question about stb analysis & iprobe component

[jimito13]

Hello,

I am trying to calculate the loop gain and phase for my design after inserting the common mode feedback loop of the main opamp.This design consists of a fully diff folded cascode opamp with gain boosters.

For measuring the above things i run stb analysis with the iprobe component but the result does not seem reasonable...a flat line with some negative kdB's gain and something similar (absolutely flat line at 180deg) for the phase plot.I tried to put the iprobe component in different places (inverting input of the opamp,inverting output of the opamp,cut the loop somewhere else and put it there) but the result is the same.Where is the right place to insert the iprobe for correct measurements??

And a final question.When running stb analysis,should i or slouldn't i inject ac diff signal at the inputs of the opamp?I suppose that answer is yes since stb seems to run ac analysis at the background but i am not certain.

Thanks in advance for any helpful answer.

 

[erikl]

... i run stb analysis with the iprobe component but the result does not seem reasonable...a flat line with some negative kdB's gain and something similar (absolutely flat line at 180deg) for the phase plot.

This looks like your ac stimulus is corrupt. Did you - inadvertently - use a common mode stimulus (same phase stimuli at both inputs, instead of inverted phase)?

I tried to put the iprobe component in different places (inverting input of the opamp,inverting output of the opamp,cut the loop somewhere else and put it there) but the result is the same.Where is the right place to insert the iprobe for correct measurements??

In the feedback path(s).

And a final question.When running stb analysis,should i or slouldn't i inject ac diff signal at the inputs of the opamp?I suppose that answer is yes since stb seems to run ac analysis at the background but i am not certain.

Yes, you are right, you should.

 

[jimito13]

Ok,i fixed the problem,i was breaking the loop at the wrong place...i inserted the iprobe component somewhere in the loop with the correct direction,looking from loop's input to it's output and i managed to measure the loop gain and phase.

I noticed that stb analysis doesn't "care" if we inject diff ac signal at the input of the opamp...In both cases (signal on and off) results are the same.Are you aware of that or am i supposing something wrong?

But...i have another problem...i am designing tha classical resistor averaged resistor CMFB circuit.Between folded cascode's output and the resistors that sense the common mode output dc voltage i put source followers as buffers so that my freq responce (gain) will remain untouched by the resistors.With or without buffers the cmfb doesn't seem to work at all.Have any idea??The bias voltage that will be fed back to the opamp is close to the desired value but all other biasing of the opamp is destroyed...

And a final question,should i insert something else between opamp's output and sensing resistors in order to prevent ac signal go into the cmfb circuit??

 

[erikl]

I noticed that stb analysis doesn't "care" if we inject diff ac signal at the input of the opamp...In both cases (signal on and off) results are the same.Are you aware of that or am i supposing something wrong?

No, I'm not aware of that. This would mean the stb analysis assigns its own stimulus. But how would it know where the input(s) is/are? At least this info has to be given.

Sorry, I can't help you with your CMFB pb.

 

[jimito13]

Since stb is running ac in the background,probably searches the schematic for ac sources and sets automatically the required values...i suppose...

Thank you very much for your answers

 

[sharpsheep]

I think the iprobe component could not be used to test fully differential loop gain and phase. You can insert large ind, eg 1GH in the feedback path, and use AC coupling capacitor to set AC input signal. The iprobe component can be used to test loop gain and phase when the feedback is at a single path rather than a differential path. For example, cmfb circuit is feedback to the gate of tail current source of opamp, you can insert a iprobe component between the feedback point and the gate of tail current source. Then run the stb analysis, and you can get the loop gain and phase of cmfb circuit.

 

[jimito13]

sharpsheep : I finally managed to measure loop gain and phase exactly as you describe.For fully diff cmfb loops the proper component to use is cmdmprobe from analoglib as i discovered but this is not my case anyway...I get about 20dB for the loop gain and above 90 deg phase margin.I think that the gain is small...do you have any idea of typical values of loop gain for op-amps?Also,for my topology exist two gain boosting amplifiers in addition to the main folded cascode.I designed cmfb circuits for them as well,am i right or it is not needed?Thanks for your reply.

 

[evilpicasso123]

you can use cmdmprobe to a diff path to find PM