[Amps] SS Amp questions

[Steve Thompson]

Manfred has said most of what I was going to say. I'd add a couple 
more comments.

RF power fets don't have quite the uniformity of characteristics 
that switching fets have. In particular, threshold voltage can 
vary, but if you have separate bias adjustment for each transistor 
then transistors with the same batch code can normally be mixed 
without any problems.

If you're seeing a big imbalance between the boards it's worth 
characterising the amplifier sections in isolation and checking 
that the splitter and combiner are working properly. When you come 
to drive it all up, you need to stop when the highest gain part 
starts saturating (or hits dissipation or temperature limit) which 
might mean you can't get max. possible output.

As Manfred said, heat is the most common problem. At 1kW out you 
could easily have 1kW of heat to shift. As well as overall 
heatsink function, the mounting of each transistor is important.

Steve