Paralleling OP amps - Page 1
Forum section: electronics / beginners

author: IvoS
author_rank: Frequent Contributor
timestamp: January 02, 2017, 09:17:07 pm
content: I am building a virtual ground splitter. I have limited space on PCB that is why I decided to go with OPamp. I need around 30-40mA of current to be available for the other circuit that it will be powering. So paralleling two OPamps would be sufficient. I breadboarded this "A" and "B" version and differences are as follow:-Version "A" works very well for voltage regulation and it can cource current of approx. 100mA. But when it runs unloaded it consumes 40mA by itself.-Version "B" does not regulate voltage well enough due to feedback resistors tied right at output pins. There is like 10-100mV drop at VG output due to 5 ohm resistors voltage drop. This version does not cunsume extra current. All it consumes is the op amp idling current, which is around 2mA per 1 opamp.Why is so much extra current consumed in version "A" when it runs unloaded?Thanks.

author: wraper
author_rank: Unknown
timestamp: January 02, 2017, 09:50:51 pm
content: You get high current consumption because outputs fight each other. Also you should not use resistors in series with inverting input, they won't do any good, and likely will even do bad. Better use opamp which can supply such current without paralleling.

author: Benta
author_rank: Super Contributor
timestamp: January 02, 2017, 10:10:41 pm
content: I'm trying to understand why you need two opamps for generating a virtual ground voltage between +12 and -12 V.A single high-current opamp will do the same with much better results.Configured as buffer, the non-inverting input connected to your voltage divider should do the job.Are there some special requirements that you have not mentioned?Benta.

author: Zero999
author_rank: Super Contributor
timestamp: January 02, 2017, 10:35:42 pm
content: It should work. Perhaps the current sharing resistors aren't big enough?At only 40mA the current sharing resistors can be much higher. Try 47R.And refer to the application note linked below:http://www.intersil.com/content/dam/Intersil/documents/an11/an1111.pdf

author: Benta
author_rank: Super Contributor
timestamp: January 02, 2017, 11:02:34 pm
content: It could probably be made to work.But again, why complicate your life? There are plenty of high-current opamps or buffers out there at the same price.Here you go:LMH6321:http://www.ti.com/product/lmh6321Same price range as the amps you've chosen. Much easier.Benta.

author: IvoS
author_rank: Frequent Contributor
timestamp: January 02, 2017, 11:36:09 pm
content: Quote from: Hero999 on January 02, 2017, 10:35:42 pmThanks, I stumbled upon this note a few days ago. This is pretty much what I did. Mine connection doesn't have any gain. I will try to increase output resistors to 47 ohm and see what happens.I will also try to connect it as shown in figure one. That may do the trick.

author: IvoS
author_rank: Frequent Contributor
timestamp: January 02, 2017, 11:37:27 pm
content: Quote from: Benta on January 02, 2017, 11:02:34 pmThank you. Reviewing its data...... That may be a good candidate.

author: BrianHG
author_rank: Super Contributor
timestamp: January 03, 2017, 10:38:49 am
content: I cant seem to find the old Motorola 150 ohm driver opamp I used to use, however, here is the equivalent:http://www.digikey.com/product-detail/en/njr-corporation-njrc/NJM4556AD/NJM4556AD-ND/673767Though it's only 70ma per channel, it's under 1$ a piece at digikey.

author: rs20
author_rank: Super Contributor
timestamp: January 03, 2017, 10:54:50 am
content: Quote from: wraper on January 02, 2017, 09:50:51 pmIsn't it sometimes a good idea to make sure the op amp sees the same impedance (or as similar as can reasonably be achieved) on both inputs, so that the bias currents sunk by both inputs produce equal (and thus cancelling) drops? This is the reason op-amp followers have a resistor on their feedback path, even though itappearstotally useless (under the idealistic but actually incorrect assumption that opamp bias currents equal zero).

author: wraper
author_rank: Unknown
timestamp: January 03, 2017, 11:16:35 am
content: Quote from: rs20 on January 03, 2017, 10:54:50 amReally? I'll tell you what, it was good for old opamps with BJT input and without internal bias current cancellation. They had high input bias current, but it was almost equal on both inputs. So by keeping the same impedance on both inputs you could cancel them out. OPA2134 has FET input. And input bias currents may be not only non equal but in opposite directions. So by adding resistor you will only increase offset (depending on particular opamp specimen) and noise.http://www.analog.com/media/en/training-seminars/tutorials/MT-038.pdfQuoteQuoteQuote

author: bktemp
author_rank: Super Contributor
timestamp: January 03, 2017, 11:27:59 am
content: OPA2134 datasheet, page 9:QuoteSo using a matched resistor in the feedback path is actually recommended for the JFET amplifier, but in this case it doesn't care, because the input voltage is constant. Therefore using a small or no resistor at all is probably the best solution, otherwise the resistor forms a low pass filter, making the amplifier potentially unstable.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 03, 2017, 12:21:17 pm
content: To get low output impedance and current sharing, there is a 3 rd, better version of paralleling the OP:Use on OP like in Version A (so getting the feedback from behind the resistor). Use the second OP as a dedicated slave, with the positive input from the output of the 1 st OP, and than wired like Version B.This is working very well at low frequencies, but may be limited at high frequencies.This way the 2 nd OP is only increasing the current, the signal output is only determined by the 1.st one.With stiff virtual GND like in version A (or better C), the capacitors have very limited effect. So one might use less here. With the not so stiff virtual ground in version B, the capacitor can take over much of AC currents and thus reduce the power consumption.Instead of making the virtual ground so powerful, I would rethink if so much current is needed and maybe avoid it in the first place.

author: Brutte
author_rank: Frequent Contributor
timestamp: January 03, 2017, 12:33:36 pm
content: Being strict that is not a virtual ground splitter. Rail splitter is the thing that splits unregulated supply in half (be it 3V or 30V) while your circuit does not have such requirements as it is a fixed 24V.So you only need a 12V source and 12V sink regulators.Depending on your requirements on load, line stabilization, power consumption and price, that can span from resistor divider, 12V zeners, ... , up to a DC/DC switchers.If that is for powering sth that can push and pull from GND then I suggest two 12V zeners in miniMELF. Under full load that is 12V*0.04A<0.5W

author: David Hess
author_rank: Super Contributor
timestamp: January 03, 2017, 08:32:11 pm
content: I prefer to add a bipolar current buffer but the circuit shown here on the right work well:https://e2e.ti.com/blogs_/archives/b/thesignal/archive/2013/03/26/paralleling-op-amps-is-it-possible

author: IvoS
author_rank: Frequent Contributor
timestamp: January 03, 2017, 09:55:49 pm
content: Just tried what Kleinstein described, which is the same as Intersil note posted by Hero999. I burned one OPA2134replaced with OP275 and it can deliver plenty of current, more than I need. 110mA sourced and 80mA sinked current.However, the voltage is dropping with increasing current. It is not rock solid. There was like 12mV drop.I searched Mouser database and discovered LM7171 which is single high speed OpAmp.http://www.ti.com/lit/ds/symlink/lm7171.pdfI don't need high speed but it can do up to 100mA. I think I will abandon all this paralleling idea and go with single OpAmp instead. I need only 2 of them for my application, so price is not an issue.

author: Benta
author_rank: Super Contributor
timestamp: January 03, 2017, 10:34:17 pm
content: LM7171 is a good find.Enough output current, voltage feedback and unity-gain stable. Go for it.