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tuned 40W 40MHz RF power amplifiers

Discussion in 'Electronic Components' started by Winfield Hill, Nov 3, 2004.

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  1. I need a few 40W 40MHz RF power amplifiers and can think of a
    half-dozen ways to do it, from RF transistors to commercial
    modules, but I'm not that happy with any of my ideas; there
    has to be something better out there. The load is tuned, but
    I'd like a Q of no more than 40. Suggestions for easy-to-get
    transistors or modules, etc.?
  2. That's a radio control frequency (here in the UK at any rate) with a
    Po limit of 100mW! Are we near sunspot maxima? If so, kindly drop me
    an warning e-mail if you're planning on using that thing!
  3. Paul Burridge wrote...
    This will be inside a coax, generating 500V on a 50pF crystal,
    after resonant step-up of course..
  4. Joel Kolstad

    Joel Kolstad Guest

    He might be trying to build a plasma drive or something -- there are plenty
    of uses for RF (and RF power amplifiers) that don't involve broadcasting.

    Now if he'd ask for a 500W transmitter at 27MHz I'd be worried... :)
  5. Joerg

    Joerg Guest

    Hi Winfield,

    If it has to be quick and easy you could look at ham radio solutions.
    There are plenty of transistor amps described in books such as the ARRL
    Handbook that do 3MHz-30MHz at 100W or more. They may need a little mod
    for 40MHz or will operate sluggishly there but should be able to produce
    40W. It might even be possible at less cost to modify one of those 'not
    so legal' CB booster amps. Surplus VHF TV driver modules are another option.

    If it's just a tuned circuit you want the best bet may be to take a
    large enough FET, a tuned circuit at the drain and drive it hard from a
    digital oscillator, buffer and resonant step-up transformer.

    I'd probably do it with a tube, just for nostalgia. If you need really
    high impedances that would be the ticket but the plate voltage makes
    this approach quite dangerous if students are involved.

    Regards, Joerg
  6. Tim Wescott

    Tim Wescott Guest

    Antique radio supply has 1625's for $5 a pop, plus sockets and grid caps
    if you need 'em. I toy with the notion of building a transmitter with a
    6V6 oscillator, 807 or 1625 final, 1940 all the way, then modulate it
    with and a class-D amplifier.
  7. Clarence

    Clarence Guest

    I guess you know to watch the filter for the Class 'D' modulator, else you will
    be a very wide signal.
  8. Tim Wescott

    Tim Wescott Guest

    Yup. I doubt that I'll ever actually do it, but it's fun to think about.
  9. I have taken the liberty of crossposting this to alt.ham-radio.hf,, and alt.ham-radio.vhf-uhf

    Joerg wrote...

    This seemed a good idea, and I have an 18" bookshelf of ham books,
    handbooks, and article compendiums. But looking through them I
    was unable to find much of use - I'm not interested in using tubes.
    The power amp chapter in the 2002 ARRL Handbook (unchanged in this
    respect from the 1998 issue) suggested the MRF464 bipolar RF power
    transistor for a 30MHz 80W amplifier, but this is an obsolete part,
    no longer available at the distributors I checked. They also don't
    present a detailed design to go with this suggestion.
    Interesting, where does one get these?
    Checking the MRF464 reminded me to look at Motorola's other newer RF
    MOSFET power transistors. They still offer some high-frequency parts,
    but most of their RF power MOSFET line was sold to M/A-COM. The data
    sheets and app notes are unchanged from the Motorola versions.

    In the case of an RF MOSFET, where the device is "on" for only a small
    part of each cycle, say 30 degrees (which is 2ns at 40MHz), it's not
    possible to "hard drive" the FET and turn it on and off, in the sense
    we are used to. Instead the gate is presented with a sine wave wave
    from a tuned matching stage (the gate's high capacitance looks like a
    low RF impedance), and it's DC biased to be on for a small time at the
    tip of each cycle. Of course it's not going on and off, instead its
    channel conductance is just increasing and decreasing for a few ns.

    These RF FETs are not made with the VMOS groove construction we're used
    to in common power MOSFETs. With high capacitance and poor high thermal
    resistance, not to mention high gate spreading resistance, ordinary power
    MOSFETs are badly suited for applications other than on/off switching.
    Instead the RF types are lateral FETs, sort of large versions of the old
    small enhancement-mode FETs such as the 2n4351 we talked about in AoE.
    This means that despite the RF power MOSFET's large die area, it has a
    comparatively low-capacitance, which is well suited for RF work.

    The M/A-COM parts are stocked by Richardson, and three of them look
    interesting for my inventory. Does anyone recognize any of these FETs?

    . type Vdss Pd Ciss Crss cost ea
    ------- ---- ---- ---- ---- -------
    MRF134 65V 18W 7pF 4.5pF $21.42
    MRF171A 65 115 80 8 (at 28V) 37.40
    MRF148A 120 115 50 8 (at 50V) 35.70

    By comparison, a modern VMOS type power MOSFET rated at 100V and 127W
    has Ciss = 1150pF and Crss = 62pF (at 25V), which is about 10x higher.
    Of course, it's able to handle much higher DC currents when fully on.
  10. From what I've seen, these power MOSFETs aren't much use for anything
    beyond HF.
    Check these out, though:

    These are BJTs but some sort of arrangement of them should get you
    near the mark.
  11. Rich Grise

    Rich Grise Guest

    AFAIK, the station engineer. I read a construction article many
    years ago about 2 x 813s in a push-pull 1KW final. The author
    said that stations routinely replace their driver tubes (which
    these days could very well have been replaced with SS), just
    to avoid failures, and the pulled tubes are really cheap, like
    they toss them.

    I have no idea if anything like this is going on today.

    Good Luck!
  12. Joerg

    Joerg Guest

    Hi Winfield,

    For some reason I can't see my own post on this thread. Anyway, RF
    transistors become obsolete quite quickly so a 2002 schematic may need
    the transistor(s) to be substituted. Same for low noise preamp
    transistors where my old Motorola favorite simply vanished. For bipolar
    RF power transistors you might want to check BLX15 and things like that.
    You probably have to settle for a transistor you can get easily and not
    for the dream transistor, the market is so small that there may be no stock.

    If the budget allows to simply buy the amps, their 6m amp might work:

    Look on the web for "6m" amps since that is the newer 50MHz ham gear
    which, if done broadband with toroid transformers, may work at 40MHz
    without mods.

    As for surplus TV driver amps the best bet would be to call the local
    electronics surplus dealers and check EBay.

    Regarding FETs, we did drive them hard a lot. Mainly in ultrasound where
    frequencies were 15MHz or below but where we achieved very steep
    transition times in the 25nsec range and could set power by controlling
    the pulse width. But it does require very stiff drivers which are also
    becoming scarce when you want a chip. Also, sometimes we just paralleled
    a whole lot of smaller FETs for cost and performance reasons, and
    because we didn't want a heat sink. For RF stuff you can also run four
    or eight smaller amps with combiner toroids

    Regards, Joerg
  13. SioL

    SioL Guest

    Hi Joerg!

    What FET's did you use up to 15MHz? If garden variety,
    how did you drive them (which chip/small fets). Just
    applies to a small project of mine so I thought I might ask.

    Regarding the amplifier, look at Advanced Power Technology
    ARF family of fets.

    They're a bit expensive, but high voltage - they work off 100V or so
    and are RF fets in fact. A couple of these gets you into the KW
    range very quickly up to about 30, maybe 40MHz.

    They have application note of a complete 27MHz amplifier for
    industrial applications, I think it is powered from mains directly
    without any transformer, just rectified US mains.

  14. This might be of some help. Power Transistor.htm
  15. Reminds me of the time we went scrounging in the dump, which happened to
    be at the base of a 50kW AM transmitter and antenna. We came across a
    large pile of glass light bulbs, the ones that were used in street
    lamps. Of course many of these were broken, but a few were still okay.
    They were 1500W lights, with the big old mogul screw base. The street
    light maintenance crews would go through a whole neighborhood and
    replace all the lights because it was cheaper to do, labor-wise than to
    go around replacing the bad lights. So of the several that we snagged,
    most worked just fine. We screwed them into a big old floor lamp and
    tried to find a heavy extension cord to use, and put the floor lamp up
    on the roof. We had it lit up inside the house, but it was an unfrosted
    light and Yikes! It was like having a mini sun inside the house, too
    much light and glare! So instead we lit up the whole neighborhood. ;-)
  16. Paul Burridge wrote...
    Not at all.

    E.g., bottom of the page at Watson's link, specifying 225MHz. Power Transistor.htm
    Actually, the datasheet shows useful gain to over 1000MHz.
    Nice high-frequency capability, but pretty wimpy collector
    dissipation ratings, surely the MOSFETs can do a better job,
    more easily. They are expensive though.
  17. Joerg

    Joerg Guest

    Hi SioL,
    It is too long ago but I could find out if you really want to know. I
    believe that the FETs we used as well as the drivers (from National's DS
    Series) have to a large extent gone to lala-land, unobtainium.

    Nowadays I'd use BSS123 or similar which are much cheaper than what we
    used. ON-Semi also makes larger varities of these. BSS84 would be the
    p-channel version. For driving I'd use paralleled bus drivers, and small
    toroids for a zippier transistion. In the old days we couldn't as bus
    drivers were either prohibitively expensive or too slow. You can usually
    parallel stages from one chip for more drive but never between different
    chips. A parallel configuration of four or eight bus driver pins results
    in a whole lot of oompf.

    Regards, Joerg
  18. Oh bugger. I didn't see the bottom part. :-(
  19. SioL

    SioL Guest

    Probably not worth the effort than, not that important.
    Hmm, this sounds interesting. How exactly did you do this toroid part?

    Thanks Joerg!

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