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CMOS and transistors differences.

Discussion in 'Electronic Design' started by Boki, Apr 25, 2006.

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  1. Boki

    Boki Guest

    Hi All,

    Q1:When I doing VLSI design, I remember that somebody told me never to
    use bipolar ( our process is standard cmos process ), because it is
    expensive ( ? really? )

    Q2:When I doing discrete design, I saw a lot of bipolar here, and
    someone told me that FET is expensive. ( about two times than npn/pnp
    transistors )

    Q3:and if I use a NOT gate chip that is also expensive than two FET (
    nmos + pmos inverter )

    Q4: I don't have too much idea about the price for FET / transistor /
    not gate ..etc, where can I get them ? ( what can I only do is to
    froogle it ... )


    Q5: What is the condition you decide to use a MOS or transistor ?

    Best regards,
    Boki.
     
  2. Pooh Bear

    Pooh Bear Guest

    This is really the chip designer's territory but maybe bipolars use more
    silicon 'real estate'. Bipolar circuitry tends to use more power too.
    Discrete fets are indeed more expensive than bipolar devices.
    An IC inverter is more expensive than a discrete one made from fets ?
    Unlikely. There are some cute 'baby' chips now that contain single
    inverters/gates instead of having to buy 4 or 6 in a package.
    Manufacturer's website perhaps ? Many give 1k qty pricing.

    e.g. http://www.fairchildsemi.com/pf/BC/BC550.html A device I use.
    Pricing is as low as 2.3 cents @ 1k qty ( see the table )
    Depends on the application. Discrete designers will typically use bipolar
    unless there's a specific reason to use a fet.

    Graham
     
  3. Hlrsr

    Hlrsr Guest

    BJTs in VLSI are used mainly for driving Current-demanding circuitry and
    they are bulkier than CMOS devices. So, the answer depents on what your
    circuitry needs to do

    GM

    PS:Sorry for my English
     
  4. Boki

    Boki Guest

    Great thanks!

    Best regards,
    Boki.
     
  5. Boki wrote...
    You would do well to have access to catalogs from the following
    distributors (you may have observed us providing specific links
    and prices here on s.e.d.), and become familiar with their websites,
    http://www.digikey.com/ http://www.mouser.com/ http://www.jameco.com
    http://www.newark.com/ http://uk.farnell.com http://www.alliedelec.com/

    There are many others I also use, but these have the best catalogs
    to learn from. You would also find AoE's component tables useful.
    Once you start finding your way around popular discrete parts you'll
    no doubt learn about local distributors who are easy to work with,
    even if they don't have huge staffs creating massive fancy catalogs.
     
  6. A1)& A2) In industrial R&D "somebody said" is not used
    A3) On mass production we listen carefully to supply department, that's
    IS their expertise.
    A5) Past experience in solving similar problems, MTBF and warranty
    costs, existing parts in stores and so on and on...

    In the end the finished part is usually sold to customer with working
    specs ~50% of the manufacturers advertising sheets.

    HTH

    Stanislaw
    Slack user from Ulladulla.
     
  7. Joerg

    Joerg Guest

    Hello Boki,
    Sometimes you have to use them.

    True. As Win said keep a distributor web site open at all times when
    designing discrete circuitry. I usually have Digikey and Arrow open.
    Then gradually learn the cost of the most often used parts by heart so
    you don't even have to look at the other PC screen. When you get good at
    this you can predict the cost of your whole design to within 20% before
    running the BOM.

    Try an example: On Digikey enter MMBT3904 and you'll find it around 1.5
    cents. Now enter BSS123 and you'll see that it costs almost 5 cents.
    That is why I agonize over the use of a FET quite some time on high
    volume products. Do I really, really need a FET here? If there is any
    way to do it with bipolar transistors I'll do it. Even with 0.8 cents or
    so added in for the burdened cost of a base resistor the BJT typically
    wins. If you go for off-brand BJT you can sometimes get them into the
    BOM at well under one cent (Asian sources, mostly).

    Go to Digikey, it's all there. You can buy a non-buffered 74HCU04 for
    under 10 cents. It contains six nmos/pmos pairs and you can do marvelous
    things with it. There is no way to buy 12 FET for that kind of money.
    Unless I need the functionality of a FET this boils down to one reason: $$$

    Regards, Joerg
     
  8. Tim Shoppa

    Tim Shoppa Guest

    Boki:

    It is more than 30 years old now, but I HIGHLY RECOMMEND that you read
    and thoroughly understand the CD4007 data sheet. If you want 3 PMOS
    transistors and 3 NMOS transistors in one package and with some
    individual terminals accessible and others bussed for complementary
    pair use, then the CD4007 fills many of the needs most of the time.

    The CD4007 is often lumped in with the CD4001 digital logic series but
    it isn't necessarily digital. Most clever uses come from using it in
    analog ways.

    Also closely examine the ULN2003 data sheet (7 common-emitter NPN
    drivers) etc. as you seem to be struggling with bipolar drivers.

    Neither represent high integration anymore but they are workhorses
    still.

    Tim.
     
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