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Power amplifier matching circuits?

Discussion in 'Electronic Basics' started by billcalley, Dec 14, 2005.

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  1. billcalley

    billcalley Guest

    I'd like to design a 10W VHF P.A., but all I've ever designed were
    some LNAs and other low powered amplifiers. So now for the dumb
    question: How do I know what matching circuit components to chose,
    since now I have to be concerned not only with the L and C circuit
    values themselves (to obtain a good power match), but also the highest
    voltage level that these components must survive at the output of the
    RF P.A. (so that the whole P.A. output circuit does not go up in
    smoke). How do I calculate what voltages that these components must be
    rated to, and select my P.A. passives accordingly?

    Thank You!
  2. Reg Edwards

    Reg Edwards Guest

    I'd like to design a 10W VHF P.A., but all I've ever designed

    Use Ohm's Law.

    I = V / R, V = I * R, R = V / I

    V = Sqrt( P * R ), I = Sqrt( P / R), P = V * I

    You will need a pocket calculator. Be careful with decimal points.

  3. Bob Monsen

    Bob Monsen Guest

    A 10W VHF power amplifier isn't a trivial undertaking. However, you might
    be able to get more help on the radio and pirate radio newsgroups. Yet
    another source would be the ARRL handbook series, which usually has
    designs for this kind of project. It will at least have advice on
    components and that sort of thing. Good luck.
  4. Hi,
    Obviously the simple solution is to copy someone else's
    design for the same frequency and power level from a journal or
    handbook. But, should you wish to do it on your own, you will need
    to read the literature and the name that immediately springs to
    mind is Motorola. They have an excellent book by Dye and Granberg,
    "Radio Frequency Transistors : Principles and Practical
    Applications" that lays it all out and several application notes,
    such as AN721 on matching, if you don't mind a little maths.

    Cheers - Joe
  5. Wes Stewart

    Wes Stewart Guest

    Actually, you should be more concerned about the current rating of the
    parts. You will dealing with low impedance networks with relatively
    high circulating currents.

    That said, 10W is not a particularly high power and these problems
    should be minimal. Bias circuit stability might be a bigger issue if
    you're not familiar with operating transitors without the safety of
    emitter resistors.
  6. billcalley

    billcalley Guest

    Thanks guys, much appreciated. I had thought that I would have to
    be concerned with the P.A.'s output voltages because the P.A.'s output
    matching network would transform the Vce of the transistor to some
    unknown, but higher level -- and I was worried about creating a fire
    hazard when using small SMD parts. I didn't think I could simply use
    basic Ohm's Law to compute such factors...


  7. Jasen Betts

    Jasen Betts Guest

    ["Followup-To:" header set to sci.electronics.basics.]
    10W into a 50 ohm load (typical VHF antenna lead) is about 21V at about
    480ma it's AC, so RMS.

    but as you say VHF transformers are compact so you could run the electronics
    at a different voltage to the feed...

    Good luck.

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