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  1. #1
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    Class AB fully differential amplifier design issue

    Dear friends,

    I am desinging a class AB fully differential amplifier like the one shown in the image below,

    If I don't use the transistors M7/M12 then I will get very high current in the output stage, but with the help of these transistor I can have a control on the output current to the level I want by tunning W/L of these transistors,

    can you please explain me how they are working ?

    Thank you

    Click image for larger version. 

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  2. #2
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    Re: Class AB fully differential amplifier design issue

    From what I see M7/M12 are just providing level shift. M6 and M5 are most probably same or ratioed. If Vds of M6 (defined by one Vgs above gnd) is bigger than Vds of M5 (defined by the output CM voltage) then the drain current of M6 is bigger than that of M5 (due to finite gds) and is mirrored into M9. The output CMFB through M3/4 will have to increase the Vgs of M5/M10 to compensate which will increase the current through them but also increase the current in M6/M11. It is kind of chasing its tail until some balance is reached perhaps at a higher current.


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  3. #3
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    Re: Class AB fully differential amplifier design issue

    This looks like a poor FD design. How does it achieve common mode control?


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  4. #4
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    Re: Class AB fully differential amplifier design issue

    Quote Originally Posted by FvM View Post
    This looks like a poor FD design. How does it achieve common mode control?
    Dear FvM, The CMFB ampliifer is not shown in this circuit, but I am using to control the Vbiasn (shown in the circuit) in such a away that if the output common mode voltage is increasing then the CMFB ampliifer decrease Vbiasn



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