Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Biasing NMOS using current mirror

  1. Nov 8, 2011 #1
    Hi

    I need a brief explanation on how a Current Mirror is used to Bias an NMOS using Constant Current Model. My Doubts are :-
    * Where the input is given ?
    * If the current remains constant how does it work ?
    * If the current does not remain constant then why say constant current ?
    Bit Confused. Little help needed. :)

    Thanks
    Salil
     
  2. jcsd
  3. Nov 9, 2011 #2

    es1

    User Avatar

    Not sure what "bias an NMOS" means. Could you be more specific?
     
  4. Nov 9, 2011 #3
    Thanks for replying es1. By Biasing I mean setting the Qpoint of the NMOS using the current mirror i.e. setting Id constant. But I am not getting how the signal would be transferred from the input to the output. If Id remains constant what is the quantity that changes with the input signal? Coz i think since Id is set using constant current source ..... it wont change. Not able to put the pieces together :-(. A Brief Explanation or a good doc will help me out :-). Thanks.

    Thanks
    Salil
     
  5. Nov 9, 2011 #4
    Not entirely sure if I understand your wording correctly. Are you wondering how there is a voltage gain given a constant current?
     
  6. Nov 10, 2011 #5
    Sorry for that :-(. But i guess i m a bit confused about how it works. May be that's why i cant frame my question well. My notes are not giving good explaination. I am just not getting the main idea how it works. If you have time sandy.bridge , could you pls explain me how there is voltage gain ? that would definitely work for me.

    Thanks
    Salil
     
  7. Nov 10, 2011 #6

    es1

    User Avatar

    Slides 2-4 explain the mirror concept pretty well I think.
    http://www.aicdesign.org/SCNOTES/2010notes/Lect2UP160_(100325).pdf

    On slide 4 is should be pretty trivial to find iI when it is set by a resistor from a constant source in the standard most basic configuration. It's just iI=(Vdd-Vgs)/Rref because the gate and drain are shorted.

    And on slide 4 you can see that if the transistors are matched (in this case that means they have the same W & L) then iO=iI.
     
  8. Nov 11, 2011 #7
    Wow, Thanks. I got how the current mirror works. Phew! But still confused how the NMOS works when it is biased using constant current. :-)

    Thanks
    Salil
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Biasing NMOS using current mirror
  1. Why Use Kohm to bias? (Replies: 13)

Loading...