A Very High Power 50v LDMOS Amplifier for 1296MHz

Update (20 March 2018); assembled/tested RF decks, basic kits, and complete turn-key amplifiers are now available for shipment. The latest technical update is shown at the end of this page. If you purchased an assembled/tested RF deck, here are some observations and the instructions for making the connections.

Many of us were waiting for an LDMOS device for 1296 which might be easy to design for, could be driven with a 10w radio, and would deliver high power. That seemed like quite an unrealistic dream, but in August (2017) 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 the testing and debugging began...and here are the results from the last prototype I built around this device:

The NXP data sheet lists it just about the same (shown below), and after building 30+ RF decks to date, I'm finding less than 0.3 db variance; many of them will produce up to 700w, some a bit less, but so far none less than 645w.

On the ones I'm producing, I'm setting IDQ at 1.7 amps for linear operation; I'm finding the IDQ setting affects input match, but 1.6 to 1.8 amps seems to be about right.


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 (2018). They were available from most of the major distributors in the $315 to $360 range as of that time; NXP recently lowered the cost to around $200.

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.

Technical update (14 February 2018):

The commercial PC boards arrived, and completed RF decks are in stock and available on the parts page; complete turn-key amplifiers can also be ordered at this time. Board kits with all necessary components (except for LDMOS) are also available.

The RF decks are playing consistently and well, some with saturated output around 700w, and P1db at 600w. Accurate measurements of efficiency at saturation indicate 58%, a bit better than the data sheet suggests.

RF Deck Information

After constructing 30+ RF decks so far, these are some of the observations I made:


Initial testing of the deck (see test setup to the right):

When mounting the deck, use all 8 of the supplied 4-40 machine screws and flat washers, or your metric equivalent (the washers will help distribute the mounting forces). Get them firm, but not so tight you damage the board material, which is soft and thin. Use heat sink compound between the copper spreader and the heat sink...not too thick, just a thin layer will do.

All assembled/tested RF decks have already been set up in the manner described below; no additional adjustments are necessary for operation at 1296 MHz.

The power supply used to set up the amplifier was current limited to 3 amps, and that's important when setting IDQ. If you are doing this yourself with any LDMOS device on any band, and it gets away from you, that current limiting will save the device. Fuses are not fast enough. The termination on the output is a 30w cellular type (no drive power for this test, of course).

While making the IDQ setting at 50V VDD, the input match is observed with a scalar analyzer and optimized at 1296 MHz (where most of us will be using this). The two parameters affecting the input match were IDQ and trimming pads. Usually, only one or two of the input trimming pads were needed with IDQ set between 1.6 and 1.8 amps. The inset below shows the measured optimization.

On the output side, most of the decks required just one or two of the output trimming pads to be connected.

Making the connections to the deck

This alternate method shows the coax pre-mounted to .020 tin mounting strips, and because I must install coax when I set up the decks, and then remove it, it is the method I use most of the time.

Cut your tin strips to .375 by 1.375 (3/8 by 1 3/8). Mark and drill the hole locations for pass-through (for the screws) by comparing the strip to the board hole pattern.

Prepare your coax and solder the shield to the tin strip, aligned as shown in the photo. Now you install your coax using the two mounting screws at either the input or output, whichever you're working on. Once the assembly is fastened in place, you can solder down the center conductor.




The next page contains an important warning about the presence of flux residue around the output connection



The output pad has considerable microwave power on it when the amplifier operates. It is absolutely essential to keep this area clean. The area shown in this photo is the output pad and its trimmer pads and is the only area on the boards requiring this special care.

Smoke may be the first warning as flux residue heats up, followed by small sparks and then by a larger arc. Stopping the transmission immediately at the first hint of trouble will prevent damage to the board, giving you time to clean the area with isopropyl alcohol. Flux trapped under the trimming pad jumpers has been reported to cause this kind of problem, so use just a small amount of the no-clean type flux and clean it up afterwards.

This also applies when you solder the output coax to this area; make certain there is no flux in the area between the output pad and the ground plane, or around any of the trimming pads.

Installing the PCB clamp on the RF deck

Some digital EME operators report that repetitive full power heating and cooling (1 minute on, 1 minute off) of the rf deck in these modes will eventually cause the thin PC board material to lift away from the heat spreader just enough to affect the match. This is noticed when the gain and output power drop down significantly from their usual levels, sometimes by as much as a third. Clamping the boards securely to the heat spreader solves this problem and restores normal function to those units affected.

Extra mounting screws could not be used for clamping due to the width and positioning of the PCB traces, so a Teflon spacer clamping system was developed. The sheet metal parts of the clamp system are made from semi-rigid aluminum sheet and will self-adjust to provide the correct pressure during heating/cooling cycles. The installation procedure is located here: