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Re: [Amps] SS Amp questions

To: amps@contesting.com
Subject: Re: [Amps] SS Amp questions
From: Steve Thompson <g8gsq72@gmail.com>
Date: Mon, 10 Sep 2012 09:41:05 +0100
List-post: <amps@contesting.com">mailto:amps@contesting.com>
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
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