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Author Topic: Linear power amplifier with 10 W output for frequencies between 100 kHz and 2000  (Read 9904 times)

Offline Pieritz

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Hallo! I want to build an AM-transmitter with a power output of approximately 10 watt for the frequency range between 100 kHz and 2000 kHz. The transmitter should be a solid-state type with an operating voltage of 12 volts and it should, if possible only use parts available in electronic stores and not too many coils to be manufactured by my own.

I want to use a linear amplifier as I want to use as oscillator and modulator a "Spitfire AM transmitter" ( http://www.vcomp.co.uk/spitfire/spitfire.htm ), as this device can be easily tuned digitally to the desired frequency and has a very excellent AM modulator, but just an output of 100 mW.

Has someone switching diagrams of a radio frequency power amplifier, which would fullfill my ideas? If yes, please post them or show my links to them. If MOSFETs and other types of FETS are required, how sensitive are these types to statics? Is it still so, that the danger of destroying a FET by soldering is so big, as one can read in most books from the 1980ies, or are there protection devices reducing the risk at modern FETs?
Can I build the desired amplifier using integrated circuits for power audio frequency amplification in a modified way? If yes, with which ICs it would work?

Offline ff

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Welcome to the board Pieritz!  I don't know of any small linear circuits for MW although I'm sure many exist.  You can Google for those.  You will be looking for a linear amp that has a gain of about 13-14 dB.  I have built something similar for HF that uses two IRF510s in push-pull, but that design uses several transformers wound on ferrites.  Just about any MOSFET will give you usable power on MW.  The IRF series performs well at low frequencies.  These units have been designed for automotive and industrial uses and are quite robust - no special handling required.  For simplicity, a single-ended amplifier would probably be best for you.  And remember, MOSFETs can be connected in parallel for more power handling.  Although this increases the capacitance problems, on MW, the frequencies are low enough where that is rarely a concern.  One last thing - when using a linear amplifier you really should always use a low pass filter at the output.  It will help to keep your operation cleaner.  And cleaner is better, and more discrete.  Good luck with your project... 73!
Hailing from the upstate boondocks region of the progressive paradise which once was New York State

Offline redhat

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There really shouldn't be much to this.  Most drive-in transmitters and the like used an LM1496 modulator followed by a push pull linear amplifier.  As FF said, use a pair of push-pull irf510's or similar running from either a 12 or 24 volt supply.  Use material 77 on the toroids so they will work in MW.  Probably T140-77 for the output wound tri-filiar, with two of the windings joined to form a center tapped winding for the primary.  Circuits like this should be easy to find on the web.

Something like this http://www.sm0vpo.com/tx/15w-pa.htm

+-RH
Somewhere under the stars...
WinRadio Excalibur/305 w/ a chi-town resonant loop, Kenwood KDC-U356 for mobile listening.
Please send QSL's and reception reports to xfmshortwave [at] gmail [d0t] com

Offline Pieritz

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Would the circuit shown on http://www.qsl.net/df3lp/projects/lftx/tx50.gif at http://www.qsl.net/df3lp/projects/lftx/ would work? Ifnot, which modifications are required?

Offline ff

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Would the circuit shown on http://www.qsl.net/df3lp/projects/lftx/tx50.gif at http://www.qsl.net/df3lp/projects/lftx/ would work? Ifnot, which modifications are required?

No.  If you need to ask, then you probably lack the expertise to get it working anyway.  My advice is to stick with tried and true designs.  I'm sure you'll have plenty of "fun" getting an amp working, even with no mods.  Redhat gave you a great link to a workable amp on Harry Lythall's site.  I suggest you give that one a shot.  And please use a low pass filter at the output.   There's a good calculator for one here:

http://www.calculatoredge.com/electronics/ch%20pi%20low%20pass.htm

Good luck with your project...

Hailing from the upstate boondocks region of the progressive paradise which once was New York State

Offline redhat

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Also keep in mind when considering amplifier ratings for amplitude modulation, a ten watt amplifier means ten watts peak, not carrier level.  For 100% modulation, the peak power is four times the carrier level, ie 2.5 watts carrier is 10 watts peak at 100% modulation.  Above ten or so watts carrier, it becomes much more advantageous to do high level modulation verses linear amplification.   A 30 watt linear am transmitter produces an awful lot of heat, while its high level cousin produces almost none.  This is ever more important for battery operation.

