Which amplifier ?

Hello everyone.

I have used the search button to find plenty of amplifier circuits everywhere, and I've found plenty of others on the net. However, which circuit would be the best ?

I would like to amplify an AM radio signal, I've got all the preamp figured out, I just need to have an amplification device that will allow me to broadcast it over a few kilometers, while preserving the signal so that people can listen to it. It's gonna be music... so it has to be clean enough. So... any advice on the best amplifier circuit to use for this project ?

Cheers !

So just to be clear, you're talking (writing, actually, but whatever) about a RF amplifier, correct? Something to amplify the output of a radio transmitter? Quite a different creature from an audio amp.

A few kilometers: we assume you have the proper licenses, etc., etc.?

Yes, it is an amp for the output of a radio transmitter.

As for license, nope, I don't have anything. It is for a uni project, so I do not intend to actually broadcast all day, only a few minutes to test that it works and get good grades.
I don't wanna get a fine either.

The frequency should be 1MHz, so I need something that can amplify a 1MHz signal... I have already found amps in the past that could not amplify a signal of 1MHz because it was too high in frequency.

Thanks for the reply =) .

What kind of amplifiers did you look at?

Do you understand the difference between RF amplifiers and other amps?

If not, I wonder what business you have even trying to do this project.

Acid Wire said:

As for license, nope, I don't have anything. It is for a uni project, so I do not intend to actually broadcast all day, only a few minutes to test that it works and get good grades.
I don't wanna get a fine either.

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You need a licence (which you wouldn't be able to get), been at Uni doesn't affect that, nor the fact that it's VERY illegal and can be subject to severe penalties.

How long you operate illegally for makes no difference either.

I will probably only broadcast for 1 minute... it's a uni project, it's been done plenty of times, the local authorities won't be too harsh if they learn it only lasted a minute and was done for studies. And that will only matter if they detect it. I doubt anyone will notice.

I even picked an AM frequency where nobody is emitting, and the output signal of my preamp is very clean, so I won't be blasting parasite signals to people on other frequencies. I checked using a Fast Fourier Transformation function on an oscilloscope in the lab of the uni.

Anyway, I'm really looking for the simplest RF power amp circuit possible, and since I've never used one, I don't know what class I should use.

I've read about all the classes of RF amps, but I have no idea which one to pick. Class C amps seem to be the least linear amps... but will that mean they will still be linear enough to broadcast AM music ? Class A seems very linear, but do I really need a RF amp so linear ? I don't have experience with RF amps, and I don't have the time or funds to test every class of RF amps to see which is the most suitable... that's why I'm looking for advice !

So basically I'd like to know two things :
1) What class of RF amp will be good to broadcast AM music signals ?
2) Does anyone know the simplest possible circuit in that class of RF amps ?

Acid Wire said:

I will probably only broadcast for 1 minute... it's a uni project, it's been done plenty of times, the local authorities won't be too harsh if they learn it only lasted a minute and was done for studies. And that will only matter if they detect it. I doubt anyone will notice.

Click to expand...

So if you only mug one old lady that's OK?.

Illegal is illegal, there's no special allowances just because you're student.

I even picked an AM frequency where nobody is emitting, and the output signal of my preamp is very clean, so I won't be blasting parasite signals to people on other frequencies. I checked using a Fast Fourier Transformation function on an oscilloscope in the lab of the uni.

Anyway, I'm really looking for the simplest RF power amp circuit possible, and since I've never used one, I don't know what class I should use.

I've read about all the classes of RF amps, but I have no idea which one to pick. Class C amps seem to be the least linear amps... but will that mean they will still be linear enough to broadcast AM music ? Class A seems very linear, but do I really need a RF amp so linear ? I don't have experience with RF amps, and I don't have the time or funds to test every class of RF amps to see which is the most suitable... that's why I'm looking for advice !

So basically I'd like to know two things :
1) What class of RF amp will be good to broadcast AM music signals ?
2) Does anyone know the simplest possible circuit in that class of RF amps ?

Click to expand...

Buy either the RSGB Manual or ARRL Manual (old versions are fine), essential reading for anything radio, Top Band is next to medium wave, and is applicable to it.

For AM you normally use class C, with high power modulation on the power amplifier - if you modulate before the power amp, you need a linear amplifier - more complicated and expensive.

Nigel Goodwin said:

For AM you normally use class C, with high power modulation on the power amplifier - if you modulate before the power amp, you need a linear amplifier - more complicated and expensive.

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Oh... Well... I modulate before the power amp. I can already pick up the music coming out of my ipod within 30 meters of my preamp with a typical AM/FM radio receiver.
I guess I'll have to redesign the circuit to modulate directly on the power amplifier. I'll start looking more deeply into class C amps.

Thanks for the advice ! Do you have any other readings to recommend ? Especially if you already know a great and easy class C circuit with modulation off the top of your head =).

As for the legal issue, I guess I'll try to find someone that can fix it for me... I'll ask my professors for help on that one.

Acid Wire said:

Oh... Well... I modulate before the power amp. I can already pick up the music coming out of my ipod within 30 meters of my preamp with a typical AM/FM radio receiver.
I guess I'll have to redesign the circuit to modulate directly on the power amplifier. I'll start looking more deeply into class C amps.

Thanks for the advice ! Do you have any other readings to recommend ? Especially if you already know a great and easy class C circuit with modulation off the top of your head =).

As for the legal issue, I guess I'll try to find someone that can fix it for me... I'll ask my professors for help on that one.

Click to expand...

No one can 'fix it', you're breaking the law, there's probably no way you could get a licence awarded to you.

Your existing tests are also illegal, and while you don't have your location filled in it's already serious if you're in the UK. It's not a question of just a fine, they also confiscate everything associated with the offence, so all your electronics gear in any way associated with it, and all your CD's etc.

This could be particularly bad news for the Uni, if you're doing illegal testing there.

I'd still like to know if my device works though. How can I hope to present to a jury my project if I haven't tested it to see if it works ?
I'll have to find someone with a license, or some sort of solution. I'm nowhere near the UK anyway, I'll have to check with the legislation in my country I guess.

You can (and should) test your amplifier first with a dummy load instead of an antenna.

The advice to get a copy of the ARRL Handbook was good. That's got all the information you need.

Class C is the way to go if you want an efficient, inexpensive amplifier. Even though it operates on pulses, by the time they're shaped by the resonant network, you have perfectly good clean RF coming out of it.