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Negative PSRR in two stage opamp

Discussion in 'Electronic Design' started by [email protected], Sep 25, 2008.

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  1. Guest

    I have problems to simulate negative psrr in two stage opamp...
    If i consider an ac Vss in series with a dc VSS, if opamp is
    configured as buffer with V+ to ground, i should have Vout/Vss=1/
    Psrr-, so Psrr-=Vss/Vout.....is this right?
    I've simulated this configuration in spice with dc VSS=2V and ac
    Vss=0.1V, so psrr- should be Vss(=0.1V)/Vout...
    My problem is bode plot of Vss/Vout: arg[Vss/Vout] has higher values
    at higher frequency and this is not possible: what's wrong? Using the
    same method with psrr+ i have the right bode plot, so i think i've
    done some error in plotting....Can you help me?
    Thanks in advance
     
  2. Hello,

    Is there any comment in the model that the PSRR is modeled?
    If not, the simulation will be useless.
    Please show the model text.

    Best regards,
    Helmut
     
  3. Guest

    thanks for the answer...
    i don't have opamp macromodel because i've done the same two stage
    opamp used in this link:
    users.ece.gatech.edu/~pallen/Academic/ECE_6412/Spring_2003/L180-
    PSRR-2UP.pdf
    I've tried to use the same method to calculate positive psrr and
    negative psrr, but for negtive prss i have obtained this picture
    http://img262.imageshack.us/img262/5314/sdgsdfgru0.jpg
    i don't understand the picture, because phase shoud descrease as
    frequency increases and not the contrary....
    For positive psrr using the same method i have a correct bode plot...
    What's wrong?
    thanks
     
  4. Hello,

    This may be just a question how you have polarized your AC-source in series
    with the supply voltage source.

    Taking care of phase in a PSSR simulation is nonsense in my opinion.

    Best regards,
    Helmut
     
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