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cmdmprobe from analogLib

illaoi

so I am looking at this component and wanna make sense of how it does calculate the loop gain.


from here we have 

(1) in1 = out1

(2) (in1 - out1)*CMDM = in2 - out2, but from (1), that yields in2 = out2.

so far, all reasonable, since we broke the loop, and these variables are equal. What I need to understand is finj, so from PCCCS, I think we can write for the current that injects to out2, 

(3) the current the "iprove vinj" has times CMDM?

i need help to understand 


and how this essentially calculates the loop gain from stability analysis?

Also is CMDM=1 differential or common-mode stability?

Thanks!

Parents Reply Children
  • in reply to ShawnLogan

    Following on from Shawn's reply, you should not use cmdmprobe. It is obsolete. The big clue is when you try to instantiate it:

    It was replaced because it didn't handle unbalanced differential loops, and the diffstbprobe is a much better way of doing this - then the choice as to whether you are measuring common-mode or differential-mode loop gain is a choice on the stb analysis form rather than being a parameter of the probe device. 

    Andrew

  • in reply to ShawnLogan
    ShawnLogan said:
    I will not speak for Frank, but I believe he also recommends the use of the Cadence diffstbprobe in lieu of the cmdmprobe

    This is of course absolutely correct. Thank you also for referencing my website; I explain the diffstbprobe there in the section Loop Gain of Differential Circuits.

  • in reply to Frank Wiedmann

    Thanks to all the replies, understood.

  • in reply to illaoi

    Since we are on the note of stability, I wanted to make sure on another thing, assuming a fully differential block, it can have cascade of differential amplifiers, or amplifier plus active filter, ..., all differentials.

    I want to know, is having two ports at input and output, and looking at k-factor sufficient for the system to be stable differentially?

    also, for common mode stability, is shorting the inputs and connecting to input port, and shorting the outputs and connecting to output port and looking at k-factor if greater than one is sufficient to make sure the system has common mode stability at all cases?

    In other words, is choosing the short route of doing so rather than doing individual block differential and common mode stability more efficient?

  • in reply to illaoi

    K-factor stability is a different concept with limited applicability, see for example www.microwaves101.com/.../Stability-Analysis-Presentation.pdf

  • in reply to illaoi

    Dear illaoi,

    illaoi said:
    Since we are on the note of stability, I wanted to make sure on another thing, assuming a fully differential block, it can have cascade of differential amplifiers, or amplifier plus active filter, ..., all differentials.

    I want to know, is having two ports at input and output, and looking at k-factor sufficient for the system to be stable differentially?

    The use of the K factor to determine if an amplifier is unconditional stable is limited to a single-stage amplifiers. It may not predict oscillation between its stages.

    illaoi said:
    also, for common mode stability, is shorting the inputs and connecting to input port, and shorting the outputs and connecting to output port and looking at k-factor if greater than one is sufficient to make sure the system has common mode stability at all cases?

    The use of K to predict unconditional stability is really limited to single-stage amplifiers where the input and output impedances can be well defined. Stability circles can be more helpful to assess multistage amplifier stability.

    illaoi said:
    In other words, is choosing the short route of doing so rather than doing individual block differential and common mode stability more efficient?

    It is more efficient, but it may not lead to a correct assessment of stability!

    As a side note, the use of solely K to predict single amplifier stability can also lead to incorrect results. There are a couple of examples in reference [1] where the specific conditions for K to predict stability are specifically defined.

    Shawn

    [1] A. Platzker, W. Struble and K. T. Hetzler, "Instabilities diagnosis and the role of K in microwave circuits," 1993 IEEE MTT-S International Microwave Symposium Digest, Atlanta, GA, USA, 1993, pp. 1185-1188 vol.3, doi: 10.1109/MWSYM.1993.277082.

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