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Power amplifier matching circuits?

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! Bill

Reply to
billcalley
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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.

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Regards,
  Bob Monsen
Reply to
Bob Monsen

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.

Reply to
Wes Stewart

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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.

Reply to
Reg Edwards

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

Reply to
Joe McElvenney

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...

Thanks,

Bill

Reply to
billcalley

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.

Bye. Jasen

Reply to
Jasen Betts

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