A Very High Power 50v LDMOS Amplifier for 1296MHz

I've been waiting for an LDMOS device to become available which might be easy to design for, could be driven with a 10w radio, and would deliver as much power as my LDF4 coax cable could handle. That seemed like quite an unrealistic dream, but in August my favorite supplier asked if I'd be interested in a new device from NXP, the PRF13750H, capable of more than 600w output at 1300Mhz. Naturally, I was delighted to hear the news, and once a sample device became available I began to experiment...and here are the results from the final prototype I built around this device:

I was told the PRF13750 is the "pre-production" device issued by NXP to it's distributors for initial sampling and sales; there were a limited amount of them made available, with full-scale production of the MRF13750 (note the prefix change) scheduled for late January or early February, though sometimes these dates tend to slip a bit. Pricing on the device is expected to be in the $330 to $360 range from most of the major distributors.

Here are two of the other particulars I measured on the prototype; this first one is gain, almost 20db:

This last one is input return loss, translating to roughly 1.25 to 1 SWR at center frequency:

Both gain and input return loss can be moved down in frequency by use of small pads located at the input and output of the PCB, which can be connected in or not, depending on which parts of the band you wish to optimize. For example, if your main interest is ATV, you may want your optimization in the lower part of the band instead of the upper as I have it here.

Here's a photo of the prototype; it's a bit on the rough-looking side because I made it here with humble tooling, as I do with all prototypes before investing in having a commercial board house make them in quantity. The board with the device installed will easily fit on a standard 3 x 5 copper spreader...the spreader in the photo is oversized, used only for experimenting. The amp itself needs to dissipate no more than the VHF KW amplifiers (400-500w), and the standard 3 x 5 spreader appears to be just right for the job.

Oh yes, this part can also produce 750w at 900MHz. Different PCB, etc. still to be prototyped when time permits.

So the project begins...I have on order an initial 50 sets of the PC boards shown on the left (not to scale), and plan to have fully assembled and tested RF decks available using the PRF13750 in late January (I purchased a modest quantity of the devices when they were still available). These will be mounted on a 3 x 5 copper spreader, ready to bolt down to your heat sink. Expected pricing for the RF decks is $850

Shortly thereafter in April-May, complete custom turn-key amplifiers (w/o power supply), the same physical size as the VHF KW amplifiers, will be available. These will have the usual safety features (SWR and overtemp protection); expected pricing is in the $3100-$3300 range (depending on options ordered).

By the beginning of February the MRF13750 should be in full production, and if there is sufficient interest I can have an additional run of boards made ahead of time, included in kits available for those wanting to build their own. The amplifier does use some rather expensive ATC capacitors, so the expected pricing for the kit without LDMOS or copper spreader is $150. Fully machined (for this rf deck) 3 x 5 copper spreaders will be available at the normal price ($100). Those wishing to machine their own can do so using this drawing; not on the drawing, but important, you will need to fly-cut the back surface of the copper spreader for flatness so it mates well with the heat sink you will be using (the surface of which should also be fly-cut for flatness).

That's all I can think of for now; if you have interest in either the RF deck, the kit, or the custom turn-key amplifier, please drop me an email, as there will be a limited number of these available until I'm able to determine whether or not to continue investing in the project.

best regards, Jim