linnix said:
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?
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