A few days ago, W8DIZ made a post on qrp-l.org announcing a new addition to his RF Toolkits line. This one is a 5 watt CW amplifier using a push-pull 2SC5739 pair as the PA (my new favorite full QRP gallon transistor). I built my own version of the circuit using parts that I had on-hand. The only substitiutions that I had to make were the cores for T3 and T4 (the output transformers), but I have the correct cores on order from Diz. With an input signal level of 0 dBm (1 mW), I was able to get nearly 5 watts output into a 50 ohm dummy load from 160 to 30 meters. The power output started to droop above 20 meters, but I suspect that is because I used the wrong core types on the PA circuit. There is also a control to adjust the power output using a PIN diode bias control on the driver amp emitter. It seemed to work fairly well on some bands, but on others was not very well behaved. Once again, I’m going to wait for my correct cores to arrive before making any final judgements about this. The great thing about using a PIN diode design like this is that you are using a DC bias to control the gain, which means you can run a long cable to a panel-mounted potentiometer. This looks like a great amp and I look forward to getting it built completely correctly once the parts get here. Stay tuned for another update with detailed measurements once that happens.
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[…] I needed to build the W8DIZ 5 watt amp correctly. You can see my previous post on this amplifier here. In my last post, I noted that I was seeing some strangeness in the drive level circuitry. I found […]