After having my eyes opened by testing various buffer transistors I began substituting output transistors. Before testing currently available transistors I thought it useful to compare 6 alternatives in my junk box. These included fake 2SC2166 and 2SC1173 transistors, some genuine recovered CB finals, and a 30 year old BD139. All surprised me by appearing to work quite well. However, I wasn't driving them to get 1 Watt at this point.
Some of the testing results surprised me so I decided to give LTSpice a go to see if I could learn why I was surprised. That was a few days in my life I wont get back. However, using LTSpice made me accept that my spreadsheets may be in danger of becoming obsolete. The biggest challenge is being restricted to transistors for which a suitable model exists. But if you can find a spice model, LTSpice allows you to quickly rule out a device once everything is set up.
I have tested a number of devices now and I could not find a device that worked to my satisfaction. Issues noted were:
- the output dropping as the temperature rose, sometimes by 20dB!,
- distortion that required large increases in standing currents, and
- a roll-off in gain that ruled out use beyond 7MHz
I found that driving 50 ohms from a 13.8V appears to be pushing the envelope. If I increased the supply voltage, or changed bias resistors to increase the standing current, I could get 1Watt out. This came at the expense of the transistor dissipating 3 watts.
Conclusion:
From my measurements the KSC3503 is worth trying. If you settle for less than 1 watt you will be happy. At 7MHz and below the TTC004B is also worth considering.
However, from results to date, a working output transistor recovered from the junk box is still better
than trying to buy a general purpose transistor for RF output stage
applications.
Next Steps:
Investigate if parallel transistors will meet the 1 Watt target. (Update - yes they did and very nicely. See posts tagged "Parallel" for details.)
73's
Richard
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