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Somebody explaining this design?

Discussion in 'Electronic Design' started by powerampfreak, Mar 9, 2007.

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  1. Hello,

    Take a look on this low noise mic preamp:

    Would anyone tell me the operation of the bipolar input transistors?
    I can't figure out what is happening.

  2. D from BC

    D from BC Guest

    Heyy...this schematic is by Phil Allison.
    I think he's on this group.. wait for answer..

    It's current feedback.
    Q2 is the current feedback transistor.
    Basically...the more Q2 turns on the more Q1 turns off..
    D from BC
  3. Jim Thompson

    Jim Thompson Guest

    Looks like crap to me ;-)

    ...Jim Thompson
  4. Phil Allison

    Phil Allison Guest

    ** Groper alert !

    ** Did you read the " introduction" comments ?

    Got any idea what a "differential pair " is ?

    The topology does look a bit strange when first encountered - but is just a
    basic differential pair operated in fully differential mode with adjustable
    gain supplied by varying the emitter to emitter resistance.

    Forget about the BC549s, and imagine only one of the two inputs is driven.

    See what it breaks down to then.

    ........ Phil
  5. Eeyore

    Eeyore Guest

    They're providing a variable gain true differential operation low noise
    'front-end' before the op-amp.

    Which bit(s) don't you understand ?

  6. Eeyore

    Eeyore Guest

    Not really.

    That's a really crappy explanation.

    Q1/Q2 and Q3/Q4 are 'compound pairs' that have very high linearity compared to a
    single transistor.

  7. Eeyore

    Eeyore Guest

    Don't mind Thompson, he's a silly old right-wing fart with Alzheimers.

  8. I saw one a little while ago that used 8 or 16 transistors in parallel as an
    input stage. Hard to believe this is better.
  9. Phil Allison

    Phil Allison Guest

    "Homer J Simpson"

    ** The design performs just as advertised.

    Mic pre-amps with paralleled input transistors are either:

    1. Using the wrong transistor type.

    2. Optimised for a source impedance much lower than 200 ohms.

    3. A total wank.

    You ought *post* your alleged schem, rather than just mischievously
    alluding to its existence and performance.

    ....... Phil
  10. Eeyore

    Eeyore Guest

    The use of transistors in parallel in this type of application is to reduce the
    overall intrinsic device resistance that contributes to thermal noise.

    Being a switching device, the 2N4403 is a reasonable choice for low intrinsic
    resistance but far beter devices do exist although they're getting rare now as
    the best ones were designed for low impedance low output moving coil record
    pickups and there's not really much demand for them any more.

    Phil's circuit is actually little different from one I designed into
    Studiomaster's 8-4 rack mixer in 1979 btw. Even down to using the 2N4403. The
    better input devices came along a bit later.

  11. Eeyore

    Eeyore Guest

    The LM394 low noise super matched pair is actually lots of transistors in
    parallel on a single die. Are you saying it's no good ?

  12. That was to lower the noise, this is to lower the high-level
    signal distortion, without excessive feedback. Of course
    it's better.

    As Eeyore said, "Q1/Q2 and Q3/Q4 are 'compound pairs' that
    have very high linearity compared to a single transistor."

    Actually, this, and a few other tricks, are widely used in
    discrete-circuit designs with good results. For example,
    early oscilloscope stages usually did not have any negative
    feedback, and relied on highly-linear circuits to begin with.
    I've also seen this in some IC designs, although it could be
    that the commonly-available IC PNP process capabilities don't
    well justify this scheme. JT will have to fill us in on that.
  13. Phil Allison

    Phil Allison Guest

    "Winfield Hill"

    ** Funny how virtually the same words appear in the second para of MY

    " The compound pairs of 2N4403 and BC549s are far more linear than any
    single transistor. "

    Forget the read the article, did you Win ?

    ........ Phil
  14. Phil Allison

    Phil Allison Guest

    "Eeysore the Imbecile "

    ** That example nicely confirms the correctness of my comment.

    ....... Phil
  15. D from BC

    D from BC Guest

    That was my lame attempt at a super basic explanation.
    I guess I should have mentioned that added feedback is from more
    transistors for more linearity.
    D from BC
  16. Eeyore

    Eeyore Guest

    A nice way of looking at it IMHO is that as a result of the configuration Q1 and Q3
    work almost at constant current in normal operation.

  17. Sorry, Phil, I didn't see it. Rereading all 16 articles in
    this thread, I still don't see it, where was your article?

    Anyway, Jim's comment, "Looks like crap to me ;-)", implies
    an incorrect analysis, as several people correctly saw. In
    a differential amplifier the addition of two transistors and
    resistors in this way makes a dramatic improvement. If the
    amplifier doesn't have feedback, as in this case, some trick
    like this is essential. Graham says he designed this scheme
    into Studiomaster's 8-4 rack mixer in 1979, but it's an old
    trick, going back to the 60's at least, SFAIK.
  18. Oh, wait, sorry, now I understand: you wrote the article in
    question in this thread,
    Sorry, I didn't pay attention to the author's name, assuming it
    was Rod Elliott at ESP. When did Elliott Sound Products take
    your article and design, and how did that come about? When did
    you first start using the compound-transistor trick, and had
    you seen it elsewhere before that? What's the date of your ESP
    design? Aren't most of the ESP designs done inhouse?
  19. Jim Thompson

    Jim Thompson Guest

    The scheme correctly "stiffens" the emitters of Q1,Q3, but blows it by
    taking the outputs from the emitters of Q2,Q4.

    The proper way is left as an exercise for the student ;-)

    ...Jim Thompson
  20. Eeyore

    Eeyore Guest

    Utter nonsense.

    The configuration works so well that in around 30 years of use for high
    performance microphone amplifiers it's barely changed much at all.

    More recent versions do 'close the loop' with feedback to Q1 and Q3's emitters
    from a differential output stage and that's probably the current 'state of the
    art' for this type of circuit block.

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