Paralleling Op Amps—is it possible?

RE: Paralleling Op Amps—is it possible?

Gabriel Jonquieres — Tue, 30 Apr 2024 01:09:39 GMT

Hi,

Can I parallel 4x opamps 1 master 3 slaves in cascade each one another ? So I can drive 600ohms resistor in feedback of my shelf filter audio circuit with 22nF high grade duelund capacitor (no smaller value existing) and get the right RC value for the frequency I need.

I saw this rule in audio circuits of not going over 1/10 linear output current of the opamp in the feedback loop, this is why I need 4x opamps to meet this spec.

So Is it better to go for 1/5 linear output current and use only 2 opamps ?

RE: Paralleling Op Amps—is it possible?

Tim Burke — Tue, 14 Aug 2018 22:38:11 GMT

Neil Albaugh - Your parallel noise reduction circuit is still in the OPA111 datasheet:

www.ti.com/.../opa111.pdf

RE: Paralleling Op Amps—is it possible?

Neil Albaugh — Tue, 23 Jan 2018 16:32:19 GMT

My original design for parallel op amps was published in1984 in a Burr-Brown data sheet for the OPA111, a then- new dielectrically- isolated op amp. It is shown as Figure 16, 'An N-Stage Parallel-Input Amplifier for Reduced Relative Amplifier Noise at the Output." I called this approach "The Lunatic Fringe Amplifier"- but not for publication. Jim Williams and apparently a few audio folks have later promoted this paralleling idea. At the time I came up with this, my friend Mark Stitt doubted that it would actually result in lower noise but was intrigued enough with this unusual architecture to breadboard the circuit and test it. It did work.

The noise reduction takes advantage of the fact that the op amps' uncorrelated noise adds as the square root of the sums while the signal, which is correlated, adds directly. This statistical trick also applied to noise and drift reduction of summed voltage references- another idea that Mark (and maybe Bruce) breadboarded and tested long-term with success. That test PCB was stuffed full of voltage references, all summed together into one low-noise, very stable output.

Speaking of Mark Stitt, a brilliant Mechanical Engineer who became an equally brilliant electronics engineer, someone should write an article about the now- disbanded Burr- Brown "cast of characters".

RE: Paralleling Op Amps—is it possible?

Andrew Coad — Thu, 18 Jan 2018 01:11:00 GMT

An older thread but ... @Charles Hansen1 - Doug Self's design parallels 4 x NE5532 with 10R isolation resistors. However, note that his design goal is noise reduction not output drive capability. The rationale behind parallel op amps to reduce noise is explained in his book "Small Signal AudioDesign."

NwAvGuy (nwavguy.blogspot.jp/.../o2-details.html) parallels 2 x NJM4556 in his O2 headphone amp design with 1R isolation resistors. NwAvGuy's design goal is increased output drive

RE: Paralleling Op Amps—is it possible?

Bob Turner — Thu, 19 May 2016 12:16:25 GMT

@ Bill Grenoble : I liked the 709, nice op amp , not the same common mode voltage range as the K2-W , but a bit faster.

re buffering: If you used a AC126 / AC127 pair as emitter followers, the cross over distortion would be negligible.

RE: Paralleling Op Amps—is it possible?

Jens-Michael Gross — Mon, 15 Apr 2013 14:28:54 GMT

Percy, your gues sis right, if there are different propagation times, then the two gates might have complementary output for a short time. Using gates from the same package should indeed minimize the effect. Output series resistors will reduce this, but also reduce the maximum current then and/or increase the output voltage drop.

On the MSP430 processors, often two output port pins are used in parallel, not so much to increase the fan-out, (the MSPs can drive up to 40mA per pin, depending on supply voltage and tolerable output voltage drop) but rather to reduce the voltage drop at medium currents.

However, for standard gates, it might be an idea to change the technology rather than paralleling gates. (e.g. HCT instead of LS)

RE: Paralleling Op Amps—is it possible?

Percy Williams — Tue, 09 Apr 2013 13:26:02 GMT

Sam Green and Jens-Michael Gross: I have just done a Google search about connecting gates in parallel to increase the fan out capability, and have come up with nothing for or against. I would guess that there might be a slight switching delay from one gate to another, which at high frequency may cause overheating, due to current passing from VCC through one output into the other and to ground. This effect could be minimised by only connecting gates in the same package. The effect of "Shoot-through" could be reduced by connecting the outputs together via small value resitors in each output. Does anyone have anything to say about this?

