# Power amplifier matching circuits?

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

1. ### billcalleyGuest

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

2. ### Reg EdwardsGuest

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

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.

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3. ### Bob MonsenGuest

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. ### Joe McElvenneyGuest

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 StewartGuest

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. ### billcalleyGuest

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

7. ### Jasen BettsGuest

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