See below...
On 10/17/2014 02:24 PM, Manfred Mornhinweg wrote:
- Heat. Solid state devices simply are very small, and don't tolerate extreme
temperature. So, a high power, class AB, solid state amplifier will ALWAYS be
problematic in terms of cooling. It will need large heatsinks, fans, heat
spreaders, and careful design of the thermal aspects, just to start becoming
viable.
PC water cooling technology is able to get rid of 150W of CPU heat from a 1 cm
die with only 10-20 degrees of temperature rise. For an RF amplifier, allowing
70 degrees of rise, the same CPU thermal block should support 1000W of heat
takeaway per transistor. The best water cooling blocks have thermal resistance
below 0.03 deg C per Watt, for example see
http://skinneelabs.com/v1-swiftech-mcr-320-qp/4/.
Hams need to think more about the difference between thermal mass and thermal
resistance. The solid state amp people often add a big heat sink with copper
and aluminum...that's thermal mass. The thermal resistance is set by the fans
blowing across the fins. There's really no need for thermal mass when a very
thin bit of copper with water cooling is a much lower thermal resistance from
Silicon die to water.
- Fragility: RF power transistors are usually run very close to their absolute
maximum voltage spec, close to their maximum current spec, and at or even
above their rated thermal capability, with the heat sink system used. Any
problem like non-perfect SWR, relay glitches, etc, and their survival depends
100% on excellent protection circuitry. Tubes instead are so forgiving that in
practice they don't need protection circuits in most cases, or some tubes need
simple circuitry to protect against excessive screen or grid dissipation, but
not much else.
Modern RF transistors have enough margin to handle 100% VSWR for some
milliseconds (Freescale MRFE6VP1k25, NXP BLF-188) which is achieved by having
enough Voltage tolerance to handle peak voltage and enough current/thermal
tolerance to give the shutdown circuits enough time to act.
- Poor linearity: Both bipolar and field effect transistors are less linear
than tetrodes and pentodes, and while better than triodes, they don't have
enough gain to use them in grounded base/gate configuration. So, they depend
on negative feedback or other external means, to arrive at good IMD specs.
Many designers still don't grasp this concept well enough, and try building
solid state class AB amplifiers without negative feedback, getting horrible
IMD performance.
Many VHF and UHF RF power transistors (typically LDMOS) have good linearity (-40
dB IMD) because the Cellular market demands it. Then amplifier designers added
additional linearity with digital pre-distort and typically get a system IMD of
-60 dB for 10 MHz of bandwidth. Practical pre-distort algorithms can improve
IMD by 15-20 dB, although lab experiments can show 30 dB improvement in any one
case.
Achieving good linearity of the transistor requires precise control of the
impedance seen by the part around the peaks of the modulation cycle...which
probably can't be achieved with ham antennas that blow in the wind and get wet.
So the solution is to use active pre-distort which measures distortion in real
time and controls the input to the amplifier.
I've been working on the design of a system which samples the RF output of the
amplifier (or transceiver) and adds pre-distort to the microphone audio to
cancel out distortion. The whole SSB signal chain is linear and the output is
just the frequency shifted input, so it should work. There are some issues: for
example a pre-distort system like this would work very hard to cancel out the
audio compression of the transceiver...so audio compression and frequency
response adjustment would have to occur in the MIC path before the pre-distort.
Now some people have tried, and are still trying, to solve these problems by
brute force methods: Use lots of transistors, on big heatsinks, run them well
below their maximum specs, use UHF transistors at HF to get enough gain that
allows using lots of negative feedback, and put in complicate protection
circuits. The results of these efforts can work reasonably well, producing
amplifiers that are instant-on, no-tune, reliable, and about as large and
heavy as tube amplifiers - but the solid state ones tend to be more expensive,
done that way. And often the implementations are simply wrong and unsafe, for
example by relying on an SWR sensor placed between the low pass filters and
the antenna.
Agree there is another way to do it.
Protection circuits are not really that complex any more because a $30 Arduino
(or equivalent) can sample the voltages, currents, and RF levels well enough to
protect the amp. Moore's Law has done it's work. Once a designer commits to
digitizing and processing many points in the amplifier design for protection, it
is a relatively short step to increasing the sampling speed and implementing
digital predistortion.
As an aside, what's the problem with a SWR sensor between LPF and antenna?
What we need to do, my dear friends, is something totally different. For
starters: Forget class AB, because it's too inefficient, and forget Granberg's
push-pull configuration, because it has no inherent protection features and
needs problematic transformers.
yes
Instead of Granberg's design, we need to place our RF power transistors in
half bridge or full bridge configurations, with effective antiparallel diodes.
This configuration eliminates all risk from overvoltage. Then we need to run
our transistors in switchmode, _not_ in any linear mode, to get rid of the
heat that causes so much trouble. Then we add simple current sensing with
quick shutdown, to protect against severe overcurrent situations. We need to
take the highest voltage transistors we can, up to a level of 400V or so, to
get rid of the ultra low impedances that result from low voltage operation,
and which are hard to handle. And instead of a broadband transformer (not very
easy at the kilowatt level), followed by relay-switched low pass filters, we
should use relay-switched resonant matching networks. That's no more complex
than the low pass filters, and the resulting Q is low enough to pre-tune these
networks to each band and then forget them.
There is an example of these kinds of "amplifiers" from the induction heater
industry. The designs operate IGFETs at line voltage and use pulse width
modulation to generate "RF" at about 50 KHz to drive the heater coil. You can
get a 1.8 KW hot plate for less than $100. IGFETs don't have the switching
speed to operate at HF frequencies, but we can probably learn from how they
design their systems.
