Hi all,
nice to see so much activity in this matter! Here are my condensed
replies to some posts by several of you. I'm merging them all into one
post, instead of posting many separate replies.
Roger,
> On the parallel, PP, the convention has been PP, parallel. Each unit
> runs two devices, PP with the outputs into a combiner.. Those new 65
> Volt devices,
>
http://www.richardsonrfpd.com/Pages/Product-Details.aspx?productId=1241241
> rated at 1800 watts ea (one page lists Max as 2KW Carrier) at $250
> ea. 4 would be $1,000 and only 375W per device. Less than a quarter
> of their ratings which should require far fewer efforts at the most
> efficient cooling per device. Actually with 4 of these, 3 KW out is
> still less than 50% leaving them running well away from the "1%
> knee"
But the efficiency would be very poor.
> NOTE the base, rather than being insulated is the source,
Yes.
> so the copper spreader would be at 65 VDC
No. The source is usually at ground potential.
But you have a point there: It's that using such transistors requires a
very good, very short, very wide connection path between the spreader
and the circuit board. The bypass caps at the bifiliar choke need to
connect to the MOSFET's flange through a very direct, short, wide path,
to get any chance at correct class AB linear operation. And the gate
drive circuit also needs this direct access to the flange. Otherwise
there is a big chance of inducing enough RF voltage from the drain
circuit to the gate circuit, to burn out the gates, which tolerate only
a very small voltage range. In addition to stability considerations, of
course.
For those reasons I feel more comfortable using transistors that have an
insulated thermal surface, and several source straps that solder
directly to the board. Like the MRF150, or the IXZ210N50L2.
> Running at those levels would require less protective circuitry and
> an ability to handle higher SWR. Of course, with that much overhead
> there would be those who would want every watt they could get out of
> it even though the circuits were optimized for the legal limit, or
> relatively close to it.
The power supply should be dimensioned so as to disallow running
excessive power.
> With 4 devices at $1,000, we are very close to the cost of tubes
> capable of running any mode at the legal limit.
Yes. I don't find it attractive to spend that much money, neither on
MOSFETs nor on tubes.
> Even at the 1800 W limit we're looking at 7200 Max which 4 devices
> should do on SSB.
No, they can't handle the dissipation associated to 1800W each, in
linear operation.
> How ever you look at it these new LDMOS are capable of working the
> legal limit from 160 through 440 although the LP filters could get
> kinda messy, but ALL bands with one amp! Now there's something to
> think about. OTOH the layout for HF and low VHF wold probably be a
> problem at high VHF and UHF.
An unsurmountable problem. For VHF and UHF operation, such transistors
are used in narrow-band circuits, with the impedance transformation
network starting right at the very body of the device. Actually the
impedance you get at the device's terminals is very different from the
one that's present just a few millimeters inside, at the die! That's
part of the fun of working with very low impedances at UHF and higher.
There are no conductors at all, everything is highly inductive! Even a
flat, wide, short piece of copper strap is an inductor. Let alone a wire.
In fact this extends down to HF, at these low impedances, although to a
lesser degree. Still that's the reason why two devices used in push pull
need to be close together. You are connecting them by de-facto coils,
not by conductors, and you have to keep these coils small enough.
Roger, Bill,
>> OTOH we should not forget that a tiny (short duration) voltage
>> spike can take out a SS device, while tubes are relatively
>> forgiving. Very forgiving when compared to SS devices
>>
>> 73, Roger (K8RI)
>
> REPLY:
>
> Good point. Induced voltage from a nearby lightning strike could be a
> serious problem.
Forget it. MOSFETs can clamp an overvoltage pulse, and absorb a pretty
large amount of energy in avalanche mode. They are
overvoltage-sensitive on the gates, but not on the drains, where a
lightning-induced transient would arrive.
I started repairing radios in 1980. I have seen my fair share of radios
damaged by nearby lightning. What fails is always the receiver section,
not the transmitter. Often it's just a protection diode or other
protective device, in some cases somehing more fails - but always in the
receiver section, or right at the antenna connector, where some
protective devices are often placed. The transmitters are robust, even
in 100W radios and often even in QRP radios!
