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    Diff pair power supply rejection ratio

    Hi,

    I run the PSRR simulation for current source load diff amp and active current source load diff amp.And the PSRR plots are different and this diferences are conditioned by the diode connection.
    Please tell how I can explain this plots. Attached picture...

    Thanks.
    Click image for larger version. 

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    Re: Diff pair power supply rejection ratio

    The first circuit has a current mirror which appears to get its bias from an external source.

    The second circuit has a current mirror whose bias is connected to the left column, therefore it is determined by what goes on in the left column.

    This might explain difference in behaviors, although I'm not sure how it affects PSRR.



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    Re: Diff pair power supply rejection ratio

    Bias networks / generators are one of the main "back
    doors" for PS variation to enter the circuit as "signal".

    How your "VP" co-varies or anti-varies with VDD is the
    key, on the left.

    Both circuits look lame as far as output drive and both
    will only work at all when input is in a proper common
    mode range, and output likewise. Start with inspecting
    the bias points of the two. If the input and output DC
    voltages are different then forget AC attributes matching.



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    Re: Diff pair power supply rejection ratio

    Apparently both circuits short inn to outp, I don't recognize a suitable PSRR test setup. I also wonder how the full test circuit looks at all. If it's a fully differential amplifier, you need to acquire differential output voltage in the test.



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    Re: Diff pair power supply rejection ratio

    Quote Originally Posted by FvM View Post
    Apparently both circuits short inn to outp, I don't recognize a suitable PSRR test setup. I also wonder how the full test circuit looks at all. If it's a fully differential amplifier, you need to acquire differential output voltage in the test.
    I run the ac analysis,put ac=1 in VDD and connect the positive output with negative input to make a negative feedback(for compensation power noise) and diff amp worked in follower mode...
    And DC psrr values are different,but in first scheme,when is go the first pole the psrr is increase,but with active load,when is go the first pole the psrr is decrease and only then is increase...
    Could you please say why?



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    Re: Diff pair power supply rejection ratio

    Whether it's a reasonable PSRR setup or not - the different behavior can be easily explained by circuit asymmetry.



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    Re: Diff pair power supply rejection ratio

    Yes it's a PSRR test setup



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    Re: Diff pair power supply rejection ratio

    Whenever I try anything new as I simulate a differential amplifier, I find I have to adjust a lot of things in the long-tail pair, until I believe it's performing true differential action. (Until I succeed it could be something else like a mixer or adder.) The output needs to make sense since it is supposed to subtract one waveform from the other. Common mode should not appear in the output.

    The aim is to find a 'good operating point' combining supply voltage, input amplitudes, current levels, device sensitivities, etc.



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