Tetrode power limit
BF982, 981 and 998 are limited to 225mW. 908, 988, 960 and 961 to 200mW. Let's
take the 12V can be imprecise by 10% to 13.2V. The maximum energy transfer occurs
when the source resistance is the load resistance, that means both the transistor
and the resistor will have half of the voltage 6.6V. 200mW@6.6V corresponds to
217.6 Ohm and 225mW@6.6V corresponds to 193.6 Ohm. Therefore we take
220 Ohm.
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