how to remove the oscillation of step response

[icmeiying]

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

 

[alfredop]

Have you checked the stability of your common-mode feedback loop?

It would be helpful if you included a schematic of your design. How are you connecting the amplifier in unity-gain configuration?

 

[dick_freebird]

You might look for transient saturation (if this is a bipolar
design) or "windup" of stages under high overdrive conditions.
These will give you large signal overshoots that small signal
analysis won't indicate.

Sometimes you will see features like cross-clamping of diff pair
outputs, to address this sort of thing.

 

[icmeiying]

Have you checked the stability of your common-mode feedback loop?

It would be helpful if you included a schematic of your design. How are you connecting the amplifier in unity-gain configuration?

Yes, I have checked the stability of your common-mode feedback loop, the AC response is OK. I use four equal resistors to form the unity-gain buffer. Two worked as input resistor and two worked as feedback resistor. Thanks!

 

[jackielin007]

Hello, I also have this problem, the common mode loop is almost ideal (DC AC are all fine) but it is oscillating in transient simulation. I don't know why? anyone have a clue?

I am quite sure it is due to the simulator, because the oscillating frequency is far beyond the unity gain frequency of the CM and DF loop. But what setting should i change in the transient sim?:?:

 

[keith1200rs]

Hello, I also have this problem, the common mode loop is almost ideal (DC AC are all fine) but it is oscillating in transient simulation. I don't know why? anyone have a clue?

I am quite sure it is due to the simulator, because the oscillating frequency is far beyond the unity gain frequency of the CM and DF loop. But what setting should i change in the transient sim?:?:

To check to see if it is a simulation artifact, reduce the minimum timestep and see if the apparent oscillation changes. If it is a simulation artifact you will be seeing a triangular "oscillation" where the time between points is the minimum timestep. Another option is to change to "gear" integration rather than trapezoidal, although that is not usually the preferred solution.

Keith.