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calculating MOSFET parameters K & Ve

Discussion in 'Electronic Design' started by [email protected], Jun 3, 2007.

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  1. Guest

    Hi,

    I'm having problems with measuring (through simulation) values for K
    and Ve, in order that I can then use those values to design with in a
    process which I have the design kit for. Something has gone wrong
    though because when I try and use these values to design a
    differential amplifier (following the methods in Sansens book "Analog
    Design Essentials"), the gain and bandwidth I get are just rubbish -
    nothing to do with the intended specifications I calculated device
    sizes for. Can anyone tell me whether my procedure for calculating K
    and Ve is sensible? It is as follows:

    To calculate K, I just used a single nmos transistor (w=1u l=1u) with
    a 50k resistor from vdd to drain. Then K'=Id/((Vgs-Vt)^2), and my
    results for a sweep of vin from 0.75v -> 1.6V gave me a decaying
    exponential with a knee at about vin=0.9v (ie. beyond (vgs-vt)~0.9V,
    K' is flattish). So I have a range of K' values to use depending on
    what the vgs-vt is.

    To calculate Ve, I found an expression rds=VeL/(Ids), so I used the
    same circuit as above but stepped Vin from 0.75V to 1.3V. My simulator
    tells me the gds (=1/rds) at each Vin, so I just computed Ve=rds*Ids,
    to get a result in Volts/micron. Plotting the results, I get a
    parabola rising from Ve=30 at Vin=0.75, and peaking at Ve=52 at
    Vin=1.2V (then decaying after Vin=1.2).

    Part of my problem could be because my Vt value seems to change a
    little for some reason, according to my simulators output. I've
    connected the bulk to source so I don't know why this is. I'm not sure
    if I'm doing something else fundamentally wrong though.

    Thanks for any insight on this,

    Oh, and thanks for the replys to my post the other day on what a Gm
    stage is, that cleared some things up!

    Ted
     
  2. Jim Thompson

    Jim Thompson Guest

    Simple-minded text book equations are just ducky for getting a "feel"
    for how things work, but rarely work very accurately in actual
    practice.

    Surely Willy (*) has some more advanced (and accurate) equations in
    his book.

    Willy Sansen and I shared the podium for a week in 1986 at the Royal
    Melbourne Institute of Technology... Willy lecturing on CMOS analog
    design, I on bipolar.

    We both like Chinese food ;-)

    ...Jim Thompson
     
  3. Guest

    He does make references to more exact equations but I don't think that
    is the aim of the chapter in question; an introduction to the design
    parameters for a mos transistor. I think he just wants to give the
    reader some easy-to-use first-order expressions so that you can
    quickly and easily see trade-offs in the system - ie. if rds=Ve.L/Id,
    then for more gain(=gm.rds), increase the transistor length.
    Additionally, the equations will give you first-order values from
    which you can start your design. The exact equations, and refinement
    of design parameters, are left to the simulator.
    I'm on the other side of the fence - I was on one of his CMOS analog
    design courses, hence the book I have :)

    Ted
     
  4. Marra

    Marra Guest

    Why not build it and see how it goes.
    Your gonna have to build one anyway?
     
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