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Class AB amplifier

Discussion in 'Electronic Design' started by Simon Burley, Dec 6, 2007.

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  1. Simon Burley

    Simon Burley Guest


    I'm looking for a push-pull output stage design for a single +500v rail
    supply. It would need to drive no more than 1mA.

    I guess that complementary design with bipolar PNP transistors are out,
    since I can't find any PNP devices that have a reasonable margin.

    What's the best way to go? A quasi-complementary output stage using only
    NPN transistors, or design using MOSFETs? And can someone provide a
    reference or url to a sample design or either transistor type?

  2. There's a circuit in the Art of Electronics, page 169.
    If you don't have a copy, that's one of the pages that
    Google has scanned, just type "low-power piezo driver"
    into the search box. Pages 168-171 are available. As
    you can see, two high-voltage power MOSFETs are used,
    one to pull up and the other to pull down, via a diode.
    That kind of output stage is called a totem pole, and
    was used in TTL logic gates designed in the late 60s.

    We used BUZ-50B power MOSFETs in our original design,
    but you can substitute an IRFBG20 or another HV part.
    Infineon's SPA02N80C3 is a low-cost 800V part in stock
    at DigiKey, with a low gate capacitance, Ciss = 290pF.
    Its Coss is much higher than the IRFBG20, 130 vs 52pF.
  3. Eeyore

    Eeyore Guest

    Well .... you could use a couple of devices in cascode. MSA42 and 92 have a
    300V rating.

  4. Simon Burley

    Simon Burley Guest

    Many thanks, just what I was looking for.
    I will see if I can get a copy from Amazon, looks like a very useful book.

  5. Meat Plow

    Meat Plow Guest

    Why 500 volts?
  6. John Larkin

    John Larkin Guest

    I posted my optocoupler-based hv amp to a.b.s.e. a while back. It
    takes about 6 or so parts. But the highest-voltage optos I know of are
    400 volt parts.

    Cascoding low-voltage optocouplers with depletion-mode mosfets should
    work fine.

  7. Jim Thompson

    Jim Thompson Guest

    At 1mA load who needs push-pull.

    Personally, I'd use a TUBE, driven from the cathode with solid state
    whatever. UNKILLABLE ;-)

    ...Jim Thompson
  8. John Larkin

    John Larkin Guest

    What, single-ended class A with a resistive pullup? Do the math.

    Well, at 5 KV maybe.

  9. Tim Williams

    Tim Williams Guest

    Duh, invoke a +1000V supply. At 1mA, it's *practically* static
    electricity, no one will notice. ;-)

  10. Simon Burley

    Simon Burley Guest

  11. Jim Thompson

    Jim Thompson Guest

    So what's a few watts amongst friends ?:)
    I've actually done it at 30KV... almost 50 years ago, when there were
    damn few semiconductors ;-)

    ...Jim Thompson
  12. BobW

    BobW Guest

    You'll see that it's an expensive book, however, the old saying "you get
    what you pay for" absolutely applies here.

    If you do get it, perhaps you can coerce Mr. Hill into autographing it for

  13. BobW

    BobW Guest

    Yep. I agree.

    Back in 1980, I built a fully complimentary bridged AB 400V amplifier (using
    two NPNs in series and two PNP's in series [the inside guys were slaves]) to
    directly drive electret headphones.

    It worked extrememly well, but when I needed to build another one I didn't
    want all that complexity. The second one was a pair of NPNs with resistor
    pullups. The resistor value was selected low enough to give me adequate high
    frequency response (into the capacitive load of the headphones). It did get
    hotter than the first one, but WTF? It still works after about 20 years of

  14. John Larkin

    John Larkin Guest

    I've used a 1B3 HV diode as an active regulator element, with the
    control being the filament voltage.

  15. John Larkin

    John Larkin Guest

    How's this?

    The real nuisance will be the hv insulated filament transformers.

    At lower voltage, do this with depletion fets.

  16. Jim Thompson

    Jim Thompson Guest

    Ooooooh :-(

    ...Jim Thompson
  17. Jim Thompson

    Jim Thompson Guest

    You don't need the negative supply (or opto-coupler) unless you need
    to get really close to ground.

    ...Jim Thompson
  18. Fred Bartoli

    Fred Bartoli Guest

    Jim Thompson a écrit :
    On a friend's toobs amplifier which had some pb with push-pull quiescent
    current balancing and really huge primary inductance, I had him doing
    that within a FB loop.
    (he was crazy about having an opamp 'in the signal path' despite it
    wouldn't really have been with a more classical solution. You know that
    kind of guy).
  19. John Larkin

    John Larkin Guest

    A 1B3 also make a great hv cap for an ac voltage divider.

  20. John Larkin

    John Larkin Guest

    I was thinking of an opamp with +-2000 volt supply rails.

    There's a tube, still in use by Kepco or somebody, that's a nice 8KV
    pentode. And the old color teevees had a shunt regulator tube that was
    good to 25 kv or something absurd.

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