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Op-amp design: Bipolar or Cmos

Discussion in 'Electronic Design' started by linnix, Jun 4, 2007.

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  1. linnix

    linnix Guest

    We are in the process of building some op-amp ICs.
    I found a reference design and layout of a bipolar op amp.
    However, the author says that cmos op-amps are more popular
    in the real world. Questions:

    #1 Anybody got a cmos op-amp design and layout to sell or license?

    #2 Anybody able to port from bipolar to cmos?

    #3 What are the drawbacks to stay with bipolar, other than powers?

    The op-amp circuit will be enable on demand, so power usages
    may not be too critical for stand-by. What about active power usages
    between bipolar and cmos?

    How about linearity? Would bipolar be better than cmos?
     
  2. Joerg

    Joerg Guest

    To be honest I never really cared. As long as noise, GBW and whatever
    else we need are fulfilled the only variable that matters boils down to
    one symbol: $

    I know this doesn't help you much here but I wanted to share what many
    design engineers think.
     
  3. Jim Thompson

    Jim Thompson Guest

    Who is "WE"?
    CMOS OpAmps are more "popular" because they're cheap. They're also
    generally low power, resulting in high noise, high VOS, and low
    gain-bandwidth product
    No. But I can design one for you once you have a set of
    specifications.
    Yes. I started designing when there was ONLY Bipolar
    None. Better offset, better gain-bandwidth product, better slew-rate,
    higher output power capability, higher operating voltage capability.
    Depends. I just designed a CMOS amplifier with 0.03% linearity for a
    ±1V (differential) output.

    ...Jim Thompson
     
  4. I have been staring at this post for a few minutes and I still can't
    decide if it is a joke....

    Maybe I'm out of touch, is this something that companies can just
    decide to do now? Perhaps I should be getting into the diode business,
    or something?!
     
  5. What about input bias current / noise?
     
  6. Tim Wescott

    Tim Wescott Guest

    If you're planning on building some chips that are _just_ op-amps this
    seems like an odd thing not to know. And if you _are_ planning on
    embedding the amp into a chip, I would think that the process of the
    chip that you're building would form a huge flashing arrow pointing at
    the process that you would have to use with your amp.

    CMOS op amps have vanishingly small input current, as a consequence of
    the fact that you're looking into an insulated gate. In general they
    have significantly higher offset voltages than bipolar amps, so you have
    to pay close attention to your requirements and decide which amp is
    right for you. Keep in mind that bipolar and CMOS op-amps are happily
    coexisting in the market today (I dunno if there are any new JFET-input
    designs out there, however); if one technology would do for everything
    the others would have faded away.

    I'm not sure that you'll find that equivalent-performance parts will
    have significantly different power consumption numbers -- unlike logic,
    CMOS analog circuitry needs some standing current to bias everything
    into the linear range. This standing current consumes power, so CMOS
    amps are certainly not "no power" parts like CMOS logic that isn't being
    clocked.

    If you're going to be enabling the op-amp on demand, then you should
    probably calculate overall power consumption the way that TI advocates
    for their microcontrollers: count the coulombs that it takes to turn the
    op-amp on, wait for it to stabilize, use it, then shut it off. If you
    have to have a microprocessor on while this is happening, factor that
    in, too. A circuit that consumes 10mA but can do everything in a
    microsecond would be better than a circuit that consumes 100uA but needs
    a millisecond before it can shut down. The above numbers are absurd,
    but I hope you get my point.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  7. Joerg

    Joerg Guest

    Yeah, but at the frequency range where that begins to matter I design
    most everything with discrete parts ;-)

    Ok, I confess that I have violated that rule once this year by designing
    in a THS4021. Promise repentance...
     
  8. Tam/WB2TT

    Tam/WB2TT Guest

    This comes about because management consists of MBAs and C programmers,
    neither of which has a clew about engineering.

    Tam
     
  9. Joerg

    Joerg Guest

    Often lower cost as well, simply because they are older and the sheer
    volume is so high. I mean, where can you get a rocket like ye olde uA733
    for around 30 Cents in CMOS?

    [...]
     
  10. I think Sphero might be talking about external modulated RF signals
    causing artifacts in quite low frequency circuits. E.g. thermocouple
    amplifier next to a transmitter.
     
  11. linnix

    linnix Guest

    Yes, it will be embedded in a device. We can go either way (bipolar
    or cmos), with least design costs and masks. So, we want to start
    with something well tested. I am not doing the design, just looking
    for contractors to do it.
    If bipolar is good enough, we will go with the reference design. It
    might not be perfect, but save time and money.
    That's what I thought, probably not much difference for analog. I am
    just trying to get some opinions before arguing against the well
    respected author.
    The microcontroller will be coming out of stand-by, turning on the
    charge pump and op-amp. I think we have seconds to do that.
     
  12. I seem to recall linnix as pretty clewful though... That's why I was
    puzzled.
     
  13. Tim Shoppa

    Tim Shoppa Guest

    Most important factors in buying op-amps:

    1. Wide color choice. Women tend to favor bright red and green, men
    usually go for the brown and navy units. Get Eddie Bauer as a fashion
    consultant, even.

    2. Cupholders. More cupholders, the more buyers.

    3. Salesmen. Salesmen with white leather belts and white leather shoes
    seem to be the most succesful.

    Good luck!

    Tim.
     
  14. linnix

    linnix Guest

    We figure the incremental costs of 10 to 20 cents, cheaper than the
    external packaging costs. Of course, we have to eat the $30,000 NRE.
     
  15. Have you looked at the price of "ye olde" LM358? Not the greatest
    opamp perhaps, but then I don't suppose the ones inside a
    "system-on-chip" are that great either. If that is what you are doing
    - it's still not clear to me.
     
  16. Input linearity can quite noticably affect EMC, and thus $.


    Best regards,
    Spehro Pefhany
     
  17. Jim Thompson

    Jim Thompson Guest

    Forgot bias current ;-) Bipolar is higher bias current, BUT input
    referred noise is MUCH better in Bipolar... show me 2nV/rt-Hz in CMOS
    ;-)

    Except for noise, BiCMOS is best.

    ...Jim Thompson
     
  18. linnix

    linnix Guest

    Yes, that's what we are designing out.

    If I can have the layouts please (GDSII or CIF files). The dice need
    to be fitted into the wafer in exact places, but not necessary
    efficient usages.
    Sorry about the misunderstanding. I am talking about adding an op-amp
    inside a chip. It will be part of a wafer cap anyway, so only
    minimum additional processing costs. It will save bonding cost and
    PCB spaces.
     
  19. Jim Thompson

    Jim Thompson Guest

    And certainly not a CLUE ;-)

    ...Jim Thompson
     
  20. linnix

    linnix Guest

    This comes about because management consists of MBAs and C programmers,
    I don't want to start a flame war (probably did). The best IC
    designers I know are C programmers, who wrote the design programs.
     
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