The test results for the 2SD882 showed it had the flattest response but at 160mW it appeared to reach a thermal limit that saw the output drop to near zero. After cooling off it appeared to work normally again.
A LTSpice model suggested this transistor should have been capable of 1W, and with a much better heatsink and increased standing current perhaps 1.5W. However, achieving this would require a smaller emitter resistor with the result that Zin falls to 38Ω at 3.6MHz and 25Ω at 7MHz.
Using two 2SD882's in parallel has some drawbacks:
- The gain is lower, so they have to be driven with a higher input
- The input impedance is lower still (30Ω @ 3.6MHZ and 18Ω @ 7MHz)
Looking at a few transistors with LTSpice returned the following ballpark numbers when operated in parallel:
Zin at
Transistor 3.6MHZ 7MHz
2SD882 30 18
MJD44H11 36 24
2SC5824 42 33
SOT89 style package2SD965 43 41
2SD1007 42 35
2SC5964 45 38
Conclusion
General purpose through hole transistors don't appear to be a good solution when operated in parallel. They don't have the frequency response of a true RF transistor and the input impedance is lower than desired for a 50Ω module approach.
Some of the SOT89 style transistors appear promising and if the heatsinking can be resolved may be worth pursuing.
73's
Richard
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