600 Watt HF Amplifier

There’s an old axiom in amateur radio which states “Life’s too short for QRP.” While I generally disagree, it certainly appears to be true for 75m AM operation here in central North America, where a one watt carrier can quickly become buried in QRN & QRM. This amplifier is a mate for the QRP 75m AM Transceiver described elsewhere at this site.

The amplifier uses four Motorola MRF150 50 volt TMOS power FETs configured in push-pull/parallel and biased for class AB linear operation. This scheme yields a power gain of 20db. The 1 watt drive from the “Wheat Box” will produce a 100 watt carrier (400 watts PEP @100% modulation) with headroom to spare.

There’s little original design here. The basic 2–30MHz PA brick is straight from Motorola’s application note EB104. The power supply, low-pass output filter, control circuitry and instrumentation are mine. Still, I would describe this project as mostly an excercise in packaging.

Some amplifier highlights:


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