RE: Paralleling Op Amps—is it possible?

Percy Williams — Tue, 09 Apr 2013 09:50:25 GMT

dox: If you take the feedback from the emiiters rather than the bases, then the overall circuit including the op-amp and the two transistors acts like an op-amp and cross-over distorsion is eliminated.

RE: Paralleling Op Amps—is it possible?

Bill Grenoble — Mon, 08 Apr 2013 19:43:07 GMT

Back 40+ years ago I was in a seminar with a leader who knew what he was talking about. He mentioned Gumpert's Law: "When contemplating the design of a direct coupled amplifier, the First thing to consider is DON'T!” I rather think paralleling opamps would also fall into that category. I did put a complementary pair of transistors on the end of an opamp. Bases tied to the opamp output, emitters tied together, and the feedback loop tied to the emitters. If was “interesting” during the zero crossing, but the output was powerful enough for the load and the opamp took care of the zero crossing. It worked and it was stable. A number of engineers looked at it and shook their heads. But it worked and in those dark days before Digikey, I had the parts on hand. It may have been a 709 (yes, we used them) or a 741.

Clearly using a power Opamp is the answer. Just drop it in and forgetaboutit. Engineering is making what you need from what you can get. Today, ELSI devices are on development boards and it is not unheard of to have several boards on the bench, working together. Or Not Working together! Pile of torn hair on the floor, walls turned blue from the frustrated profanity, reading the manual through again...

Best Regards

Bill Grenoble

Remember the CK722 ?

RE: Paralleling Op Amps—is it possible?

Jeff Melvin — Mon, 08 Apr 2013 17:44:15 GMT

Sam Green - Did you link to the wrong datasheet? No paralleled op-amps here.

shantanu dasgupta - your coil driver example isn't the same as it is only Hi/Lo, like logic paralleling.

Percy Williams - don't forget the problem of cross-over distortion, unless things are optimally biased.

Bruce Trump - I'd like to see further discussion, possibly with more elegant approaches, to address the problem of achieving good slew rates driving highly capacitive loads while ensuring stability.

RE: Paralleling Op Amps—is it possible?

Jens-Michael Gross — Mon, 08 Apr 2013 11:56:08 GMT

Sam, paralleling gates to increase fan-out is not a problem at all. Gates will either try to pull to GND or up to VCC, not to a specific voltage in-between. So as long as all gates are pulling into th esame direction, their output can be easily added with no side-effects.

Multiple OpAmps, however, might have a different opinion about the output voltage and therefore fight each other if you do not allow a small output voltage difference between them by coupling resistors.

RE: Paralleling Op Amps—is it possible?

Percy Williams — Mon, 08 Apr 2013 09:24:10 GMT

The application sections of most operational amplifier data books show emitter followers. I think this is a good solution. One disadavantage is that rail to rail output operation will be compromised.

RE: Paralleling Op Amps—is it possible?

Leendert Prevo — Mon, 08 Apr 2013 08:45:40 GMT

In 2010 I used 2 * PA93 in parallel to drive a 500 nF load, 110 Volts pk-pk, DC to 10kHz, using the isolation resistors, required for opamp stability.

RE: Paralleling Op Amps—is it possible?

shantanu dasgupta — Mon, 08 Apr 2013 07:17:10 GMT

We are using a op-amp based relay driver in a electronic starter where 2 stages of LM324 quad op-amps are paralleled without any current sharing resistances. The relay consumes around 35mA which is a bit too much for a single stage. We have sold thousands of these without any complaints whatsoever.

Of course this is not an amplifier application.....more of a high current buffer which operates on only two voltage levels HI & LO.

RE: Paralleling Op Amps—is it possible?

Sam Green — Sun, 07 Apr 2013 17:43:34 GMT

I have done this with no ill effect at modest gain of 2 t0 4 to increase output drive current in Figure 5 of datasheets.maximintegrated.com/.../MX7543.pdf. I do the same thing with logic gates to increase fan-out. I will keep your cautions in mind in the future.