Giving my circuit symetrical drive??

Discussion in 'Homework Help' started by Denny1234, Oct 29, 2008.

  1. Denny1234

    Thread Starter Member

    Feb 17, 2008
    27
    0
    Hi I've done a really easy inverter design which i have attached (sorry about the quality). However after making it my tutor has told me I have to give it symetrical drive?

    I've read about it somewhere because I know it means giving the same delay for pull up or pull down but I dont have a clue how to implement that practically, what would have i have to do to my design to give it symetrical drive?? I've looked on google and my textbooks and there is surprisingly very little about it.

    Thanks for any help
     
    Last edited: Oct 29, 2008
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
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    Here is a hint: P-channel carrier mobility is about 1/3 that of n-channel.
     
  3. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    To expand on Ron's comments, the key to creating symmetrical gates is understanding the proportion of mobility.

    Typically, mobility is ~25uA/V^2 for NMOS and 10uA/V^2 for PMOS. So, the hole mobility of PMOS is about 40% of the electron mobility of NMOS.

    You can relate this to the W/L ratio of the transistor. You would size either the NMOS or PMOS transistor for a rated output current, which is inversely proportional to Ft. Anyways, if you chose a W/L of 10/1 for NMOS, then you would need to use a 25/1 W/L ratio for PMOS to make it symmetrical. For other arrangements, it gets more complex.

    Steve
     
    Last edited: Oct 30, 2008
  4. Denny1234

    Thread Starter Member

    Feb 17, 2008
    27
    0
    Thanks for that, we were given the mobility of the pmos and nmos on my worksheet and I should have realised that they would have been a reason for it!

    Cheers.
     
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