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Simulators >> Circuit Simulators >> Help in using stb
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Message started by RobertZ on Jun 17^th, 2009, 11:10pm

Title: Help in using stb
Post by RobertZ on Jun 17^th, 2009, 11:10pm

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

Title: Re: Help in using stb
Post by wave on Jun 18^th, 2009, 8:17pm

RobertZ wrote on Jun 17^th, 2009, 11:10pm:

First, clarify if you are simulating DM or CM.
Either way, a differential circuit can have a rather strange behavior, if a large signal changes the gain or load into triode on either side.
If you are doing things properly, it shouldn't matter where in the loop it is broken --- assuming it's really the same loop.

Wave ::)

Title: Re: Help in using stb
Post by RobertZ on Jun 19^th, 2009, 12:00am

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

RobertZ wrote on Jun 17^th, 2009, 11:10pm:

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

Title: Re: Help in using stb
Post by Frank Wiedmann on Jun 19^th, 2009, 5:16am

I am afraid that neither this method, nor the alternative one using ideal baluns that I present on my webpage http://sites.google.com/site/frankwiedmann/loopgain are designed for circuits that are not completely symmetric (and balanced), so I would not expect them to work correctly in other cases.