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[AMPS] close ratings

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Subject: [AMPS] close ratings
From: (Tom Rauch)
Date: Tue, 17 Jul 2001 12:12:32 -0400
Hi John,

> The plate swing probably will be slightly less than the DC voltage, 
> for a reasonable efficiency amplifier. Otherwise it will be even less and
> the amp will be underloaded. So figure that the voltage goes from 0.2 to
> 1.8 x the DC or something like that, at the plate. The experts here
> probably know the correct figure. Then on the other side of the blocker,
> you only have the RF swing without the DC component. So the cap passes the
> RF through, and blocks the DC, right?

That is true only for a properly loaded amplifier. In a misloaded or 
mistuned PA,. or a greatly over-driven PA, the peak RF voltage can 
greatly exceed the supply voltage on each half of the RF cycle!

I can demonstrate, with an amplifier biased to the low edge of class 
AB, that peak voltages many times the supply rail can be 
produced. Switching supplies work on this principle.

If there is not a very fast load-fault circuit, the PA should have a 
spark gap of some form on the anode side of the tank. 

In any amplifier biased much less than class A, and unless I had a 
load-fault trip that would work faster than heat would damage the 
component that arced, I would include an intentional spark gap of 
some form.

73, Tom W8JI 

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