Hello,
Yeah well guess what? I *know* that. Apparently *you* are the one that doesnt know that
In a regular 4 or 5 watt amplifier circuit this will work. There's no reason to think he's going to use a circuit which isnt designed right in the first place. If the circuit isnt designed right it is even possible that a 1000 watt transistor wont work in a 1 watt application.
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My most humble apologies MrAl. I can see now that I made a mistake in reading your statement "So yes it should work up to 5 watts." as being in response to the OP's question "*May I use it on 5 watt FM RF Amplifier?" Reading it again, I see that you were not answering his question at all, but just saying that the 2SC1971 can dissipate 5 watts. (with a proper heatsink, of course)
At the time I wrote my first response, there was no reference circuit posted. I did not know if the OP was building from someone elses circuit, designing a new amp, or trying to find a substitute for another transistor. And even now that he has posted a circuit, I have no way of knowing if it was, as you say "designed right in the first place" The internet is full of poorly designed circuits. Even the OP was skeptical, or he wouldn't have asked the question in the first place.