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Fully differential op amp: How to reduce ACM ?

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gn7623233

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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
[ATTACH=CONFIG]135551._xfImport[/ATTACH]
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
16145761_1575906445759777_799280320_o.jpg
 
Last edited by a moderator:

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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