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I try to incorporate the constant-gm bias circuit (Figure 6 in Improvements_in_biasing_and_compensation_of_CMOS_opamp) into the PFC (positive feedback frequency compensation) opamp, but it resulted in erroneous bode plot

Which part is wrong in the constant-gm bias circuit ?

constant gm bias circuit

Note that the PFC opamp itself does not cause the bode plot to go wrong as you can see below :

enter image description here

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  • \$\begingroup\$ What does the operating point simulation show? Likely issues include a faulty connection and/or a stage that is saturated. \$\endgroup\$ – W5VO Apr 17 at 4:57
  • \$\begingroup\$ @W5VO yes, the added constant-gm bias circuit itself caused the problem. Even after I removed miller mosfet (M21) and connect gate of M17 to some other node voltage, I still face similar problem. Could you help to debug ? mosfet_018.lib \$\endgroup\$ – kevin Apr 17 at 6:38
  • \$\begingroup\$ show your math analysis of the expected "constant GM" function \$\endgroup\$ – analogsystemsrf Apr 17 at 13:03
  • \$\begingroup\$ @kevin My recommendation is to use the operating point simulation (.op) to identify DC voltages and currents at various points in the circuit. It will help you track down which parts of the circuit are biased correctly, and which parts are not. Using the .op simulation, annotate the schematic with relevant voltages and currents, and that will help a lot more than asking us to blindly look at a schematic. \$\endgroup\$ – W5VO Apr 17 at 13:43
  • \$\begingroup\$ @W5VO See this published opamp circuit \$\endgroup\$ – kevin Apr 23 at 2:20

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