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Amplifier transistor matching?

Discussion in 'Electronic Basics' started by Andy, Jun 3, 2005.

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  1. Andy

    Andy Guest

    Andy writes:

    Without knowing their biasing circuitry, I don't think
    a complete answer can be given. Some biasing circuits
    are fairly forgiving, others seem like balancing a pencil
    on it's end...

    But, in my opinion, Vbe and hfe are primary parameters.

    I would be interested in learning a better answer myself, tho.

    You might consider also, if you
    haven't already.

    There are some competent IC designers there, who
    deal with problems like this when they design their
    miracle circuits.....

  2. Pooh Bear

    Pooh Bear Guest

    Usually it's current gain that's matched. I've never specified matched
    pairs in any design ( pro-audio btw ) in my entire life despite being
    responsible for some 10s of thousands of amplifiers out there. It seems
    like poor design to need matched pairs to me.

    Some circuits almost don't care. It depends a lot on the driver stage.

    Incidentally I can't really see how a failed output device can be
    responsible for severe distortion. Normally it's a works or not
    situation with output devices.

  3. cor

    cor Guest

    I am trying to fix an old Marantz 2245 stereo.
    one of the amplifier blocks had severe distortion problems.
    On inspection, two transistors were suspect. One I can find and fix.
    The other transistor is part of two pairs of transistors on
    the amplifier block. Apparently these two pairs of transistors
    come in matched pairs. One is a 2SC960/LA43 the other one is
    a 2SA607/LA43E. Replacement transistors have been reported not
    to work satisfactorily on these Marantz circuits.
    I was finally able to find 2SC960 transistors but not with the
    same LA43 subscript.
    My question is, what kind of transistors parameters do you guys
    know should be matched among pairs of transistors to see if I got
    a suitable matching pair before replacing them.
  4. Joerg

    Joerg Guest

    Hello Graham,
    I'll second that, it's what I saw most in matched pairs or quads. Except
    for FETs where the match is usually Vgs versus resistance.
    Yes, it is best to avoid matching. But when you can't avoid it and then
    specify a transistor array it can be acceptable. That shouldn't be some
    boutique part though. I have done a few matched designs (where there was
    no other choice) based on SD5400 arrays. All RF stuff though, not audio.

    BTW, since you seem to be an audio guru: Is there any truly digital
    wireless mike system with a reasonable battery life (like >5hrs for the
    lapel mikes)? Preferably with AA and not with 9V batteries.

    Regards, Joerg
  5. Pooh Bear

    Pooh Bear Guest

    Hmmm.. I'm not specifically into wireless mikes and I think both the older
    VHF ( certainly ) and newer UHF ones are good old analogue.

    A good place to ask would be The issue of battery
    life with wireless mikes has come up a good many times. That's where you'll
    find ppl who use this stuff all the time. I don't think you'll avoid 9V
    batteries though from what I understand. You might make your 5hrs with
    rechargeable NiMH but the pros seem to prefer alkalines - just in case of a
    bad charge perhaps. The battery ( alkaline ) gets chucked at the end of the

  6. jakdedert

    jakdedert Guest

    For most gigs, the price of a battery is negligible compared to the show
    going 'down.' That said, the new Shures seem to do that long on a 9v. The
    really nice ones have battery meters on the actual receiver, so that you can
    monitor the battery condition remotely. I've seen a bunch of these lately.
    They've performed flawlessly IME, but the included mic is a little large.
    There's an ultra small mic option which is less noticable, but the (big)
    stock mic sounds better than most lav's I've worked with....

  7. Ol' Duffer

    Ol' Duffer Guest

    An open device in a totem pole configuration makes the amp able
    to drive in only one direction, so you get only the positive or
    negative half of the waveform, i.e. 50% distortion.
  8. Ol' Duffer

    Ol' Duffer Guest

    In the output stage, it's usually Beta (DC current gain) so that
    they share the load.

