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Nonlinearity Separation

itos

This is with 95% a question without solution but I still try it .. maybe there is something out there.

For noise the different sources are visible in the results browser and noise summary so I can identify the devices/sources which kill noise performance.

I would be looking for something similar for linearity (e.g. IIP3, HD3, IM3, ...).

The issue: A simple differential pair (PMOS with NMOS loads or reverse).

The IIP3 versus VGS, VDS, GM/ID etc looks arbitrary and I have no control what is limiting linearity.

I can get a relatively accurate expression for IIP3 for drain current linearity but this is 30-40 dB (!!) better than what I get. So I assume I am limited by ids/vds linearity. But is it the gm transistor or the load? And what know to dial to get a handle on this?

Some sort of "nonlinearity" separation could confirm that I may neglect the common ids/vgs nonlinearity and just plot IIP3 vs. VDS of a single transistor.

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  • Frank Wiedmann

    The compression distortion calculation only takes the amplitudes at the fundamental frequency into account.

    For 1 mV input, you get 20*log10((10 mV - 100 nV)/10 mV) = 20*log10(0.99999) = -86.86 udB.

    This matches the result I get with SPECTRE16 ISR1 in IC617 ISR8:

    AC Compression Distortion Summary                             

    Results in        ac_distortion    

                                        Nonlinear Mag(Phase)[V(Deg)] at 1st 2nd & 3rd harm of linear freq                                       
    Instance          Distortion(dB)    freq=1            freq=2            freq=3           
    Total             -86.86u           100n(180)         1.031a(180)       33.33n(180)      
    G0                -86.86u           100n(180)         1.031a(180)       33.33n(180)      

    The phase is shown as 180 degrees, so the values are negative.

    For 1 V input, I get the following result:

    AC Compression Distortion Summary                             

    Results in        ac_distortion    

                                        Nonlinear Mag(Phase)[V(Deg)] at 1st 2nd & 3rd harm of linear freq                                       
    Instance          Distortion(dB)    freq=1            freq=2            freq=3           
    Total             19.08             100(180)          11.23f(-161.6)    33.33(180)       
    G0                19.08             100(180)          11.23f(-161.6)    33.33(180)       

    This is calculated as 20*log10(|(10 V-100 V)/10 V|) = 20*log10(9) = 19.08 dB.

    Your older version does not take the phase into account and calculates for 1 mV 20*log10(1.00001) = 86.86 udB and for 1 V 20*log10(11) = 20.83 dB.

Reply
  • Frank Wiedmann

    The compression distortion calculation only takes the amplitudes at the fundamental frequency into account.

    For 1 mV input, you get 20*log10((10 mV - 100 nV)/10 mV) = 20*log10(0.99999) = -86.86 udB.

    This matches the result I get with SPECTRE16 ISR1 in IC617 ISR8:

    AC Compression Distortion Summary                             

    Results in        ac_distortion    

                                        Nonlinear Mag(Phase)[V(Deg)] at 1st 2nd & 3rd harm of linear freq                                       
    Instance          Distortion(dB)    freq=1            freq=2            freq=3           
    Total             -86.86u           100n(180)         1.031a(180)       33.33n(180)      
    G0                -86.86u           100n(180)         1.031a(180)       33.33n(180)      

    The phase is shown as 180 degrees, so the values are negative.

    For 1 V input, I get the following result:

    AC Compression Distortion Summary                             

    Results in        ac_distortion    

                                        Nonlinear Mag(Phase)[V(Deg)] at 1st 2nd & 3rd harm of linear freq                                       
    Instance          Distortion(dB)    freq=1            freq=2            freq=3           
    Total             19.08             100(180)          11.23f(-161.6)    33.33(180)       
    G0                19.08             100(180)          11.23f(-161.6)    33.33(180)       

    This is calculated as 20*log10(|(10 V-100 V)/10 V|) = 20*log10(9) = 19.08 dB.

    Your older version does not take the phase into account and calculates for 1 mV 20*log10(1.00001) = 86.86 udB and for 1 V 20*log10(11) = 20.83 dB.

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