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Design >> Analog Design >> the oscillation of step response with large signal
https://designers-guide.org/forum/YaBB.pl?num=1264002465

Message started by icmeiying on Jan 20^th, 2010, 7:47am

Title: the oscillation of step response with large signal
Post by icmeiying on Jan 20^th, 2010, 7:47am

Hi all:
Now I am designing a fully differential amplifier with unity-gain buffer connection. The fully differential amplifier is a two stage miller compensation structure. When I simulated AC response, the bode plot was ok and the phase margin was above 75 degree. Also, when I simulate the step response with small signal, it worked well and did not oscillate.
But when I simulate the step response with large signal, it oscillated.
So could you tell me how to solve the problem? I appreciate your help.

Best wishes

Title: Re: the oscillation of step response with large signal
Post by Frank Wiedmann on Jan 20^th, 2010, 8:11am

Take a look at http://www.designers-guide.org/Forum/YaBB.pl?num=1182388268, perhaps your circuit has a similar behavior.

Title: Re: the oscillation of step response with large signal
Post by icmeiying on Jan 20^th, 2010, 8:31am

Frank Wiedmann wrote on Jan 20^th, 2010, 8:11am:

Take a look at http://www.designers-guide.org/Forum/YaBB.pl?num=1182388268, perhaps your circuit has a similar behavior.

hi Frank

Thanks for your reply! My bode plot was not the same as that one the post mentioned. But it seems that my bode plot was a little different from the the theoretical one(two poles). That may be the problem.

Title: Re: the oscillation of step response with large signal
Post by buddypoor on Jan 20^th, 2010, 11:41am

icmeiying wrote on Jan 20^th, 2010, 8:31am:

.........
........ But it seems that my bode plot was a little different from the the theoretical one(two poles). That may be the problem.

No, I don`t think so. The Bode plot gives you only information on the small signal response of the system.
When your circuit response differs between small and large input signals, I don`t think that this will be caused by pole positions.

Title: Re: the oscillation of step response with large signal
Post by raja.cedt on Jan 20^th, 2010, 8:12pm

hi icmeiying,
as buddypoor said..i am sure the problem becuase of the small signal parameters variation due to large change in the input common mode voltage. So do .op analysis on the opamp with input common mode equal to your large swing and check for gm variation.

Thanks,
rajasekhar.

Title: Re: the oscillation of step response with large signal
Post by Mayank on Jan 20^th, 2010, 8:55pm

Hi,
You could try this approach -- Obtain Bode Plots for min, max & center input CM levels corresponding to your large signal swing.
If any of these Bode Plots indicate instability, then you can have problems in large signal swings.

Bode Plots can be trusted only if the phase function's monotonic.

--
Mayank.

Title: Re: the oscillation of step response with large signal
Post by buddypoor on Jan 21^st, 2010, 1:26am

Hi icmeiying,

have you ever heard about the "wind-up" effect?
May be the Miller compensation causes something similar.

Title: Re: the oscillation of step response with large signal
Post by icmeiying on Jan 21^st, 2010, 2:42am

hi buddypoor, raja.cedt and Mayank
Thanks a lot for all your suggestions! Today I replaced a pmos transistor that works in the linear region with a real nulling-zero resistor during the miller compensation, and found that everything was OK including the large signal response. The value of miller capacitor used by a pmos did not change.
To be honest, I was still a little confused about this phenomenon.

best wishes

icmeiying

Title: Re: the oscillation of step response with large signal
Post by raja.cedt on Jan 21^st, 2010, 3:50am

hi buddypoor,
whats that wind-up effect?

Thanks,
Rajasekhar.

Title: Re: the oscillation of step response with large signal
Post by buddypoor on Jan 21^st, 2010, 5:03am

raja.cedt wrote on Jan 21^st, 2010, 3:50am:

hi buddypoor,
whats that wind-up effect?

Thanks,
Rajasekhar.

Hi Rajasekhar,
Integrator "wind-up" is a phenomenon which can be seen in control systems which have an integrating controller. Very often, the actuators driven by the controller (electrical,mechanical,...) come to their limits although the integrating controller further integrates. Now, when the the error signal changes its sign (due to the control action) the integrator must be discharged up to a certain degree before the actuator can respond to the new condition. Thus, the integrator has itself wound-up. This leads to a delay within the loop and causes overshoots.

Title: Re: the oscillation of step response with large signal
Post by raja.cedt on Jan 21^st, 2010, 5:23am

hi buddypoor,
tx for your reply and i felt through u r explanation that this effect is like saturation non linearity.

Thanks,
Rajasekhar.