Hi all,


i'm trying to design a low-voltage pseudo-differential amplifier. There are no tail currents. It seems that no matter what i do, I cannot stop the CM oscillation. At the desired operating point, the phase margin is well above 60 degrees, but i suspect that the circuit is simply too sensitive to CM disturbance. I found that if I use CMFB without trying to define the CM voltage, then it works ok. However, in that case it is very susceptible to variation due to process variation. Does anybody have any experience in this matter, and care to give some feedback?


thanks,
Aaron

Here's the schematic. Common-mode is the voltage i want to set the CM level to.

Thanks for the reply,


The output CM voltage is determined by the current through the output stage (at CM the load is diode connected), which is controlled by the CMFB. I can't see a problem there. Actually I can get this circuit to work without any problems if the supply voltage is higher and i use a tail current at the input stage.

Based on phase margin analysis of the feedback loops, there should be no problems. I've tried a few variations of this circuit and they all seem to have stability problems.

thanks,
Aaron

Hi subgold,


I think i've found out why it was oscillating (managed to get it to stop), and i'll see if i can post it after I analyze it to make sure. As for the analysis, I broke the loop with an iprobe and ran an stb analysis.

I realize with three gain stages I should have three poles and therefore a poor phase margin, but I guess due to the sizing of the stages, the PM didn't need compensation.

You mentioned that the CMFB should be able to regulate the circuit even when the operating points are deviated from the desired values. I was wondering about that too since I usually only ensure good PM at the operating point (my circuits input and output CM are fixed). However, on start-up, the PM is probably very different from what it is at the desired operating point. So how do we normally deal with this?


thanks,
Aaron