Re: µBITX & QRP labs 50W PA

ajparent1/kb1gmx
 

Mark,

For SSB high idle current are not a factor (unless unrealistically high) as
the average current is likely going to be higher and if heat is an issue
than larger heatsinks or fans are the solution.

Keep in mind speculation is not measurements.  For 50W out 
your input DC at 20V has to be in the range of 70 to 90W
assuming 70 to 50% efficiency.  The difference between output
power and input power is resulting in heat.  Power amplifiers 
above the milliwatt range are all concerned with heat and
managing it.

I have used the same heatsinks for the QRPLabs 10W SSB amp 
at higher voltages and power there is a lot of heatsink there.  Running
bias in the 100Ma per device range is fairly trivial at 4W.  Even at 250ma
we are at under 10W.    Considering at 50W out the likely current  is
3.6A at 20V and converting 30% of that to heat for classC and much
Less (typical is 55%, average) for class AB1.

For SSB running to 50W peak the average is likely closer to 20W
and the heat load is maybe near 20W average as well assuming
50% efficient and that would be low.  So heat should not be an issue
with supplied heatsinks and reasonable airflow (no sealed chassis)
and not over driven.

It should be noted that for SSB the peak power is around 50W the
average will be lower and should be to avoid splatter from clipping.
That means the SSB drive power should be lower than CW.

Allison
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