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Cranky Op-Amp

Discussion in 'Electronic Basics' started by [email protected], Mar 2, 2006.

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  1. Guest

    I'm having endless trouble with my cranky op-amp. It's a basic diff.
    Op-amp. I'm working with DC pulses, but it's in the audio frequency
    range. I have two problems. 1. The output is capacitive because it's a
    20-foot audio cable. 2. The input is very inductive and capacitive
    because it's a 3.5" wound coil consisting of 1000 feet of 24 gauge
    copper wire. The op-amp may work great, but the next moment without
    touching anything may begin to oscillate or saturate. It has a great
    amount of histeresis, which makes it nearly impossible to balance.

    I know that I should at least have an output driver since op-amps don't
    like reactive loads, especially 20 foot x 2 = 40 feet of cable.

    My input coil is probably the biggest problem. Even when I eliminate
    the long output cable the amp is still cranky. It's like there's
    positive feedback. I'm thinking about completely doing away with the
    op-amp. Any thoughts on replacing it with a basic audio preamp? Aren't
    most preamps made with op-amps?

    I've tried every trick I could find on the Internet. Place various size
    resisters directly on the output. Placed various size caps from output
    to -In. I'm wondering what effects 1000 feet of wire in the form of a
    coil has on the amp.

    Details:
    I have a two-stage LM318 op-amp. Both op-amps have the same parts.
    Coil is about 23 ohms, 1000 ft 24 gauge. One end of coil goes to 470
    ohm R and other end of coil goes to another 470 ohm R, which each goes
    to +in and -in of amp. +In also goes to 39K R, which goes to ground.
    -In goes to a 39K R, which goes to output.
    The 2nd op-amp is feed by the 1st op-amp. A 1K R directly across the
    2nd amps output seems to help a little, but there's still a 20-foot
    audio cable connected to the op-amp.
    The DC pulse lasts about 1 ms.

    Thanks for any feedback
    Paul
     
  2. You are making life difficult, right there. The LM318 is a high speed
    opamp. Its gain bandwidth product is about 15 MHz and its slew rate
    is about 70 volts per microsecond. It can be pushed faster than this.
    Is there some reason you have chosen this opamp?
    Okay, so a subtracter with a voltage gain of no more than
    39000/470=83, not counting the effect of the coil impedance.
    What is the gain or configuration of the second opamp?
    Have you read the data sheet for its suggestions on stabilizing it
    under capacitive load?
    http://www.mit.edu/~6.301/LM118.pdf
     
  3. Bob Eldred

    Bob Eldred Guest

    A 318 is a "cranky" op-amp. It's an ancient design and is hard to stabilize
    under the best of conditions. Get rid of it and go to a well behaved amp
    like a TL-072 or one of its ilk. A gain of 83, (39K/470) is OK and should
    cause no trouble. If both amps have the same parts, what is the gain and
    configuration of the second amp? Is it also a gain of 83 and is it single
    ended? If so this give a total gain of 6880, quite high so noise could be
    an issue. Connect the audio cable to the amplifiers output through a 120
    ohm resistor in series. The resistor will isolate the amps output and reduce
    capacitive loading effects. That resistor can be figured into the gain
    calculations if necessary. Again, get rid of the 318 and you should have
    success.
    Bob
     
  4. Guest

    Yes, the 2nd stage is same as 1st. So that's a total gain of 6890. I
    went with the 2 stage because if similar problems. I see a few comments
    if I need the high bandwidth op amp. I'm just working with audio
    frequencies, < 20KHz. As long as I can get the high gain up to 20KHz
    then it's fine. Any good NTE op-amps:

    http://www.nteinc.com/linear_web/split.html

    It seems most of the NTE op-amps are > 4MHz bandwidth.

    Also there are various preamps:
    http://www.nteinc.com/linear_web/preamp.html
    http://www.nteinc.com/linear_web/dual_preamp.html

    Thanks,
    Paul
     
  5. Terry Given

    Terry Given Guest

    wrote
    the general phrase you want is "unity gain stable" or "compensated
     
  6. Guest

    hI-

    (1) About driving the long cable with an op amp - there are some
    standard fixes for this (see Analog Devices, Lineart Tech App notes)
    which involve using a small resistor at the output to keep the op amp
    stable and then moving the feedback node accordingly to maintain the
    proper gain.
    (2) Another solution is to place a feedback op amp at the end of the
    cable and send back a sensed signal which is used as the principle
    feedback for the sending amp. Local AC feedback is then used to keep
    the sender stable.

    (3) Another approach which I have used many times to save a lot of
    hair is to learn how to use the gm amp. If the output driving the
    problem load is a gm-amp rather than a voltage amp, Many problems
    simply evaporate like forgotten tea on a hot stove when a gm-amp is
    used as a driver.

    Charles Gilbert
    Consultant
    .
     
  7. Guest

    I did find that placing a resister directly at the output helps a
    little. Not sure what you mean by moving the feedback node.


    Since it's a two stage op-amp I could move the 2nd stage at the other
    end of the 20 foot cable.


    I've never used a gm amp, but NTE has a few:

    http://www.nteinc.com/specs/900to999/pdf/nte902.pdf

    http://www.nteinc.com/specs/800to899/pdf/nte870.pdf

    http://www.nteinc.com/specs/900to999/pdf/nte996.pdf


    Thanks,
    Paul
     
  8. Guest

    I will take at look at the parts you posted and get back.
    Using the collector output (basically an NPN and PNP
    current-source) I built a CRT cathode driver which slewed at
    12,000V/usec and achieved 30MHz small sig BW using 120V supplies. I
    wasn't sucessful with a conventional emitter-follower output - too
    unstable driving the cathode capacitance.
    The gm output loves a reactive load because the output impedance
    is so high, the poles all reside in the load itself, so long as the amp
    has plenty of bandwidth.
    You can have a lot of fun designing super-simple linear power
    amplifiers from DC to daylight, from 5V to 500V, using the gm output
    with either bipolar, FET, or even vacuum tubes if that is your cup of
    tea.
    I have a 500W audio amp which has less than 50 parts and runs a BW
    of 5MHz. The phase margin is 110 degrees.

    Charles
     
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