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  1. #1
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    Fully differential op amp: How to reduce ACM ?

    Hi everyone~

    I've designed a fully differential op amp

    but my common-mode gain is large

    how could I change the parameter to decrease my ACM to -20dB at DC

    Thanks for the reply..

    below is my hspice code

    Code:
    .subckt  op  vdd  vss  vip  vin  vop  von  vcm  clk
    
    M1      3      vip      1      vss     n_18     W=20u     L=1.5u   m=80
    M2      4      vin      1      vss     n_18     W=20u     L=1.5u   m=80
    
    M11     1      vb0     vss     vss     n_18     W=10u     L=1u     m=10
    
    
    M3      3      vb1     vdd     vdd     p_18     W=10u     L=1u     m=80
    M4      4      vb1     vdd     vdd     p_18     W=10u     L=1u     m=80
    M5     vop     vb2      3      vdd     p_18     W=20u     L=1u     m=30
    M6     von     vb2      4      vdd     p_18     W=20u     L=1u     m=30
    
    
    M7     vop     vb3      5      vss     n_18     W=5u      L=1u     m=20
    M8     von     vb3      6      vss     n_18     W=5u      L=1u     m=20
    M9      5      vb4     vss     vss     n_18     W=20u     L=1u     m=10
    M10     6      vb4     vss     vss     n_18     W=20u     L=1u     m=10
    
    
    ***************************CMFB**********************************
    
    
    Mcf1   d1      v+      d5      vss     n_18     W=50u     L=0.3u   
    Mcf2   vb0     vcm     d5      vss     n_18     W=50u     L=0.3u   
    Mcf3   d1      d1      vdd     vdd     p_18     W=50u     L=0.3u   
    Mcf4   vb0     d1      vdd     vdd     p_18     W=50u     L=0.3u   
    Mcf5   d5      vb5     vss     vss     n_18     W=50u     L=0.3u   
    
    R1     vop     v+      1x
    R2     von     v+      1x
    Click image for larger version. 
    
    Name:	16145761_1575906445759777_799280320_o.jpg 
    Views:	1 
    Size:	58.3 KB 
    ID:	135551
    Vb1   vb1     vss   1.1v
    Vb2   vb2     vss   0.63v
    Vb3   vb3     vss   1v
    Vb4   vb4     vss   0.53v
    Vb5   vb5     vss   0.6v
    .ends
    
    x1   vdd   vss   vip   vin   vop    von   vcm   clk   op
    
    Vdd   vdd    gnd    1.8v
    Vss    vss    gnd    0v
    E+     vip    vcm    7      vss    0.5 
    E-      vin    vcm    7      vss   -0.5 
    VID     7      vss    DC    0v     ac   1
    VIC    vcm   vss    DC    0.9v  
    
    
    C1     vop    gnd    4p
    C2     von    gnd    4p
    
    *.tf v(vop) vid
    .tf v(vop) vic
    Click image for larger version. 

Name:	16145761_1575906445759777_799280320_o.jpg 
Views:	1 
Size:	58.3 KB 
ID:	135551
    Last edited by andre_teprom; 22nd January 2017 at 13:54. Reason: Title related to the subject in question

    •   Alt22nd January 2017, 09:40

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  2. #2
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    Re: fully differential op amp

    Did you check if all transistors are in saturation?

    Also common mode gain as absolute number isn't significant. You'll either determine ratio of differential to common mode gain, or CMRR with external feedback.



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