And then, of course we need to add circuitry around the amplifier block, to
obtain a linear transfer function despite the switching operation of the RF
transistors. This can be done by RF pulse width modulation of the drive
signal, power supply modulation, bias modulation, a combination of two or
three of these, or any other method. This is far more complicate than a
traditional tube amplifier, of course, but it uses cheap, small, widely
available components, and so it's inexpensive to implement.
A SS high power amplifier consists of a three high power stages; Power factor
correction, Power Voltage generation, and RF amplification. It's useful to
consider how to use each stage. To some extent, you can consider the power
supply as an RF amplifier operating at HF, the frequencies and design issues are
similar.
Instead of using PWM for controlling RF output in the RF stage, the amplifier
can the use the existing PWM in the power supply stage to control the main power
voltage to the RF amplifier stage. The RF amplifier only sees enough supply
voltage to meet the power output needs of modulation cycle and so operates at
maximum efficiency at all times with effectively near zero idling current.
Modulating the supply voltage has a hard to predict effect on linearity and
IMD...but we let the digital pre-distort measure and compensate for that in real
time.
Two other high efficiency techniques from cellular industry should be
considered. Doherty amplfiers and out-phasing. A Doherty amplifier uses a
linear stage and a separate peaking amplifier running in class C.
Outphasing amplifiers modulate the current draw to control RF output (see
"Ampliphase"). The amplifier consists of two output stages in parallel
operating out of phase. When perfectly out of phase the power cancels and each
amplifier sees a very high impedance load, as the phase shifts to in phase, the
power output add and the amplifiers see a match load.
The last thing to consider is whether to implement an amplifier or to implement
a complete transmitter. Power supply modulation and out-phasing techniques lend
themselves to separating the modulation into amplitude and frequency components
and it's easier to perform that separation at audio than at RF. The predistort
system works best on MIC audio so we have control of that part of the system.
Lastly, the RF power amplifier has 30 dB of gain and doesn't need a 100W
transceiver driving it. We could include something like a Softrock for the
transmitter (and use the receiver for the RF sampling of the output)
The result would be an instant-on, no-tune, small, lightweight, silent, highly
efficient, reliable _and_ inexpensive legal limit amplifier.
yes
Anyone actually developing this concept to market maturity can put all
existing ham amplifier manufacturers out of business. A scaring thought - for
them!
no, the old farts will resist ;-)
Do you notice the logic in this? Going from class AB to a switching mode
achieves several important advantages:
- Cooling becomes very much simpler, cheaper, and silent.
- Power supply requirements are drastically cut down, producing advantages in
cost, size, weight, etc. A 1700W power supply can power a 1500W amplifier.
- Power consumption is reduced a lot, an important selling point in many
countries that have expensive electricity. Maybe not in the US, where it is
almost free.
- The transistors needed are very much smaller and cheaper than those needed
for class AB, due to low dissipation requirements.
A niggle, but the die will be the same size, and peak thermal density will be
the same. So, cheaper only if produced in very high quantity because they are
used in power suppliers or other applications.
- A good active linearization circuit can produce far better linearity than
class AB with 10dB of negative feedback, and even better than that of tetrodes.
yes. Another aside. In crowded band conditions, the - 30 dB IMD from many
transmitters across the band is the most significant limitation on
communication. Buying an $8000 transceiver with 100 dB of dynamic range doesn't
do any good when the splatter from 10 nearby stations raises the noise floor.
I would be in favor of requiring improved IMD in transmitters and power
amplifiers...say -40 dB for 100W transceivers and - 50 dB for 1000W
transmitters. This would drive the state of the art, improve the bands dramatically.
And the difficulties involved in this approach:
- Finding ways to get around the limitations of present-day RF power
transistors, in terms of voltage-dependent internal capacitances, slew rate
limitations, and high voltage handling.
Solved at VHF with unmatched LDMOS FET's from Freescale and NXP. Will those work
at HF?
- Summonning the determination to do all the detail design work, and break
free from the idea "if Granberg did it that way, that must be the best/only way".
Any idea, anyone?
The most complex part might be the linearity management system. But that a
separable component with it's own value. We need a box which samples the RF on
any HF band with 2KW passing through it. A box which generates predistort on
the mic audio. And the software to process it.
Second most complex part, building an 90% efficient RF stage based on switch
mode (class C) transistors and a modulated power supply voltage. This would be
coupled with a separation of amplitude and phase in either RF or audio. The DC
power supply would put out 0 to 50V (or 0 to 400V) with a bandwidth of 3 KHz.
The modulated power supply would have to run at 1 MHz or greater switching
frequency to achieve this bandwidth. The input to the modulated power supply
would be 400 VDC from power factor corrected direct line rectification.
Where to put the line isolation transformer? The 400 VDC from line
rectification and PFC is not isolated. The modulated power supply is simpler if
it doesn't have a transformer (would need 2 KW transformer for 1 MHz). Is it
cheaper to put the transformer in the RF path?
Radical but scary thought, what about just capacitively coupling the RF output
and skip the whole line isolation completely? The water cooling loops would
have to isolate the RF transistor case from safety ground, that used to work for
plate voltages and this would only be a few hundred volts.
Maybe we should start a collaborative open project, developing this thing! The
final goal: A solid state amplifier no larger nor heavier than a typical HF
radio, that can produce solid legal limit output in all modes, with no time
limit, with good IMD performance and high reliability, a total parts cost
around $500, and selling to those who are too lazy to build it, for around $1000.
I'm willing to help. I've wanted to do something like this for a while now.
jeff, wa1hco
I'm just waiting for the right transistors to show up, and then I will do it
myself. With the transistors I know right now, I would get up to the 40m band
only, or at most to 20m, but not to 10.
Manfred
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