[Amps] Freescale and NXP VHF LDMOS Fets

[John Lyles]

The newest LDMOS high power transistors are being grabbed up by 
commercial amplifier production, for HDTV (band II VHF and into UHF) and 
FM broadcast amplifiers and for scientific. I didn't mention military, 
but I'm sure its there too. For instance, Harris broadcast has been 
using the Freescale parts, and had done a lot of development testing for 
them. Engineers at Argonne National Laboratory have been developing very 
high power solid state units, over 100 kW, using them at 350 MHz. Again, 
lots of interaction with the manufacturer to work out the bugs. 
Overseas, I have heard from reputable designers who like the NXP 'XR' 
better, one story suggesting that the Freescale parts were originally 
too nervous and would oscillate at high frequency (UHF/Lband) easily.

In December we took delivery of a first piece of a 5.5 kW pulsed amp at 
200 MHz, to use as a predriver. They will eventually replace our six 
1995-vintage units used a box full of MRF151G-like devices at 24 VDC, 
with linear power supply, 3 phase. These are obsolete amplifiers now, as 
the mfr is out of the biz and no longer supports parts. So the new 
Communications Power Corporation amp uses (I think) the Freescale LDMOS, 
and only needs 8-12 devices compared to 64 of the older DMOS devices at 
lower voltage. We are just about to install it to drive the first tetrode.

These new LDMOS transistors are rather costly, comparable with a 3-500Z 
for power in a device, more fragile but higher Gain x Bandwidth.

For HF and below, of course, plastic switching MOSFETs are still the 
least expensive way to get high power, and can be run above 50 V. You 
can see many results on the Class E amplifier Forum online, where a 
number of hams are building kW-level modules for AM transmitters with 
PDM modulator. Neat stuff!
73
John
K5PRO




> ORIGINAL MESSAGE:
> On Fri, 22 Feb 2013 14:51:36 +0000, Mike wrote:
>
>>
>> Now, this would be interesting but at the same time I'm swapping all my
>> amplifiers out from tubes to BLF578 or MRFE6VP61K25H that need the PSU
>>from (1) above...
>
> REPLY:
> Ok, how about some design info on the above?
>
> A few months ago there was a post about the BLF578XR transistor from NXR.
> Very high power and very resistant to damage. Haven't heard anything since.
> Is anyone doing anything with this?
>
> There are several YouTube videos on this. Just do a search for BLF578XR.
>
> Bill, W6WRT