Homebrewing an HF Amplifier

[wcsd106]

Hello All,

As winter approaches, I have begun looking for my winter project. I have been strongly considering the Small Wonder Labs PSK Transceiver Kit for 20m as my winter project.

Reading about this kit, however, got me thinking about the feasibility of constructing a small amplifier that I could place with it when I needed a little more power than 3-4 watts PEP.

The reviews of the ramsey amplifier kits are horrible, and I have scratched them off of my list.

I have thought about trying to construct an amplifier that utilizes the IRF520 Mosfets due to their low cost. Maybe a 2 FET amplifier to start out.

Anyone have any suggestions or a starting point for me to look into? Thanks.

 

[Shockwave]

Since the IRF520 works at 27 MHz. it would certainly work well on 14. I think you might have a problem locating a schematic using this transistor in either broadband push pull or tuned input and output for 14 MHz. I'm thinking you would have a much easier project building a push pull amp using a pair of common bipolar transistors for this application. There are several published designs for construction going back to at least the 1980 Amateur Radio Handbook.

 

[Captain Kilowatt]

Why not go single ended with a single 2SC2879 or SD1446. No I/O transformers to deal with then.Much simpler to build.

 

[Shockwave]

Only reason I didn't recommend that is because it's going to require him to make a input and output tuned circuit to match the transistor. Getting the value of these inductors and capacitors correct may be more complicated then just saying "wrap three turns on the input and output transformer". Then if he decides to build another one of those QRP transmitters for a second HF band, the amp will still work.

 

[Captain Kilowatt]

True but he would still likely need to add some harmonic filtering whereas with a single ended amp the tuned input and output would likely take care of that problem.

 

[Crusher]

If I remember correctly, the ARRL amp classic book has a section that utilizes an amplifier using the IRF510 mosfets. It also has input filtering and matching for output as well. You maybe able to use this or at least use it as a basis for the 520. Hope this helps.

 

[wcsd106]

Thank you for all of the replies. I have been doing a little reading this evening and have come up with a schematic and a pretty good write up about a single IRF-510 based amplifier with band switching.

5-band RF Power Amplifier by dxzone.com

Power output on 20m with 13.8v power supply & 2.5w input power seems to be around 15 or so watts. Not bad, but not exactly what I'm after.

Any of the amp gurus care to enlighten me on what would be needed to couple two mosfets together?

Thanks again for all the help I've been receiving here.

 

[dudmuck]

Such a beast is difficult to buy, because standard HF rigs put out 100watts "barefoot".
So the only market for such a kit is the QRP crowd, and they dont want to run much power or theyre not QRP anymore.

Probably the biggest issue with any amp kit is the greater chance of spurious emissions. Your average radio enthusiast doesnt have a spectrum analyzer on the bench. Its not difficult for a mistake or bad tuning to result in out-of-band spurious emissions, which cant truly be seen without the spectrum analyzer.

 

[Unit 148]

Here;s a selection of FET homebrew amps:

www

 

[dxhound]

HF Projects - HF Packer-Amp

Sounds like what they have to offer is what you want. Very nicely built with variable degree of difficulty. This should give you a good idea for how to build your own amps in the future.