# Op-amp design: Bipolar or Cmos

L

#### linnix

Jan 1, 1970
0
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?

J

#### Joerg

Jan 1, 1970
0
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?

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. J #### Jim Thompson Jan 1, 1970 0 We are in the process of building some op-amp ICs. Who is "WE"? 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. 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 Questions: #1 Anybody got a cmos op-amp design and layout to sell or license? No. But I can design one for you once you have a set of specifications. #2 Anybody able to port from bipolar to cmos? Yes. I started designing when there was ONLY Bipolar #3 What are the drawbacks to stay with bipolar, other than powers? None. Better offset, better gain-bandwidth product, better slew-rate, higher output power capability, higher operating voltage capability. 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? Depends. I just designed a CMOS amplifier with 0.03% linearity for a ±1V (differential) output. ...Jim Thompson J #### John Devereux Jan 1, 1970 0 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? 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?! J #### John Devereux Jan 1, 1970 0 Jim Thompson said: None. Better offset, better gain-bandwidth product, better slew-rate, higher output power capability, higher operating voltage capability. What about input bias current / noise? T #### Tim Wescott Jan 1, 1970 0 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 J #### Joerg Jan 1, 1970 0 Spehro said: Input linearity can quite noticably affect EMC, and thus$.

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...

T

#### Tam/WB2TT

Jan 1, 1970
0
John Devereux said:
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?!

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

Tam

J

#### Joerg

Jan 1, 1970
0
Jim said:
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.

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?

[...]

J

#### John Devereux

Jan 1, 1970
0
Joerg said:
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...

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.

L

#### linnix

Jan 1, 1970
0
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.

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.
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.

If bipolar is good enough, we will go with the reference design. It
might not be perfect, but save time and money.
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.

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.
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.

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.

J

#### John Devereux

Jan 1, 1970
0
Tam/WB2TT said:
This comes about because management consists of MBAs and C programmers,
neither of which has a clew about engineering.

I seem to recall linnix as pretty clewful though... That's why I was
puzzled.

T

#### Tim Shoppa

Jan 1, 1970
0
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?

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.

L

#### linnix

Jan 1, 1970
0
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?

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. J #### John Devereux Jan 1, 1970 0 linnix said: 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.

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.

S

#### Spehro Pefhany

Jan 1, 1970
0
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: $I know this doesn't help you much here but I wanted to share what many design engineers think. Input linearity can quite noticably affect EMC, and thus$.

Best regards,
Spehro Pefhany

J

#### Jim Thompson

Jan 1, 1970
0
What about input bias current / noise?

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

L

#### linnix

Jan 1, 1970
0
Have you looked at the price of "ye olde" LM358?

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.
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.

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.

J

#### Jim Thompson

Jan 1, 1970
0
This comes about because management consists of MBAs and C programmers,
neither of which has a clew about engineering.

Tam

And certainly not a CLUE ;-)

...Jim Thompson

L

#### linnix

Jan 1, 1970
0
This comes about because management consists of MBAs and C programmers,
And certainly not a CLUE ;-)

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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