Topband: W7iuv preamp

[Michael Tope]

----- Original Message ----- 
From: <ron21246 at msn.com>
To: <Topband at contesting.com>
Sent: Friday, February 16, 2007 12:55 PM
Subject: Topband: W7iuv preamp


> I've built the amp and to say that the 2n5109 runs "warm" isn't enough, it 
> runs HOT! I now am running 8.5 volts, not 12. No problems with AM 
> stations. I have a heatsink but still too hot at 12 volts. Has anyone else 
> had heat problems with this preamp. I am changing the bias to try and fix 
> it. Thanks ...Ron

Ron,

The maximum semiconductor junction temperature on this
device appears to be around 200 degC. I had a hard time
finding the thermal resistance data, but from what I was
able to surmise, it appears to be around 50 to 60 deg
C/Watt (junction to TO-39 case) and 175 degC (junction
to air). For the values given in Larry's circuit diagram:

http://w7iuv.com/ (see rotatable flag link for pre-amp
schematic)

the collector current should be around 120mA with a bias
voltage of +12V. This corresponds to a device dissipation
of ~1.2 watts. Therefore you can't run this device at room
temperature without a heatshink (1.2 watts x 175 degC/watts
+ 25 degC = ~215 deg C). I recommend that you install
a heat sink big enough so that the case of the TO-39 can runs
no hotter than 60 deg C (140 deg F). That will keep the device
junction temperature down around 130 deg C or better where
the device could easily last longer than you will -:)

BTW, cooler is going be slightly better for noise figure, gain,
and linearity and since you are presumeably not making these
things en maase to sell commercially where you might care
about shipping weight, I suggest you err on the cool side
with a large heat sink (<< 60 degC  TO-39 can temperature).

73, Mike W4EF...............................