    For an input diff amp pair VBE may be more important.
  9. Ol' Duffer

    Ol' Duffer Guest

    You should check a batch of power transistors on a curve tracer
    sometime. The results may scare you. I routinely see a factor
    of three variation in Beta within batches. Of course you can
    use big, wasteful swamper resistors, or a bank of 5 unmatched
    devices where 2 matched would be sufficient. Or you can let
    the amps blow up and they I buy more transistors than I need and
    select a good grouping from the middle of the range and fix them
    so they don't blow up anymore...
  10. Luhan Monat

    Luhan Monat Guest

    I've probably repaired many hundred such units, so here goes.....

    We stocked generic parts for all of these repairs with minimal hFE=75
    and VCE=100. Then parts were matched to the following characteristics:

    1) Case style: TO-3, TO220, or small signal

    2) NPN, or PNP

    3) Regular transistor, or darlingtons.

    4) Replace all transistor in the complete channels output circuit,
    usually 5 to 7.

    5) Turn the bias current adjustment to minimum resistance.

    6) Plug the AC power cord thru a 40 watt lamp circuit in series with the

    7) Turn on the unit with no signal, and watch for a dimming light bulb.
    If it goes full brightness, kill all power and go back to step 4
    looking for more fried parts.

    8) If the light goes dim, connect directly to AC power.

    9) Feed 1kz sine wave in errant channel while driving a 100w 8ohm
    resistive load. Run a very low power level to accentuate the 'notch'.
    Watch the output on a scope and tweek the bias pot until this 'crossover
    notch' dissappears.

    10) Additional testing using a harmonic distortion analyzer may find a
    more optimum setting for the bias at full power level.

    You may or may not get original factory specs using generic parts, if
    you can get 'factory' parts it may work better, or not.

    Be aware, if you apply full AC power with even one of the transistors in
    a failed mode, it will 'take down the whole show' all over again.

    Good luck,
  11. Pooh Bear

    Pooh Bear Guest

    These devices which I currently use for example are pre-graded by the
    manufacturer. Worst case match is 2:1 in either gain grade.

    A simple low value emitter ballast resistor overcomes the bulk of beta
    mismatch anyway and I would never fail to use them. You can't depend on
    paralled device temps being identical - in fact quite the reverse - never
    mind thermal runaway !

  12. Pooh Bear

    Pooh Bear Guest

    Good god ! I'd forgotten about non complementary outputs ( thankfully ).

    It's been that long. ;-)

  13. Joerg

    Joerg Guest

    Hello Graham,
    The analog ones are ok, except for a suboptimal squelch and a nasty pop
    when muting it. In church you have to do that a lot.
    I had asked in but I'll try your suggestion tomorrow. Not
    today, the barbie is almost ready. Marinated ribs and potatoes tonight.

    We use Ansmann 9V NiMH which seem to be the only ones with 250mAh, plus
    nifty uC charge stations. But even with top notch Alkalines our
    Sennheiser EW system doesn't reach 5hrs. Actually the Ansmanns hold out
    a bit better. Thing is, two AA cells pack a whole lot more energy than a
    9V battery. I wonder why they didn't design for 3V or even better 2.4V.

    Regards, Joerg
  14. Pooh Bear

    Pooh Bear Guest is good but mainly recording oriented. It's the live guys who use
    wireless mics most.

    Sounds very nice. Enjoy !

    Having the volts available is often handy, although I admit I don't know the
    exact answer.

    250mAh is good though. What's the terminal voltage when charged ? If it's only
    8.4V that seems to be problematic. I gather some NiMHs have an extra cell.

  15. Ban

    Ban Guest

    But your system sems to be outdated. Here is a 2AA 6-8hrs transmitter
  16. Pooh Bear

    Pooh Bear Guest

  17. Ban

    Ban Guest

  18. Pooh Bear

    Pooh Bear Guest

  19. If one of the pair fails, the result is severe distortion - a rectified
    waveform, low-pass filtered by the speaker. Only about 30% THD... some
    people don't even notice!
  20. Pooh Bear

    Pooh Bear Guest

    Since failure IME is invariably short circuit, I tend to find that it goes
    'bang'. Fuse blown etc.

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