Steve,
It would also be interesting to get Manfred's views on the Hardrock 50:
https://docs.google.com/viewer?a=v&pid=sites&srcid=ZGVmYXVsdGRvbWFpbnxoYXJkcm9jazUwYmV0YXxneDoxOGMwZTJiMDIzYjZmOWE
Oh boy, I get no rest! :-)
This one uses four RD16HHF1's at 12 volts in a parallel/push-pull
arrangement. Similar design for drain DC voltage and RF output.
5 watts in/50 watts out at a slightly lower price than the hfpacker.
Okay. I just had a look at the documentation.
First thing: The RD16HHF1 indeed seems to be the best choice for
amplifiers of this kind. It's made for the purpose, designed to work
from 12V, has the source at the case, is available, and quite
inexpensive. Several weeks ago I placed an order for a few of them, to
use as drivers in my own project, but they still haven't arrived
(customs processing in Chile takes forever and a day), so I have no
hands-on experience with them yet.
Several comments come to mind while looking through the docs:
The output transformer has a 1:4 turns ratio. At the nominal 50W output
that means 12.5V RMS across the primary, so that's 6.25V at each drain,
or 8.84V peak at each drain. One would think that this is a somewhat
poor utilization of the 12-16V supply voltage, resulting in poor
efficiency - but I don't know what the actual saturation characteristics
of this MOSFET are! Maybe it can't do much better.
Reducing the turns ratio to 1:3 would need 11.8V peak at each drain,
which clearly would require more than 12V at the supply. It might be OK
with a 14V or higher supply, but not with 12V. So, unless one would want
to use a transformer with two primary and seven secondary turns, the 1:4
ratio is a necessary evil. And 7 secondary turns would probably prove
unworkable due to delays, leakage, etc.
With the 1:4 trafo, the maximum theoretical efficiency for pure class B
operation would be 57.8% at 12V, and only 43.4% at 16V, instead of the
theoretical 78.5% for a perfect class B amplifier. Of course when
operating in class AB the efficiency will always be lower, and
additional imperfections of practical components reduce efficiency even
further. So we cannot expect this amplifier to be a model of high
efficiency. I would expect 50% at 12V and 38% or so at 16V, on a "good"
band, and slightly less on a "bad" one, if the feed arrangement provided
perfect drain-drain coupling (which it doesn't).
Instead this amplifier should at least be acceptably linear, given the
relatively small voltage excursions at the drains, and the use of some
negative feedback.
Since the single turn primary of the output transformer doesn't allow a
true center tap, a bifiliar feed choke is required, and is indeed
present. The bifiliar feed choke used in this design has a single turn
on each side, on a relatively small core having the optimal shape. From
the point of view of leakage inductance this is pretty much the best one
can do. Only strap winding would be better. But the transistors are
placed pretty far apart, introducing significant stray inductance into
the drains-choke-bypass-sources circuit. It's hard for me to tell how
much, but looking at the board layout and the choke's construction, my
educated guess would be around 50nH per side. Given this amp's drain
load of only 1.6 ohm per side, a maximum stray reactance of no more than
0.3 ohm or so would be required for good clean functioning of this
choke. 50nH has 0.3 ohm reactance at close to 1MHz, and from there up
its performance starts falling apart! So I would expect this amplifier
to exhibit somewhat recognizable drain-to-drain coupling only on 160
meters, falling apart on the higher bands. From 40 meters up or so the
coupled feed arrangement doesn't work, so we are back to the usual
problem of these amps, with lower-than-expected efficiency and
linearity, and horrible drain voltage waveforms.
A better, very tight board layout would improve this slightly, but there
is just no way to get it right on the higher part of the HF spectrum. We
have to live with this problem and accept the compromised performance,
as we have been doing since solid state broadband HF power amplifiers
first started.
The thermal aspects: The specs are 12 to 16V input, 10 to 12A current
drain, 50W nominal output. That's a power input between 120 and 192W.
After discounting a few watts needed for control circuitry, let's assume
105 to 180W input power to the drains. If in all cases we get 50W output
(which is unlikely, it's probably more on low bands and less on high
ones), this would equate to an efficiency between 48% and 28%. This
seems plausible, with the actual values probably falling closer to the
middle of this range. The worst-case dissipation in the four transistors
should be 130W, while typical dissipation would be well under 100W, in
key-down testing. Let's take the worst case of 130W, which is 32.5W in
each transistor.
The RD16HHF1 has an internal thermal resistance of 2.2 K/W and a maximum
junction temperature of 150°C. It doesn't need electrical insulation to
the heatsink, so the thermal resistance of the interface should be
around 0.1 K/W. So the heatsink mounting surface just under each
transistor can be allowed to heat up to 75°C while 32.5W of heat are
being extracted from each transistor. With the heatsink used in this
amplifier, whose data I can only estimate because I don't have its exact
specs, brick-on-the-key operation for a sufficiently long time will
likely overheat the transistors, while normal ham operation at the usual
duty cycles should fall into the green range. If anybody wants to
transmit an RTTY bulleting using this amplifier, I would suggest to
place a fan on the heatsink. Even running very slowly and quietly at
reduced voltage it should prevent overheating.
I don't like the way the designer paralleled the FETs very much.
Resistive loading of each gate would have been safer than series
inductors. But as stated above, I don't yet have working experience with
these particular FETs. Maybe they have enough internal gate damping to
remain stable in that circuit.
The single turn per side on the bifiliar feed choke can look suspicious.
So I calculated how things are. It turns out that at 1.8MHz and 50W this
core operates at 13.4 millitesla flux density (or 134 gauss, for old
fashioned people). That's a little higher than usually recommended as
rule of thumb, but not terribly so. I would expect this core to run
warm, but not excessively hot.
So, overall, I think this amplifier is pretty decent. It does suffer
from the usual problems of solid state broadband push-pull linear
amplifiers, but that lies in the nature of the thing, not in any
specific problems of this particular design. To avoid unnecessary loss
and heating, it would be a good idea to operate this amp from the lowest
supply voltage that produces normal output and good IMD. Very likely
that's much closer to 12V than to 16V, and possibly even a tad lower
than 12V.
I didn't look at the control circuitry or the low pass filters. I
suppose you all are mostly interested in the amplifier section proper.
Manfred
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