hi, there,

If I unbalance an ota, meaning non-zero differential output and input, the stb analysis result is depending on where i place the cmdm probe.  If i place cmdm probe before the ota, it gives me a reasonable result; if the cmdm probe is placed after the ota, it gives much higher dc gain. Actucally, dc gain with unbalanced opamp is lower than the gain in balanced condition, which is proved in my dc simulation.

if the same ota is balanced, the stb result is then not dependent on the location of cmdm probe.

it seems to me the cmdm probe has trouble if it sees large differential dc signal. Anyone has similiar experience?

Thanks,
Robert

wave wrote on Jun 18^th, 2009, 8:17pm:



Hi, Wave,
I am simulating DM loop, cmdm is set to -1. I applied differential signals from small to large. The loop gain starts increasing at the beginning, then decreasing. No transistors go into triode region in all the cases I tried. I also check dc gain (by sweeping differential  input/output and take their ratio) and I know dc gain should monotonically drop with increasing differential output for my circuit. I break the loop at posistion A, since it is a common point.
I know I am not supposed to break at B, but somehow, stb analysis with this choice gives me the result I expected, meaning decreasing low frequency loop gain.

Thanks,
Robert