Mosfet Analysis

Discussion in 'Homework Help' started by dalam, Sep 8, 2014.

  1. dalam

    Thread Starter Member

    Aug 9, 2014
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    [​IMG]
    How should I analyse this circuit?
    I did it assuming PNP MOSFET on and NPN off and then change the assumption. It became too complex. Can I solve it simply be looking at options available?
     
  2. crutschow

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    Mar 14, 2008
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    NPN and PNP refers to the polarity of BJTs. MOSFETs are defined as N-channel or P-channel.

    The circuit is in the linear region when both MOSFETs are conducting at the same time. So under what input voltage conditions is that true? Do you know what Vtp and Vtn means?
     
  3. Jony130

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    Feb 17, 2009
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  4. dalam

    Thread Starter Member

    Aug 9, 2014
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    Yes, I know what Vtp/Vtn means. It specifies the minimum |Vgs| needed to turn the mosfet on. This is required to create the channel.
    Which region should I choose ? Linear or sturation?
     
  5. dalam

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    Aug 9, 2014
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    Last edited: Sep 8, 2014
  6. #12

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    Nov 30, 2010
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    Because A only defines one end of the region in question. Right, Jony?
     
  7. Jony130

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    Good question. But forget about that, I wrongly understood the question. They ask only about M1 transistor. And ask about the linear region (triode region). So in this case I choose A as a right answer.
     
  8. crutschow

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    Answer A might be correct if it was >1.875V but it says <1.875V.
     
  9. Jony130

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    M1 is a P channel so for Vin<1.875V M1 will be in triode/linear region.
     
  10. crutschow

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    Egg on my face. :oops: I was thinking of M2.
     
  11. dalam

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    Aug 9, 2014
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    Jony, I understand that A is correct answer among these. But what does the value of 1.875V signifies here? Why it is a boundary? Or is it to be solved by just looking at options?
     
  12. Jony130

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    Well for Vin>1.875 the M1 transistor leaves the linear region and enters the saturation region.
    So, M1 is in linear region and M2 is in saturation.

    ID1 = ID2
    Id1 = Kp*(Vgs - Vtp)*Vds - Vds^2/2

    Id1 = 0.5*Kn*(Vgs - Vtn)^2

    Then
    (Vin - Vdd - Vtp)*(Vout-Vdd) - (Vout - Vdd)^2/2 = 0.5*(Vin - Vtn)^2

    And M1 leaves the linear region when Vds = (Vgs - Vtp) hence Vout - Vdd = Vin - Vdd - Vtp
    then
    Vout = Vin - Vt

    And after we solve this for Vin I get 1.8333V

    http://www.wolframalpha.com/input/?i=(X+-+5+-+(-1))+*+(X+-+1+-+5)+-+(X+-+1+-+5)^2/2+=+(X+-+1)^2/2

    But I made a error so this solution is not true.

    http://oldeee.see.ed.ac.uk/private/ee3/med/rc/week5MOSFET.pdf
    http://www.prenhall.com/howe3/microelectronics/pdf_folder/lectures/tth/lecture11.fm5.pdf
    http://nptel.ac.in/courses/117101058/downloads/Lec-15.pdf
     
    dalam likes this.
  13. dalam

    Thread Starter Member

    Aug 9, 2014
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    Thanks a lot Jony!! You have helped me a lot here.
    I guess in case of PMOS the Vgs,Vds should be Vsg and Vsd respectively.
     
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