Mike W4EF, well done on the analysis of the W7IUV preamp and very informative.
I also calculated nearly the same numbers indicating a need for more heat sink
or less current etc. Also do not forget that most 12 volt supplies are 13.8
volts or sometimes even more. Using 13.8 volts takes the power to 1.7 watts and
the calculated junction temperature to over 300 deg. The bottom line here is
that this amplifier is too close to a thermal runaway situation than most
designers would allow in commercial use. The solution is fairly simple and that
is to increase the size of the 10 ohm emitter resistor to 27 or 30 ohms while
leaving the 4.7 ohm resistor as is. This will reduce the quiescent current in
the device to around 60 ma. and as a result the power level. With a medium
sized heat sink and lower current the result should be just a warm preamp with
reasonable performance in most situations.
Lee K7TJR Oregon
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