Richard,

Let me be the lurker on for the day. Actually, you have a good example. I
see the parallel equivalent resistance for the device at 1.5 MHz is about 2
Ohms. At 12.5V with a Vcesat of an optimistic .5V, the power that can be
delivered at conjugate match is 12^2/(2 * 2) = 36W. Not the rated 100.

The 421 is a fairly inefficient transistor. The 50W MRF450 has a *series*
equivalent output impedance at 30 MHz of 174 - j.5. The parallel resistance
will be about 175. Matched power will be 12^2/(2 * 175) = 0.4W. There is
nothing wrong with that.

One of their ap notes states that on newer devices they specify the
conjugate of the desired load, rather than the intrinsic output impedance.
On some devices you simply can't tell which they mean. On the very popular
MRF150, it is footnoted as being "conjugate of optimum load". It may be that
the FET resistance is too high to be meaningful.

Tam/WB2TT
"Richard Clark" wrote in message
...
On Mon, 1 Sep 2003 10:19:39 +0100, "Ian White, G3SEK"
The MRF421 is used in two of my HF rigs and you have yet to offer what
you have. You didn't look did you? Have you ever looked? Have you
ever repaired a Finals' deck? Ever design one that is comparable?

From that specification sheet(s):
Figure 7 - Series Equivalent Impedance, Zin
shown with both Smith Chart and tabular results over the range of 2
MHz to 30 MHz:
Freq. Zin
30 0.7 - j.5
15 1.39 - j1.1
7.5 2.8 - j1.9
2.0 5.35 - j2.2