[hpsdr] FET amplifier design

[FRANCIS CARCIA]

Hi All,
I spent many years building receiver circuits and also loved Ulrich's articles over the years. 
I have been on an IMD quest and find most information out there a copy of poor designs.
I have a little simulation going and quickly found. 1. The coupling betweeen drain phases must be tight better than 98%. 2. Same thing in the DC shunt inductor. 3. Take that little core used in most shunt inductor designs and throw it in the trash. I got a good IMD change by replacing it with a T106-2 with 10 turns bifilar (50volts). Iron core takes a DC offset better. 4. TLT transformers  are the best way to go if you want to  cover 160 through 6M. 
Transformer core selection is key. Check ON9CVD's site on transformer design. Bob sure set me straight.
Hint, IMD degrades long before the core gets warm.
I have my modified 300 watt MRI strip doing -30dB IP3 at 200W PEP. When I get it where I want it to be I will
do work on my 1200 watt SD2932 boards. I built a heat exchanger that will hold 4-1200 watt boards that will be run at reduced power. Be careful with FET specs. Most degrade IMD above 2/3 power rating due to gain roll off. That new Feeescale 1200 watt part doesn't have great IMD above 600 watts. Freescale sent me a linear amp APP that showed it. 
Frank WA1GFZ
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