On Thu, 4 Sep 2003 20:25:01 -0400, "Tarmo Tammaru"
wrote:

Richard,

Do yourself a favor and try to get a copy of Motorola ap note AN762. It
describes several amplifiers, including a version with the MRF421. It is
clearly stated that the MRF has an output rating of 100W, PEP OR CW. Also,
the amplifier has negative feedback.

Tam/WB2TT


Hi Atm,

"Continuous collector current could go as high as 21.3 A at
13.6 V operated into any load."

By Ohm's law, and variations, P = 289.68W and with a collector
efficiency 55% (however, only spec'd at 180W) results in 159W in RF
products (not all within band) over a full cycle. This spec is for
the tandem configuration common to most finals' decks found in amateur
equipment, not a single transistor and not at 12V. Further, the
current is shared by each transistor through alternation as the two
are in series feed to the primary of T3. At the drive levels offered,
the transistors each offer ballpark 1.2 Ohms T3 is specified at 1:5
(again, just as I described in past messages) and offers a 25:1
impedance transform would at a first pass evaluation offers exactly
what I said it would.

Motorola (in their confusion) offers:
"For example, in the 180 Watt version the input
transformer is of 16:1 impedance ratio, making
the secondary impedance 3.13 Ohm with a 50 Ohm
interface."
...
"It should be noted that in the lower power versions
[common to the experience and quality of gear found
in amateur application - rwc] the input and output
impedances are higher...."

At the end of AN762 they offer a design for low pass filtering (to
remove some of those out of band RF products) which specifically
includes the source specification of (-gasp!-) 50 Ohms, and a load of
50 Ohms. Of course, this all occurred prior to some accounts of the
great Motorola confusion of the early 90's that rendered all such
advice - um, well, who knows?

As for the negative feedback. Again, this is exactly what I said it
was "The Input Frequency Correction Network." This hardly qualifies
in the classic Bode sense of Z stabilization so commonly found in AF
amplifiers.

Now I did you a favor by reading it to you.

73's
Richard Clark, KB7QHC