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[Amps] Solid state amp

To: "" <>
Subject: [Amps] Solid state amp
From: "Georgens, Tom" <>
Date: Tue, 31 Dec 2013 20:55:24 +0000
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I am working on a solid state amp project.  After a few false starts, and 
damaged parts, I am trying a new approach and not pushing the parts anywhere 
their limits.

I have a 300W boards purchased online from rfsource.  They accept either BLF278 
or MRF151G parts.  The boards arrived (from Greece) and appear well made.

A BLF transistor was installed on the first board with a 48V supply and Idq of 
1A.  The board would put out about 100W and then the drive power needed to push 
it further would rise dramatically.   The current would not rise with the extra 
input power and power out divided by DC power in looked fine.  I built a second 
board with another BLF278 and it was modestly worse.  The board has a 3db pad 
in front of the amp and I could feel the resistors getting hot so I suspected 
the input matching as it simply uses a 9:1 transformer.  After a lot of 
experimenting with input matching yesterday, I could get the first board to 
look good.  Efficiency was typically about 50% at 150W and got better as 
frequency increased from 1.8  to 28Mhz.  Output power and gain looked OK.  
However, the same approach did not work on the other BLF278 board.

There were two MRF151G's left from a prior experiment and I put them on two 
other boards.  They performed noticeably different than the BLF parts, but 
similar to each other.  At 150W, the gain was over 20db (including the pad) but 
the efficiency was very poor.  It was just under 50% on 160, but dropped 
steadily as frequency rose to about 20% on 10.   I presume this will get worse 
when I have real antennas and not a dummy load.

Overall, the BLF efficiency makes for a part that cools nicely on the head 
spreader and heatsink, but I can't get a repeatable design.  The MRF parts seem 
to amplify fine but will be hard to cool and will require much more DC current. 
 Given the lower than rated power,  I was expecting some issues with the output 
matching, so the MRF symptoms seem more logical, but I have not made adequate 
progress on either

Other notes, 48V supply is solid under load, the parts are not getting too hot, 
the load is close to 50 ohms and I can monitor input power, output power and 
current simultaneously.  The transistors were purchased from China on eBay, but 
that is another thread.

Any suggestions appreciated.

73, and Happy New Year

Tom W2SC

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