Dana wrote:
Couldn't the TO-92 part (2N3904) be soldered down instead?
* * *
John speaks of high dissipation, which is a good point.
The first- and second-stage NPN amplifiers in the TNS-BUF dissipate
about 225mW each, and the output stage NPN dissipates about 425mW.
The TO-92 part is rated to dissipate from 500 to 625mW at 25C ambient,
and from 175 to 250mW at 50C ambient (in both cases, depending on which
manufacturer's datasheet you consult). With around one watt being
dissipated inside, 50C is probably a low estimate of the worst-case
ambient temperature inside an enclosed TNS-BUF. My personal design
limit would be a dissipation of no more than 150mW for each TO-92 (less
if h/w tests showed internal ambient temperature > 50C).
Even the TO-223 part (PZT3904) in the output stage exceeds its
comfortable dissipation limit unless it is mounted on a substantially
larger foil pad than the 2.5 sq inches specified in the ratings.
Best regards,
Charles
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