+-RH
Somewhere under the stars...
WinRadio Excalibur/305 w/ a chi-town resonant loop, Kenwood KDC-U356 for mobile listening.
Please send QSL's and reception reports to xfmshortwave [at] gmail [d0t] com

Offline ff

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Redhat makes an excellent point Pieritz.  If your Spitfire AM output is rated as 100mW PEP and the linear has a 13-14 dB gain figure, the exciter will drive it to about 2 watts PEP output.  If the exciter is rated as 100 mW carrier output, its PEP will be 400 mW, if fully modulated.  This will drive a linear with a 13-14 dB gain figure to about 8-10 watts PEP.  The overall point is to ensure that your linear amp will safely develop the power that its being driven to.  If not, POP goes the MOSFETs.  Of course, even before that point is reached, you will be scraping off more and more of your modulation envelope, the more you overdrive the linear.

Also what Redhat said about producing heat and power wastage is absolutely true.  You will have to decide whether a linear is really the way you want to go.  Although I have dedicated rigs for 43M and 48M, I use linears a lot to get a pirate signal up on the higher bands.  I find it easier to build and tweak many band-specific exciters and low pass filters (and dipoles), and bring the power level up with a broadband linear.  I can easily run a couple hours from a 90aH Marine Battery.  It works for me.  Only you can know if a linear is the right way for you to go.
« Last Edit: July 25, 2014, 1238 UTC by ff »
Hailing from the upstate boondocks region of the progressive paradise which once was New York State

Offline Pigmeat

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There is a guy on Ebay whose handle there is pbsn6040. His company is called BootsParts. (I think?) He uses clock oscillators for his transmitters and sells the audio transformers to feed them. He's also got diy linear amps for them. The best thing about his stuff is it's dirt cheap.

You could easily buy a clock oscillator that's resonant at say, 1710 khz., and amplify the thing to a couple or three watts.

The big problem with MW is the antenna. It's tough to build or buy one that's efficient down there and meets FCC Part-15 regulations. It doesn't mean it can't be done, but it's a PIA.

Offline ff

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I'll have to check out pbsn6040, Pigmeat.  There are a few ebay gems out there amongst the ripoffs.  I buy a lot of my RF transistors from "weezle66" over in the UK.  He is a distributor for Eleflow, a company that manufactures legacy RF semis that exceed the original specs.   I'm always looking for other reliable parts suppliers.  Thanks for the tip!
Hailing from the upstate boondocks region of the progressive paradise which once was New York State

Offline OZNRH

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Hi Pieritz  :).

Here is another option from Germany. Works very fine linear (2.5W AM = 10W PEP). It is kit you have to assemble, I use even 3 of them in 3 different designs.

http://www.qrpproject.de/UK/qrppa2008.html

Remember.. You most have 40W PEP to make 10W AM  ;).
Maybe this one:

http://www.rfsource.gr/pm62fdc-am-pallet.html

One thing more.. You must change your Spitfire output to 50 Ohm..

Best 73 from Ole (Denmark)  :)..
« Last Edit: July 26, 2014, 0815 UTC by OZNRH »
The romantic on MW and SW is still alive.
Take a look on my webside: http://radiooznrh.webnode.com/

Offline Pigmeat

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Don't sweat it FF.

Ole, that's some nice stuff you posted. The beginner stuff is priced right and the plug and play reasonable.

BTW, I'd wondered where the suppliers for Elecraft disappeared to. Thanks for the info.

Offline ff

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BTW, I'd wondered where the suppliers for Elecraft disappeared to. Thanks for the info.

A correction on the Eleflow guy: "weazle66"... my bad...
Hailing from the upstate boondocks region of the progressive paradise which once was New York State

Offline OZNRH

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A good station setup it's not all, you must also have a good antenna. Look on this site:
http://www.k7mem.com/Electronic_Notebook/antennas/shortant.html

You can learn more about my setup here and look in the 'PhotoGallery':
http://radiooznrh.webnode.com/

Best 73 Ole :)..
The romantic on MW and SW is still alive.
Take a look on my webside: http://radiooznrh.webnode.com/

Offline ff

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Hi Ole!  Really nice website with lots of interesting info and pix ("Polka to you all" - LOL!)  Congrats on getting the MW antenna repaired... 73!
Hailing from the upstate boondocks region of the progressive paradise which once was New York State

Offline Pieritz

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@OZNRH: How sensitive are the MOS-FETs used in the kits described at http://www.qrpproject.de/UK/qrppa2008.html and http://www.rfsource.gr/pm62fdc-am-pallet.html?
Which impedance has the output of the spitfire AM transmitter?
Can I realize the impedance matching well with standard inductvities ( I mean that inductivities, which look in size and design normal resistors) or should better coils be used manufactured by oneself?