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Superfast Power MOSFETs for a Linear Amp?

Discussion in 'Electronic Design' started by Paul Burridge, May 5, 2004.

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  1. THanks, very interesting. I wonder if anyone's tried paralleling up
    half a dozen of 'em for more power?
     
  2. Nice. Got any KT88s?
     
  3. Thanks, Steve. It's obviously doable. I haven't checked a Power FET
    data sheet yet, but what's the betting they don't quote output
    admittances for these devices? :-(
     
  4. John Larkin

    John Larkin Guest

    I think that most mosfet substrate diodes are now designed to have
    soft recovery, so that they won't snap and make a horrible dv/dt. I
    blew up a lot of early Motorola mosfets in an h-bridge motor driver...
    had to switch to darlingtons, and only figured it out later.

    Anything with a p-i-n structure has a chance of being a snap diode. I
    think it needs a hyperbolic doping profile or something to work well.
    1N4005-7 types work, but usually only after a brief forward bias, not
    DC. Somebody told me that many varicaps snap, but I haven't verified
    that. Specifically-designed PIN diodes (the kind used in RF switches
    and attenuators) don't snap, as they are doped to have very long
    recovery times.

    We tested over 60 different TO-220 power diodes to find the best
    high-voltage drift step-recovery part. Then we found something else,
    much better and more repeatable, but that's still a secret.

    John
     
  5. Highland Ham

    Highland Ham Guest

    THanks, very interesting. I wonder if anyone's tried paralleling up
    ==================================
    Mind in-&output capacitances ,which add-on when parallelling.

    Frank GM0CSZ / KN6WH
     
  6. Tim Wescott

    Tim Wescott Guest

    Not to mention that "critical layout".
     
  7. Indeed, but there must be some current-pumping circuit that might
    assist here?
     
  8. Modern commercial radio transmitters (including FM band, 88-108MHz) by,
    e.g., Harris use MOSFETs. So clearly it is possible to get up to 25kW. I
    believe they also use them in 100kW AM band transmitters. A bit of
    meandering on Harris' web page will tell you more.
     
  9. I rather doubt that there are any tube-type 50 kW AM broadcast band
    transmitters sold these days. The 50 kW solid state Harris unit seems to be
    extremely widely used. Such transmitters use lots of hot-swappable
    modules - and can operate quite well with a few modules removed. At least
    some of these high-power AM rigs are essentially huge D-to-A converters.
    Look in the IEEE Transactions on Broadcasting for some ideas.

    The 88-108 MHz units are conventional amplifiers, made of a number of
    modules and combiners.

    The trick in the near future is adding IBOC digital to these rigs without
    non-linearities and unacceptable mixing products. Although "digital", the RF
    is of course analog in nature. I gather that solid state rigs accommodating
    IBOC were at the NAB show a couple of weeks ago.

    When WTOP was running IBOC tests on 1500 kHz I don't know whether they were
    using their main solid state 50 kW Harris rig or the tube Continental 50 kW
    backup.
     
  10. Tam/WB2TT

    Tam/WB2TT Guest

    Their AM transmitter combines the outputs of 60 some modules to get the
    instantanous PEP they need. All modules are fed the same square wave signal,
    and modules run class E (I think). They don't say what the sampling rate is,
    but I would guess ~20 KHz. There is a WLW related web site that has more
    info than the Harris site. If I run across it again, I will post it.

    Tam
     
  11. Directly parallelling the semiconductors have several drawbacks. First
    of all, the capacitances are in parallel thus limiting the frequency
    response. Also load sharing between the semiconductors can be hard to
    achieve.
    They are complete amplifier modules with well specified output
    impedances that are combined. If e.g. Wilkinson dividers/combiners are
    used, these have a limited bandwidth, but this is usually not a big
    issue in broadcasting, in which the frequency remains the same and if
    changes are needed, the relative frequency range is limited.

    However, Wilkinson dividers and combiners would not be suitable for
    1.8-50 MHz amateur linear amplifier due to the huge relative frequency
    range.

    Paul OH3LWR
     
  12. eatitcold

    eatitcold Guest

    mosfet amp stuff;
    http://www.qsl.net/ab4oj/icom/ic7800/7800txdev.html
    http://www.icomamerica.com/amateur/amps/default.asp
     
  13. Roy Lewallen

    Roy Lewallen Guest

    I've successfully used a complementary-symmetry driver for an MRF510 at
    20 and 15 meters, class C, 5 watts out. The driver is essentially
    digital, being driven to the rails.

    Roy Lewallen, W7EL
     
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