[Amps] Solid State Amps

[jeff millar]

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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