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| 20 RD15HVF1 HF Power Amplifier |
Between Field Day and other volunteer activities this past weekend I managed to complete a build of a 20 watt RD15 amplifier for the hombrew sBitx transceiver. I spent the last week or two studying these amps and looking at various implementations. I had previously built EI9GQ, Eamon Skeleton's amp from his book "Building A Transceiver which I documented earlier. That build was copied directly from his schematic and I didn't give much thought to the design.
Push-pull MOSFET RF amplifiers pretty much all share a common architecture. The transitors are configured as common source amplifiers and biased for class AB. The differences are in how the input an output transformers are configured and how the bias circiut is designed. Compared the the EI9GQ amplifier I simplified the input and output transformers and tripled the value of the feedback resistor to get more gain. I also learned that adding a cap across the drain side of the output transformer will flattend the gain curve and increase gain at higher frequencies. Using cut and try I ended up at 330 pF which results in flat gain from 3 MHz to 21MHz and about 3db more gain at 28 MHz. This was a very satisfying build experience. Essentially everything worked as expected almost right off.
73 from Great Falls
Dean
KK4DAS
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