 0dB crossing there seems to be some slight flattening of the magnitude response and at the same time the phase drops. This suggest for RHP zero. If your amplifier is Miller compensated, did you take care for mitigating the effects of the RHP zero that usually appears in that kind of compensation? Also, it will be better if you simulated your loop gain to higher frequency, this way you can see what happens after it crosses 0dB and where the non-dominant poles are.

    Dear Suta,

    Thank you for your reply

    I am using fully differential folded cascod amplifier with class AB output buffer stage , as you can also see the second stage of it in my below image, I used Ahuja or indirect compensation, the capacitor is fed to the cascode transistors, where it is supposed not needed to add resistor with it in series for the RHP compensation, as far as I know

    Click image for larger version. 

Name:	rhp_circuit.png 
Views:	10 
Size:	4.7 KB 
ID:	158120

    Here I have simulated for you at higher frequency

    Click image for larger version. 

Name:	rhp.png 
Views:	4 
Size:	45.2 KB 
ID:	158121



  11. #11
    Advanced Member level 3
    Points: 11,130, Level: 25

    Join Date
    Nov 2001
    Location
    US
    Posts
    992
    Helped
    377 / 377
    Points
    11,130
    Level
    25

    Re: GBW id different from -3dB frequency

    OK, I see. If you do Ahuja compensation, you don't need the resistor in series with the Miller capacitor, that's true. However, Ahuja compensation needs careful design. It can lead to complex conjugate poles in your loop gain if not designed correctly. I think you have a bit of this showing in your plots - the slight bump in the Bode plot near crossover frequency. And it, of course will roll-off your phase faster. Did you check the stability of the internal feedback loop of the Ahuja compensation?


    1 members found this post helpful.

  12. #12
    Advanced Member level 4
    Points: 6,552, Level: 19
    Achievements:
    7 years registered

    Join Date
    Jan 2012
    Location
    Italy
    Posts
    1,169
    Helped
    44 / 44
    Points
    6,552
    Level
    19

    Re: GBW id different from -3dB frequency

    Quote Originally Posted by sutapanaki View Post
    OK, I see. If you do Ahuja compensation, you don't need the resistor in series with the Miller capacitor, that's true. However, Ahuja compensation needs careful design. It can lead to complex conjugate poles in your loop gain if not designed correctly. I think you have a bit of this showing in your plots - the slight bump in the Bode plot near crossover frequency. And it, of course will roll-off your phase faster. Did you check the stability of the internal feedback loop of the Ahuja compensation?
    Thank you Suta for your reply,

    Indeed I didnt get what you mean by "Did you check the stability of the internal feedback loop of the Ahuja compensation?",



  13. #13
    Advanced Member level 3
    Points: 11,130, Level: 25

    Join Date
    Nov 2001
    Location
    US
    Posts
    992
    Helped
    377 / 377
    Points
    11,130
    Level
    25

    Re: GBW id different from -3dB frequency

    There is an internal loop when using the Ahuja compensation, which needs to be stable. You probably forgot but we have already discussed this point before.

    https://www.edaboard.com/showthread....-or-transistor


    1 members found this post helpful.

  14. #14
    Advanced Member level 4
    Points: 6,552, Level: 19
    Achievements:
    7 years registered

    Join Date
    Jan 2012
    Location
    Italy
    Posts
    1,169
    Helped
    44 / 44
    Points
    6,552
    Level
    19

    Re: GBW id different from -3dB frequency

    Quote Originally Posted by sutapanaki View Post
    There is an internal loop when using the Ahuja compensation, which needs to be stable. You probably forgot but we have already discussed this point before.

    https://www.edaboard.com/showthread....-or-transistor
    Thank you Suta for reminding me with that post

    You have presented a solution as shown below,

    Click image for larger version. 

Name:	Ahuja.PNG 
Views:	3 
Size:	136.7 KB 
ID:	158213

    To increase gm M10 I have to increase the current and or W/L, since I dont want to disturb the current in the output branch I would go to increase W/L, however this will increase my design area, I am already using 20 µm for the NMOS and 60 µm for the PMOS

    I read back that post and your discussion with Frankrose, looks like your prediction was right since that time, Ahuja is creating an issue of peeking in the closed loop configuration

    One more question please, how can I see my former posts in the forum ?

    Thank you once again



  15. #15
    Super Moderator
    Points: 266,359, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    46,544
    Helped
    14165 / 14165
    Points
    266,359
    Level
    100

    Re: GBW id different from -3dB frequency

    how can I see my former posts in the forum ?
    Many ways:
    1. Use advanced search
    2. Click on user name in a post, select View Forum Posts

    Click image for larger version. 

Name:	view1.PNG 
Views:	1 
Size:	14.4 KB 
ID:	158225

    3. In top menu, select Important Links/My Posts or /My Posts as Threads
    The latter shows only the threads you have started


    1 members found this post helpful.

  16. #16
    Advanced Member level 3
    Points: 11,130, Level: 25

    Join Date
    Nov 2001
    Location
    US
    Posts
    992
    Helped
    377 / 377
    Points
    11,130
    Level
    25

    Re: GBW id different from -3dB frequency

    Do you use min L for M10? Also, in that older post I pointed to a reference where there are other ways to compensate for the Ahuja loop.


    1 members found this post helpful.

    •   AltAdvertisement

        
       

  17. #17
    Advanced Member level 4
    Points: 6,552, Level: 19
    Achievements:
    7 years registered

    Join Date
    Jan 2012
    Location
    Italy
    Posts
    1,169
    Helped
    44 / 44
    Points
    6,552
    Level
    19

    Re: GBW id different from -3dB frequency

    Quote Originally Posted by sutapanaki View Post
    Do you use min L for M10? Also, in that older post I pointed to a reference where there are other ways to compensate for the Ahuja loop.
    Dear Suta,

    I am not using minimum length since the gain was dropping severely,

    for other methods, I tried to follow those links but was not working



  18. #18
    Advanced Member level 3
    Points: 11,130, Level: 25

    Join Date
    Nov 2001
    Location
    US
    Posts
    992
    Helped
    377 / 377
    Points
    11,130
    Level
    25

    Re: GBW id different from -3dB frequency

    Your gain shouldn't really depend strongly on the cascode device. You should use small L if you want to push the zero ot increase gm10. Maybe not the min L but close to it.



--[[ ]]--