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Bruce Trump

Is it possible to parallel two op amps to get twice the output current?

We get this question periodically on our E2E forums. Though we may answer with a qualified “yes,” it tends to make us shudder just a bit. It can be done… but with great care. So let me come quickly to a key point. Don’t use the simple circuit on the left. Directly paralleling inputs and output of two op amps is sure to start a serious argument between the two. Differing offset voltages will cause them to fight over the correct output voltage. They may burn all their output current capability in the battle with one output current (sourcing) flowing into the other (sinking current).

Figure 1b has a chance. Op amp A1 is the “master” and A2 is the so-called “slave,” replicating the output voltage of the master. R3 and R4 promote reasonably equal sharing of the load current, even though A2’s output may be slightly different. Feedback is connected on the load-side of R3 and R4 so their voltage drop is corrected. You lose some output voltage swing capability in the I∙R drop on these resistors so you will be tempted to make them low in value. But the offset voltage of A2 will cause extra quiescent current equal to Vos/(R3+R4). It’s a tricky tradeoff.

Be very cautious with high speed signals. You want A2 to accurately replicate the output of A1. If the signal moves too fast, the phase shift of A2 will cause differing output voltages and wasted current. It’s important to avoid slewing. If necessary, add an R-C filter at the input so the fastest rate of change on the output of A1 is well below slewing speeds. The dynamic behavior of two amplifiers may not match well during slewing.

Don’t use older generation op amps that have output inversion (phase reversal) behaviors. If A1 can overdrive the input common-mode range of A2 and its output inverts the result is ugly.

Above all, check the behavior of your circuit thoroughly. SPICE may tell you whether you have a basic working circuit, but op amp macro-models may not accurately predict the quirks that could befall this circuit. Build a breadboard and check all signals and conditions carefully. If your op amp is multi-sourced, consider that not all manufacturers’ devices behave exactly the same. (But, of course, you have only one source for op amps, right?)

Do you think I’m a bit leery of paralleling op amps? Well, yes… call me leery. It can be successful but proceed with caution. I recommend that you consider an easier path—using an op amp with more output current. Here are a few possibilities:

  • TLV4111  300mA, 6V. CMOS Op Amp.
  • BUF634   G=1 buffer, 200mA, 36V.  Used inside the feedback loop of standard op amps.
  • OPA547   500mA, 60V Op Amp. Adjustable current limit.
  • OPA564   1.5A, 24V Op Amp, 17MHz GBW.
  • OPA548   5A, 60V Op Amp. Adjustable current limit.

Have you successfully paralleled op amps? Or do you have scars from trying? Comments welcome.

Thanks for reading,

Bruce       email:  thesignal@list.ti.com

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Anonymous

  • 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 ?

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

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

  • 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".

  • 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

  • @ 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.