+ Post New Thread
Results 1 to 7 of 7
  1. #1
    Advanced Member level 2
    Points: 2,706, Level: 12

    Join Date
    Feb 2016
    Posts
    504
    Helped
    1 / 1
    Points
    2,706
    Level
    12

    zero AC gain of CMOS inverter

    Why is the following AC analysis of CMOS inverter resulted in zero AC gain (vout/vin) ?


    •   AltAdvertisment

        
       

  2. #2
    Advanced Member level 5
    Points: 40,247, Level: 49
    Achievements:
    7 years registered
    LvW's Avatar
    Join Date
    May 2008
    Location
    Germany
    Posts
    5,770
    Helped
    1724 / 1724
    Points
    40,247
    Level
    49

    Re: zero AC gain of CMOS inverter

    The feedback resistor has a value of 1E20 ??? Typing error?



    •   AltAdvertisment

        
       

  3. #3
    Super Moderator
    Points: 253,898, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    44,234
    Helped
    13462 / 13462
    Points
    253,898
    Level
    100

    Re: zero AC gain of CMOS inverter

    vout/vin isn't zero, it's about -42 at 1 Hz starting frequency according to the plot. At GHz frequencies, vout is shorted by 1 nF Cout. Unreasonable setup, inappropriate plot parameters. You may want to plot with log frequency scale to better see what's happening.

    The feedback resistor has a value of 1E20?
    The resistor has no effect during first part of AC analysis, the inverter is effectively operated open loop. By chance, it's biased almost to maximum gain, see DC analysis plots.



    •   AltAdvertisment

        
       

  4. #4
    Advanced Member level 2
    Points: 2,706, Level: 12

    Join Date
    Feb 2016
    Posts
    504
    Helped
    1 / 1
    Points
    2,706
    Level
    12

    Re: zero AC gain of CMOS inverter

    See the alter section which basically trying to switch on and off certain branches of the circuit.

    By the way, removing cout solves the zero AC gain issue. Note that the RC time constant = 100G*1n = 100s , or 0.01Hz filtering.



  5. #5
    Super Moderator
    Points: 253,898, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    44,234
    Helped
    13462 / 13462
    Points
    253,898
    Level
    100

    Re: zero AC gain of CMOS inverter

    Note that the RC time constant = 100G*1n = 100s , or 0.01Hz filtering.
    Wrong, the gain is limited by the time constant of inverter output impedance and 1 nF Cout, I guess µs range respectively some 100 kHz cut off. You still didn't plot the gain with reasonable frequency scale?


    1 members found this post helpful.

    •   AltAdvertisment

        
       

  6. #6
    Advanced Member level 2
    Points: 2,706, Level: 12

    Join Date
    Feb 2016
    Posts
    504
    Helped
    1 / 1
    Points
    2,706
    Level
    12

    Re: zero AC gain of CMOS inverter

    You still didn't plot the gain with reasonable frequency scale?
    I do not understand what you mean ? You mean in logarithmic scale ?

    Or you mean something else ?



  7. #7
    Super Moderator
    Points: 253,898, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    44,234
    Helped
    13462 / 13462
    Points
    253,898
    Level
    100

    Re: zero AC gain of CMOS inverter

    Yes, as suggested in post #3.



--[[ ]]--