Re: µBITX & QRP labs 50W PA

ajparent1/kb1gmx
 

Since I run an EBY amp since 2006 you must have mis-read.  In the files section
I've even posted his articles back around 2010 and copied to this iteration years
ago. Still there.

The ubitx is not run that low, spec is 100ma per. 

The irf510 like all fets has a considerable hook at low gate to source voltages.
If the gate voltage is platted against the drian current the linear window is above
100ma and below saturation or about 4A.  Below that in the low end under 100ma 
drain current he gate voltage is not linear at all from start of conduction to the 
100ma range and  that makes for very non linear near cross over.  Class AB1
is more like 80-150ma but it is very toplology dependent.

His (WA2EBY) minimum was 50 ma per device for CW and testing put it higher
near 100ma per device for SSB to get a decent IMD at 28V.  I find mine
comfortable at 125 ma per device and the two tone testing is good enough at
about -28dbc.  However that design has the features needed for linear operation
including independent device bias,  current sourcing transformer for the drains
and large ferrite for the output transformer.

At 100ma per you only talking a standing power of 2.8W at 28V per device,
if that gets warm the heatsink is inadequate. 

So saying a specific current, especially low under 100ma, is inadequate
for linear use without testing questionable.  Especially for an amp designed
for class C use.  I do not say it cant be linear but at low bias its highly unlikely.

I have one on the way so testing will happen.

Allison

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