I am using an LM358 as an audio amp in a battery powered application. According to the data sheet, it is capable of sinking 20mA. Given that this is a dual package, what is the possibility of feeding both amps with the same signal and paralleling the output for more current?
These are fairly low frequencies, and I am thinking any discrepencies between the two would simply mix and alter the waveform slightly if at all. Is this correct?
How would the circuit be configured? For example, could the same feedback resistor be used for both amps?
I could buy a higher output device, but would like to know if this is possible in practice.
I was going to say pretty much the same thing, but you already did. :-) One difference, I call it tinkering. Yesterday was a very bad day for a PIC _and_ a 4*20 LCD display. I can confirm that 12V is too much. :-(
Newsgroups: alt.binaries.schematics.electronic Subject: Parallel op amps current (from S.E.D) - ParallelingOpAmps.pdf Message-ID:
...Jim Thompson
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Thanks to Google, here's my post of a few years ago, detailing the classic solution for higher current by paralleling ballasted opamps. Advice for high-frequency use is included.
All of the current-equalizing ballast resistors are the same value.
Newsgroups: sci.electr> Just parallel 2 to 4 fast opamps.
At low frequencies this can be nicely done using the old ballast-resistor current-sharing trick,
But at high frequencies slew rate becomes an important issue. For example, 20Vpp at 10MHz requires a better than 630V/us slew rate, while delivering a full load. That's a rarefied territory for +/-15V opamps. Generally opamps slewing that fast require a large error voltage before their slew rate approaches that speed.
One has to be sure each opamp has the same high error voltage so it slews in step with its neighbor. This could mean each opamp to be shared needs its own independent feedback network, plus the output ballast resistors shown above. Or some other trick would be needed yielding identical circuitry for all the output opamps.
Well, the 2nd opamp is inside the feedback loop along with the first. We've added an output-current-sense resistor to the first opamp and made the second one deliver the same current. For half of an LM358 with RC feedback it'd be something like this.
A 4.7-ohm resistor will drop 100mV at 20mA, and for an LM358 with 7mV input-offset voltage max, the standing opamp-opamp output current would be Vos / 2Ro = 700uA.
Doing this with faster op-amps will sometimes lead to unstable operation. Changing the location of the feddback capacitor so that the output of the first op-amp is used for it instead of the over all output will often solve this. The LM358 is slow enough that I doubt it will be an issue.
More common is connecting as a 'bridge amplifier' so that one drives the (+) side of a load and the other the (-) side of a load. That is effectively connecting the outputs in series...
If you really think you may be near the upper current limit, consider the heating of the op amp; low frequency errors can result. I'd never want to draw more than 2 mA from an op amp (transistors are cheap, use a couple instead). You can get a lot more current-output headroom than a piddly factor of two.
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