Here are some test results I did with the 2SD882 (from Tayda), and the 2SC5706 (from kitsandparts.com) on 40M. I used 12.8 V supply, and the Norcal 40A buffer, driver and power amp circuit with a 40M LPF with two harmonic trap capacitors. The Rutledge book "Electronics of Radio" uses these circuits for several Problems for the student--he drives the circuit with a function generator so that's how I did mine--that works very well plus you can record the input level for different output wattages, and compare it with other power amp transistors. I used a J309 for the buffer and a 2N3904 for the driver; I added an extra 2N3906 keying transistor, same as used on my earlier RF Amp Test Fixture and not the Norcal keying circuit. The Norcal 40A output drives 50 ohms directly and is suited for 0-3 watts output. I also tested for 4 and 5 watts output, changing the 18 uH RF choke on the output collector to the usual 1:4 step up transformer. I've attached the schematic for all this.
The bottom line is both transistors work very well as single-transistor power amplifiers at 7 MHz, both giving up to and including 5 watts output from the 12.8 VDC supply. No heat sinks were used; very briefly keying the circuit allows for this. Both transistors, being quite small with small heat sink area, immediately begin to heat at key-down and power immediately begins to drop, so I have to catch the initial peak voltage across the 50 ohm dummy load on the Siglent oscope. The Rutledge book shows how to measure power amp transistor gain, converting the base RF voltage to dBm and subtracting that from the output wattage/dBm. I checked at 2 watts, 3 watts, then changed to the output transformer and checked at 4 watts and 5 watts--doing this for both transistors. I was surprised that the 2SD882 would give 5 watts output, at least on 40M; and gain was still holding up at 5 watts for both transistors. Note the phasing dots on the driver transformer--this is important (of course also shown in the official Norcal 40A schematic, and in the Rutledge book--I just followed the schematic). I experimented with reversing the secondary phase, monitoring the output power set to 2 watts with the proper phasing, and the output decreased by 0.1 watts when the phasing was reversed. NA5N's "Data Book for Homebrewers and QRPers" also mentions this, see section around Fig. 4-7.
Chuck Adams has already mentioned to the list about the 2SC5706 being used in Diz's 5 watt transceiver and separate 5 watt amplifier, both use balanced pushpull output for the 5706's and cover up to 30 MHz. Of course proper heat sinks are used.
73,
Steve AA7U