A direct lightning hit will fry a radio beyond repair, and possibly the
operator too. Regardless of whether it was in TX mode, RX, or off. But a
nearby hit might damage the receiver frontend, not the transmitter. If
nearby lightning happens during TX, most likely nothing bad will happen
to the radio.
Leigh,
> For adventurous homebrewers here's an interesting article on an
> alternative novel approach to SSB generation and its efficient
> switched-mode amplification by K1LI and K1KP in the March/April 2017
> QEX magazine, pages 3 to 9. It uses a modern digital implementation
> of novel modulation concepts long ago proposed by Leonard Kahn.
I would be interested in a copy of that article.
> Whilst I like the polar IQ modulation and efficient switching RF
> amplifier architecture and its novelty I must admit I'm a bit old
> school when it comes to SSB generation and its associated low
> efficiency linear amplification...I also don't mind having a lot of
> "big iron" to crank out legal limit PEP and above for copious
> headroom.
That position seems to be shared by most hams, which places me in a
rather lonely position! :-(
> Unlike radio broadcasters, for the modest amount of time an amateur
> station is on-air, efficiency and the electricity
> consumption/conservation is not such a serious driver...
That's right. But it doesn't apply to me: I don't have a connection to
the power grid, because I literally live in the woods. In exchange I
have a great, manmade-noise-free radio environment, complete with
hilltop propagation. I generate my power with a turbine fed by a nearby
creek. In winter I have plenty of power to run my old NCL-2000 amp. But
during the other three seasons of the year, I can't use it! Instead I
do have enough power available year-round to run a legal limit high
efficiency amp in SSB (not in RTTY). This is what makes me so keen on
improving amplifier efficiency!
But while my case is special and rare, all those hams interested in
mobile or portable QRO, including DXpeditioneers, are much better served
by a small, lightweight, low power consumption amp, than by a big, heavy
and inefficient behemot. These hams are the ones that might join me in
my quest for high efficiency.
Kevin,
The guys running those LDMOS devices full tilt, meaning past 1dB
compression, will have them fail,
Not necessarily. When driving an amplifier into compression, its
efficiency improves so much that in a certain range the dissipation
gets LOWER rather than higher. And the current ratings of these
MOSFETs are plenty high enough. So, as long as the low pass filter
caps, etc, keep going, the MOSFETs will, too. It's wishful thinking
that a MOSFET failure will take those splatterers out of the air!
A friend of mine has an Ameritron 1306. 8 MRF-150's, amplifier rated
for 1200W out SSB and CW. He runs it at 1KW. He doesn't vary. When
doing RTTY he knocks it back to 700W.
The dissipation at 700W might not be much lower than at 1200W. The
best way to implement a power reduction for RTTY is to reduce the
supply voltage, instead of only throttling back the drive. That will
reduce the power while maintaining efficiency, thus reducing the
dissipation.
That's old news, it was done in the tube era too. Many tube amps, like
my NCL-2000, have "SSB" and "CW" settings, the main change being lower
voltage in CW mode. The NCL-2000 is rated for 2kW PEP input in SSB,
and 1kW DC input in CW. I understand that this was in line with FCC
rules and regulations in those years.
RF design of an SSPA is pretty straight forward,
The devil lies in the details...
The most expensive part of any SSPA is the LPF...by a long shot.
Ceramic chip caps capable of taking the high current involved are
more expensive than silver if sold by the ounce.
We should roll our own. There is no reason why these HAVE to be chip
caps. Metal-clad mica caps work fine, and these are easy to homebrew.
I'm not willing to pay 20 dollars for each capacitor, if I can build a
perfectly usable although physically larger equivalent for three cents
in materials plus 5 minutes of my time. I enjoy doing it!
Why do we need four LDMOS devices for 1500W out?
It's useful for relaxing the otherwise too stringent cooling
requirements, as I explained above and before.
What's the 1dB compression point on the devices?
It's undefined. The 1dB compression point can only be specified for a
complete circuit, not for a bare transistor.
The LDMOS devices I know about have a 1KW compression point
That must be a specification given for a specific test circuit. It
will change depending on what circuit you use.
Well, enough for today! Let's leave something for tomorrow! :-)
Manfred
========================
Visit my hobby homepage!
http://ludens.cl
========================
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps