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variable Vth

Discussion in 'Electronic Design' started by jason, Aug 19, 2005.

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  1. jason

    jason Guest

    Hello All

    Could anyone tell me why cmos circuit with variable Vth is considered
    an advantage for low power technology?

    Kindly advise.
    Thank you

    best regards
    Jason
     
  2. Drop Vt when you need speed. Raise it when you want to save power (low
    Vt devices leak like hell).
     
  3. Jim Thompson

    Jim Thompson Guest

    Come on, Jason! Read your text, or at least use your head!

    ...Jim Thompson
     
  4. jason

    jason Guest

    Hi Keith
    Thank you for writing in short but with very useful info.
    Thanks a lot

    Jason
     
  5. jason

    jason Guest

    Hi Jim

    Thanks for the comment. I have read text and used my brain :)
    I know how Vt affects the performance.
    But I wish to know more how Vt be varied in application in bulk cmos
    and soi cmos.
    How can we make proper changes in certain things that we can control
    the Vt and would not make the whole systems too complex to be achieved.
    Thank you all

    Jason
     
  6. Jim Thompson

    Jim Thompson Guest

    Variable in the circuit design, or variable as a processing choice?

    ...Jim Thompson
     
  7. jason

    jason Guest

    Thank you Jim

    It will be great if you can share ideas on both domain.
    Circuit level and also process flow and integration
    :)

    Hear from you and all

    Thanks a lot
    Jason
     
  8. jason wrote...
    One can "tune" Vt over a limited range by changing the MOSFET's
    body voltage. This can be done with active circuitry, or with
    integrated capacitors to hold a programmable charge (voltage),
    similar the use in EEPROM cells. It's a specialized technique.
     
  9. Jim Thompson

    Jim Thompson Guest

    Now there's a vague question for you! What do you mean?

    ...Jim Thompson
     
  10. Joerg

    Joerg Guest

    Hello Winfield,
    Now I just wish that "modern" laptop designers would understand that
    general concept. My old Contura could vary its processor clock according
    to load. The new ones can't and in consequences eat their batteries in
    under two hours. What a progress.

    Regards, Joerg
     
  11. Jim Thompson

    Jim Thompson Guest

    That doesn't have anything to do with clock speed.

    In general body effects can only be applied to P-channel devices
    since, in most commercial processes, the N-channel body is the
    substrate.

    But only PhD's would do such a dumb-ass unpredictable thing in the
    first place ;-)

    ...Jim Thompson
     
  12. Joerg wrote...
    The Compaq Contura? What operating system did it use?
    What program told the processor to slow down? How?
     
  13. Joerg

    Joerg Guest

    Hello Jim,
    Yes, I know. I just meant the general concept of offering both sides in
    a trade-off. Lots of power at huge battery drain versus sluggish
    performance at better battery economy. Kind of like the overdrive button
    in cars. Old portable computers could do that and it seems most new ones
    can't.
    I am not a chip guy but this variable Vth concept somehow always had
    that "molasses feel". It wouldn't surprise me it that could make the
    yield tank because now they'd have to spec two operating modes on one
    process.

    Regards, Joerg
     
  14. Jim Thompson

    Jim Thompson Guest

    VTH doesn't have a fast effect on power consumption.

    The only time I've used the effect is for "kick-start" circuits, when
    normal VTH is reached the kick-start device is "off".

    A few processes I've used have had a depletion mode FET... sloppy but
    useful for starting regulators.

    ...Jim Thompson
     
  15. keith

    keith Guest

    Define "fast".
    It's perhaps a useful technique in SOI (the fouth terminal is available to
    the bold). There are a few patents that teach how to control these things.
     
  16. Jim Thompson

    Jim Thompson Guest

    Change VB by 0.1V and observe speed change.
    Define "useful" ;-)

    ...Jim Thompson
     
  17. jason

    jason Guest

    Hello All

    Thanks for discussing
    I mean how can we actually implement circuit with multiple Vth? How a
    circuit is control this way?

    For integration wise, I refer to process integration between cmos and
    cmos soi

    Hear from you all
    Thank you

    Jason
     
  18. jason

    jason Guest

    Hello All

    Thanks for discussing
    I mean how can we actually implement circuit with multiple Vth? How a
    circuit is control this way?

    For integration wise, I refer to process integration between cmos and
    cmos soi

    Hear from you all
    Thank you

    Jason
     
  19. Guest

    Joerg skrev:
    Isn't that just because you bought a laptop with a regular pentium4 and

    not a (more expensive?) pentium4-mobile?

    afaik all mobile pentiums and maybe also newer regular pentiums support

    "Speedstep" and XP has build in support for it.


    -Lasse
     
  20. Joerg

    Joerg Guest

    Hello Winfield,
    Yes, the Compaq Contura. I used DOS and sometimes Windows 3.2 but the
    latter only when I had to go on the web.

    I don't know how it worked in detail but it had a fully static 486, the
    SL version I believe. These could be used from full to very low clock
    rates. The programs in those days did not have power save features. IIRC
    it was the BIOS that did all that. As a user you could select hi-med-lo,
    plus a variable "on demand" kind of setting which is the one I used. It
    was done via clicking on some silly dripping faucets but worked fine.
    Out of curiosity I measured Icc while running SPICE and then again while
    slowly plugging away on a Word document. Icc dropped to less than half
    with Word and that was pretty much corroborating the battery runtime. On
    long flights the folks next to me with their fancy Thinkpads were blown
    away when theirs quit and I kept writing two hours longer. Especially so
    because they paid more than twice of what mine cost.

    Regards, Joerg
     
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