ORIGINAL MESSAGE:
On Sun, 30 Aug 2009 21:11:54 -0500, "Gary Schafer" <garyschafer@comcast.net>
wrote:
>
>But the input power of the amplifier is NOT linear with respect to drive
>power or output power. As an example at maximum power output say it has 60%
>efficiency. For 1000 watts out it would have to run 1666 watts input. At 250
>watts out the efficiency will be 1/2 that or 30%. So to get 250 watts out it
>would take 833 watts input. We have dropped the input power in half but the
>output power has dropped to 1/4. We have not touched any tuning controls. We
>have only reduced drive to 1/4 of what it took to produce 1000 watts output.
REPLY:
Gary, your analysis of tank circuits in the rest of your post is very well done,
but I take issue with the above paragraph. You are confusing gain linearity with
efficiency linearity. The two are not related. As long as the gain is linear,
the efficiency can vary all over the place and the amp will work fine. It's just
how tubes work.
A practical example: Watch your plate current as and power output as you slowly
reduce the drive from full power to minimum. You will see the power falls off
faster than the plate current does, just what you would expect. The lower drive
level is less efficient. Less efficient means more DC current is required to
produce the same RF output. No surprise there, just exactly as predicted.
In fact look at the extreme example, a class A amplifier. The DC plate current
does not vary at all from max power to minimum, yet the efficiency varies from
its high point at full power to zero efficiency at minimum power. Again, it is
the gain linearity that is important, not efficiency linearity.
73, Bill W6WRT
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