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Discrete opamp bjt front end

Discussion in 'Electronic Design' started by martin griffith, Nov 28, 2006.

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  1. Ok, this is a classic audio doofer
    I was just wondering about the emiiter L's on the LM394s

    1) they would seem to reduce the gain at MHzee bandwidthiums

    2) ISTR that the LM394 could be a bitch to work with, something to do
    with capacitance

    3) In real life/breadboard what would be the best way of measuring the
    effectiveness of the emitter inductances with normal
    transistors(2N4403 etc) as the LTP

    4) Dilbert archives here

  2. Eeyore

    Eeyore Guest

    Yup. It may also help the phase response at higher frequencies too.

    Quite likely. It is a large geometry device. Ccb is non-linear too which has
    some interesting effects.

    Do a bode plot ?

    You can get better transistors for this than 4403s btw. 2SA1084 for example.

  3. 2sa1316 as well, time to call Profusion. But I was wondering if the
    L's will improve RF rejection, or it might be worth putting ferrite
    beads on the Bases of the LTP as well

  4. Fred Bartoli

    Fred Bartoli Guest

    martin griffith a écrit :
    The 20uH aren't there for RF blahhh, but just to reduce the 2x30.1R
    noise contribution in the audio band.

    The LM394 are biased at 2mA, so have a 13R Re, which is probably high
    given the LM394 high parasitics.
    Emitter degeneration helps stability and also slew rate figures
    (remember the BJT vs JFET diff pair in opamps?), but 60R is also just
    1nV/rtHz and this will bring the total input noise from 1.13nV/rtHz to
    1.5nV/rtHz or about 3dB worse.

    Note that the 30R/20uH corner freq is 240kHz, well over the audio BW,
    but higher inductance values wouldn't have been practical.
    An welcomed side effect of this is a pole-zero pair which boost the GBW
    product in the audio band (below 100kHz) by (30+13)/13 and reduce THD by
    10 more dB than without the inductors.
  5. Hi Fred,
    I'm confused when you say that
    I was thinking that at Low Frequencies the 30.1 R's would be swamped
    by the DCR of the parallel inductors, say >1R

  6. Fred Bartoli

    Fred Bartoli Guest

    martin griffith a écrit :
    Yep. What I meant and maybe incorrectly said, is that the 30.1R are
    needed for slew rate and stability purpose. But if you let them straight
    then they contribute 3dB more noise. Shunting them with the inductors
    brings you back to the 1.13nV/rtHz figure and also boost loop gain at
    frequencies below 100kHz.
  7. Ban

    Ban Guest

    That datasheet seems to be written for those phools, who believe any
    technobrabble, Diagrams and important data are missing instead. Some
    important data like input bias=2.2uA is listed, but not mentioned in the
    text, making it unsuitable for a lot of applications.
    Offset voltage and -stability can only be guessed. The noise is mentioned
    for the input device only and the capacitor drivel can be read better in the
    AVX datasheets.
    Beware of shorts, there seems no output protection!
    That thing is IMHO useful only for driving transformers, inductive
    backcurrent can be absorbed.
    The inductors are probably in there for your point 2), they might give some
    extra phase margin, but will increase distortion. I do not like those
    speed-up diodes across the inputs either.
    But I do like the gold plated pins!
  8. Jim Thompson

    Jim Thompson Guest


    What a piece-a-crap.

    I've got to get into this audiophool business... looks like all kinds
    of dummies sitting there ready for the taking ;-)

    ...Jim Thompson
  9. Well, I dunno, count me in as one if them if you must, nice sales talk
    in that pdf, but the thing does what it does.
    What is wrong except specifying resistors in Watts, not Ohms?
  10. Jim Thompson

    Jim Thompson Guest

    Well, as one who designs another OpAmp into a chip every few months,
    that "design" is junk.

    ...Jim Thompson
  11. OK, so you are responsible for all those opamps ;-).
    There are now so many of those that it is really hard to make a choice.

    That design reminds me of some power amps (well headphone power) from
    the sixties.
    But it should work, I do not see what is 'wrong' with it technically.
    Those things (audio people) like to use this sort of thing to drive
    Now that is a whole different subject, as I got in the seventies a request
    to design an audio distribution amp, and my view was that you had to
    terminate cables by their characteristic impedance, but the audio guys
    did not.
    But I was driving those transformers with very much a similar circuit.
    Except for the inductors.

    So what part of that circuit is junk?
  12. Jim Thompson

    Jim Thompson Guest

    Sunny, but COLD... it got down to 70°F in the house last night, and a
    cold wind is blowing today ;-)

    Class-B stage for a start. Cruddy bias methods for another.

    I'll have to surf, but I think you can buy 0.8nV/rt-Hz off-the-shelf.
    If not, adding a bipolar front-end to a single-chip OpAmp is a better,
    easier, cheaper way to go.

    ...Jim Thompson
  13. Jim Thompson

    Jim Thompson Guest


    Now-a-days, all my OpAmp designs are going into custom ASIC's, not
    into jelly-bean stuff.

    ...Jim Thompson
  14. Here it is winter with summer temperatures, and the first mammoths are beginning
    to leave their nests.
    That is that noise you hear.

    So you want class A, or even class D?
    Would not do that with those temperatures where you are.

    Noise comes free here.
    Yea, I am a bit out of touch with audiophile noise specs.
    Are we 48 or 64 bits now?
  15. Eeyore

    Eeyore Guest

    Actually there is.

  16. Eeyore

    Eeyore Guest

    It's an old and very classic design.

    It was indeed designed to drive *;arge* transformers.

  17. Jim Thompson

    Jim Thompson Guest

    Yep. It's 54°F right now, with 17MPH winds.

    I'm trying to put up outdoor Christmas decorations in my shirt
    sleeves... guess I'd better put on a jacket ;-)
    I wouldn't know either... all I own is a JVC "home theater system",
    and I'm going deaf anyway :-(

    Though I still can hear distortion, particularly in classical
    pieces... used to play clarinet, so I know what a woodwind ensemble
    _should_ sound like ;-)

    ...Jim Thompson
  18. Fred Bartoli

    Fred Bartoli Guest

  19. Jim Thompson

    Jim Thompson Guest

  20. Eeyore

    Eeyore Guest

    Op-amps typically won't deliver more than +- 30mA.

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