Problem with very small amplitude oscillation

[Ipanema]

oscillation

Hi,

My circuit consists of a long Limiting Amplifier stages and a closed cloop temperature and process compensation biasing circuit. For AC analysis, the phase margin is 100degree, after applying miller compensation.

My problem is the close loop compensation circuit shows a very small amplitude oscillation (I said oscillation and not coupling noise because it does not exhibit the same pattern as the input signal) when I applied ac signal to the input. When I removed the AC input signals and replaced with a dc signal, the oscillation is gone.

In this case, do you think that my circuit is stable? Why would it show oscillation only when there is an AC input signal? Pls Advise.


Thanks.

 

[CDRCDR]

oscillation

I guess it is still because of the Phase margin problems.In general PM > 50 for stability.If at startup,you see some amplitude osillation,then it is normal,but if after the transient slewing and still see the oscillation,you need to check whethter there is positive feedback in your biasing loop.

 

[purefen]

Re: oscillation

Dear:
I think that it should not be oscillation. Because as u apply input to ur amp, the unwanted signal will appear. But as u remove the signal source, the unwanted signal vanish. For "oscillation", even remove the signal source, the unwanted signal will still exsit.
U mention that u using miller compensation. I don't know what type of ur compensation capacitor. If u use MIM cap, u should check whether the MIM cap is overlay on ur input transistor or high impedance node of ur amp.
BR. rayray