Bill,
One intriguing possibility with these is direct coupling (through an
LP filter and DC isolation of course) to 50 ohm coax.
Forget the low Q lowpass filter. This transistor is rated for 300pF
output capacitance. At 30MHz that's a reactance of 17.7 ohm. But the
load resistance for 300V and 750W operation is roughly 60 ohm. This
means that you cannot possibly absorb the high capacitance of this
device into a low Q lowpass filter. Instead you have to use a medium Q
circuit between the device and the load - exactly like is the case with
tubes.
Instead of the Pi circuit practical with tubes, a T or LCC circuit is
better suited in this case. And while tubes typically require a Q factor
around 10 or 12, and often even higher on the 10m band, this transistor
could work with a significantly lower Q, like 6 or so.
Broadband operation would be possible to around 10MHz only. Better than
tubes, but not good enough for full HF coverage.
> No transformer needed.
That's right. And a single-ended amplifier is entirely practical. This
eliminates the main cause for "typical" low efficiency of semiconductor
amps. And the "conventional" layout of a single-ended stage with a tuned
output network might even appeal to people used to tubes!
Something to think about.
I have been thinking about this for years. I hope to be DOING something
about it soon - but I make no promise. It's just a good intention.
Joe,
Only on 160 meters for 750 Watts output. By the time one gets to 27
MHz the optimum output impedance is down around 5 Ohms ...
You are misinterpreting the datasheet. It says that the conjugate of the
optimal output load impedance at 27MHz is roughly 5-j15 ohm. That means
a series connection of a 5 ohm resistor and a 393pF capacitor, but the
actual transistor is NOT such a series circuit! The impedance values are
given as series impedances only because that facilitates the design of
matching networks. The actual behavior of the transistor is like a
current source having a capacitance IN PARALLEL, not in series. If you
convert the given values of series-expressed impedance to their parallel
expressions, you will find that
- the equivalent source resistance is roughly constant over the
frequency range;
- and the output capacitance is also roughly constant over the frequency
range!
That makes life much easier, when you are considering a broadband circuit.
it would be a real trick to design a broadband output.
As far as my understanding goes, it would be impossible to design a
broadband output circuit that covers to 30MHz. Broadband output circuits
are possible only up to the frequency where the device's output
capacitance, plus strays, can be absorbed into a lowpass network having
a Q of 1. That would be roughly 10MHz in this case. If anybody here
knows otherwise, please teach me!
> The best approach might
be impedance matching in the output filters for each "band".
Yes. Either using fixed-tuned networks per band, switched by relays just
as if they were lowpass filters; Or using a single, tunable network,
which could be manually or automatically tuned.
The switched network approach is no-tune, but requires lots of high
current, high voltage capacitors and coils - much more so than lowpass
filters, due to the higher Q. It would probably prove too expensive and
bulky. Which leaves us with a single, tunable network, very much as in a
tube amp. Using variable caps, tapped/switched inductors, etc.
Steve,
they were
originally designed for narrow band saturated cw use and one
common theme is the problems of trying to use them as linear amps.
Bias control is a problem and reduced supply voltage is
recommended, even then linearity is poor. One 'linear' design
quotes IMD as -25dB ref pep or -19dBc!
I dare say they are capable of better than that, but it won't be
simple and straightforward.
And I would say that instead of trying to force them to behave well
enough in a linear circuit, we would do better using them in the way
they like: Saturated, biased well below the conduction threshold,
running at high efficiency (there I go again!), and then we can achieve
the linearity by other means: Power supply modulation, along with
predistortion.
In other words, I'm proposing to use devices like this in a tuned
amplifier, running class C, E or perhaps F or inverse F, in an EER
amplifier driven by an SDR.
Peter,
Tokyo High Power used a pair of ARF1500 in their HL2.5KFX.
http://www.ab4oj.com/dl/thp/hl2_5kfx/manual_hl2_5kfx_complete_schematics.pdf
Two amps with those transistors were built in Germany:
https://www.yumpu.com/de/document/view/21269897/kw-pa-arf1500-mods-hamcom
All of these use the ARF1500, rather than the ARF1505. That's the lower
voltage version, to be run from 100-150V supplies. The output
capacitance is nearly the same, but at the required 4 times lower load
resistance the resulting bandwidth for broadband operation is
correspondingly larger. So these transistors do allow broadband
operation to 30MHz, unlike the ARF1505.
But they need transformers in a push-pull arrangement, and thus suffer
from all the troubles that come from insufficient drain-drain coupling.
Mainly low efficiency and high IMD, both of them a result of significant
common-mode voltage swing, causing somewhat peaky drain voltage waveforms.
Manfred
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