G.P. Transistors in RF Amplifiers - Parallel Ouput Transistors Preliminary

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 package 

     2SD965              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