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Possible new project w/ LDMOS transistor

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Author Topic: Possible new project w/ LDMOS transistor  (Read 646 times)
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« on: September 20, 2016, 09:26:22 PM »

Anyone here build an AM amp using one or two of these?

I was amazed that such a MOSFET existed! Might be fun.
Also, less noise, smaller size, less heat than the ceramic tube home-brew amp I normally use for 75M.
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« Reply #1 on: September 21, 2016, 07:55:49 AM »


That's an interesting find. Wondering what would be needed to drive it.

Brad K4RT

Brad K4RT

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Posts: 152

« Reply #2 on: September 21, 2016, 08:30:11 AM »

Frank that's a very good question, one I have also considered.  
These FETs and the BLF188XR which are almost the same make wonderful 50V linear amps. I have built two of them with BLF188XRs.  If driven with a sign wave from an exciter either SSB or AM modulated they work well at a carrier level of about 250W with mod peaks up to1 KW or so but being class AB1 linear the efficiency will be 65% at best so long AM transmissions make some serious heat. For the high bands 14 mhz and up its certainly a workable solution. One could also use a linear amp deck biased class C and excited with a CW carrier and modulate the Vdd supply and get slightly better results.
To run these FETs in class D or E high efficiency switch mode presents some interesting challenges. A major hurdle is the Drain to Source breakdown voltage of 135V.  This would require limiting Vdd to about 20V at carrier so that the drain pulses under full modulation never exceed 135V.  This requires a very low impedance high current design which would provide relatively low efficiency at high cost with little headroom.  There are many other design challenges also with this approach. So for 14 Mhz and below six 900V or 1200V 10A $5 FETs will do a much better job for $30 vice $300 plus.  Just my thoughts.
73s  Nigel

A "Tube Free" zone.
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