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bipolar analog IC design ?

Discussion in 'Electronic Design' started by Svilen, Jul 22, 2003.

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  1. Svilen

    Svilen Guest

    Hi,


    Up to now I have
    designed analog circuits only in CMOS. Now I have
    to
    do some stuff in BiCMOS and I'm just trying to make
    the link between bipolar and CMOS approach to
    design.
    My question is basically how to size the bipolar
    transistors? Generally speaking one sizes a MOS
    transistor based on the current that flows through,
    choosing the appropriate W/L. If gain and high
    output
    resistance is needed than choose bigger L; if more
    speed is required then L is shorter. But how to
    size
    the BJTs - it is the area of the emitter that
    matters
    there. I have looked in the books but didn't find
    any
    guidance about sizing the transistors. Everything
    there is based on the exponential relationship
    between
    Vbe and Ic and the area is represented by Is but it
    is
    a technological parameter and I'm not sure it can
    be
    used for sizing. For example if I want to have a
    current of certain value going through a bjt, what
    should be the size of this bjt or what is the
    criteria for choosing the size?

    Sorry if I annoyed you with such a stupid remarks,
    but
    I guess I need just a little bit of help.
     
  2. Jim Thompson

    Jim Thompson Guest

    "IS" is *not area*... it is "transport saturation current", a device
    parameter that you cannot change. Use parameter "Area" to re-size
    devices, *if* that's allowed by your process. Some processes have a
    "library" of devices that you must pick from, or use multiple
    *instances* of devices.
    I *generally* size devices so that they are operating at the peak of
    their beta curve.

    Do this simulation of a bipolar transistor:

    Hold the collector voltage at a constant appropriate for the device.

    Do a .DC analysis varying a current source into the base.

    Plot IC/IB (Y-axis) of the device with X-axis chosen as IC.

    You will observe that beta is low at low and high currents, but peaks
    in the middle... use this "sweet spot".

    [And learn how to set up your newsgroup reader to improve the
    word-wrap of your posts :-]

    ...Jim Thompson
     
  3. Svilen

    Svilen Guest

    Thanks to both of you for your replies. I think I've got the idea and
    now I have a guiding point to start from.

    Svilen
     
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