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Parallel op amps current

Discussion in 'Electronic Design' started by Richard Crane, Apr 9, 2007.

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  1. 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.

    Thank you,

    Richard Crane
     
  2. I'd configure them identically, but isolate them with a small resistor
    on each output, say 1R or so, feeding the load.

    But you could always try using a common feedback R etc. and see what
    happens, its called playing......


    martin
     
  3. 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. :-(
     
  4. Jim Thompson

    Jim Thompson Guest

    See....

    Newsgroups: alt.binaries.schematics.electronic
    Subject: Parallel op amps current (from S.E.D) - ParallelingOpAmps.pdf
    Message-ID: <>

    ...Jim Thompson
     
  5. Hmm, can you have a good day with a PIC?
    <duck>


    martin
     
  6. Winfield

    Winfield Guest

    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.electronics.design
    From: Winfield Hill
    Date: 26 Jun 2005
    Subject: I am looking for an output amplifier for a signal generator

    John Larkin wrote...
    At low frequencies this can be nicely done using the old
    ballast-resistor current-sharing trick,

    .. -- feedback network ---,
    .. normal opamp |
    .. ------| >--+--/\/--+-+----
    .. _______| |
    .. | add-on opamps |
    .. | as followers |
    .. +--| >-----/\/--+
    .. | |
    .. +--| >-----/\/--+
    .. | |
    .. '--| >-----/\/--'

    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.

    .. -- feedback network -------,
    .. |
    .. gain followers |
    .. ----| >--+--| >-----/\/--+-+----
    .. | |
    .. +--| >-----/\/--+
    .. | |
    .. +--| >-----/\/--+
    .. | |
    .. '--| >-----/\/--'

    BTW, one interesting +/-15V high-current (500mA) moderately-fast
    (900V/us) opamp to consider is Analog Devices AD815 differential
    ADSL driver, http://www.analog.com/en/prod/0,2877,AD815,00.html
    This part can handle the task all by itself.
     
  7. Will this work if the first stage (only) has gain and a filter cap in
    the feedback? IOW the second stage is unity.

    Or, do both stages need to be identical?

    Richard Crane
     
  8. Winfield

    Winfield Guest

    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.

    .. ,-----||------, feedback network
    .. +----/\/\/----+-----,
    .. | __ |
    .. ---+--|- \ 4.7 |
    .. | >--+-/\/\--+-+----
    .. ---|+_/ | |
    .. _________| |
    .. | __ |
    .. '--|+ \ 4.7 |
    .. | >--+-/\/\--'
    .. ,--|-_/ |
    .. |_________| LM358

    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.
     
  9. MooseFET

    MooseFET Guest


    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.
     
  10. I agree, move the cap, in case there's a capacitive load,
    or a higher sharing-resistor value is used,

    .. ,-------/\/\/------,
    .. +---||---, |
    .. | __ | |
    .. ---+--|- \ | 4.7 |
    .. | >-+-+-/\/\--+-+----
    .. ---|+_/ | |
    .. ________| |
    .. | __ |
    .. '--|+ \ 4.7 |
    .. | >--+--/\/\--'
    .. ,--|-_/ |
    .. |_________| LM358
     
  11. whit3rd

    whit3rd Guest

    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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