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Re: [Amps] MOSFET amp filtering - was: auto-tune

To: amps@contesting.com
Subject: Re: [Amps] MOSFET amp filtering - was: auto-tune
From: Manfred Mornhinweg <manfred@ludens.cl>
Date: Mon, 12 Dec 2016 15:44:15 +0000
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Several good points were brought up.

Some comments:

Indeed a diplexer filter looks very much like a dummy load to the amplifier, aiding stability, while a normal lowpass filter looks highly reactive at harmonic frequencies. A narrow-band resonant network looks highly reactive at even more frequencies, specially those lower than the operating frequency. So the stability situation is even worse in that case. But it should be quite possible to make stable amplifiers, that remain stable even when facing these loads.

When somebody develops an amplifier while using a dummy load without any filters, indeed a diplexer filter avoids some surprises, compared to a lowpass filter. What remains is the "surprise" of lower output power, since much of what was measured into the dummy load was in the harmonics. That's specially the case when experimenters try to get good figures and drive their test amps into saturation, getting great output and efficiency. But 30% of that output is in the harmonics, so after adding a low pass filter the numbers look less spectacular. And when measuring IMD and staying at an IMD-acceptable power level, the output and efficiency are both back to "normal", which means "poor".

Conceptually there is no problem with using a diplexer filter, when the amplifier is designed and operated such that its harmonic output is low anyway. Let's say, a clean class AB amplifier with lots of feedback might have the third harmonic down by 30dB. This means that 99.9% of its output power is on the desired frequency. It needs the low pass filter only to meet FCC rules, and dissipating the 0.1% of harmonic power in a diplexer filter's dummy load is perfectly fine. On the other hand, an amplifier designed for saturated operation, far example used in FM, or externally linearized, would be terrible with a diplexer filter! Naturally it generates a waveform that has a lot of harmonic contents, and dissipating all this in a diplexer filter's dummy load kills the efficiency advantage of such an amplifier. Such an amp MUST use a plain filter, and must be designed to cope with the reactive load at harmonic frequencies.

Consider the example of a switchmode amplifier: A half bridge of RF MOSFETs (one with source to ground, drain to the source of the second MOSFET, that one having the drain connected to +B), with its output (the connection of the two MOSFETs) connected to a filter that has high impedance at harmonics (an inductor-input low pass filter, or a series-resonant tank). Now drive this half bridge with an RF square wave, or simply overdrive it with a sine wave. The MOSFETs will basically switch on and off in opposing phase. The voltage waveform will be pretty much a square wave on the lower bands, degrading into a trapezoid on the higher bands. That waveform contains a whole lot of harmonics. But the output filter only allows a fundamental-frequency current to flow. So the current in the MOSFETs will be clean half sines, while the voltage is sqaure. The efficiency will be close to 100% - how close depends on how fast the FETs switch, how well they saturate, and how low their capacitances are. In this case the "reflection of harmonic energy", if anybody wants to analyze it that way, doesn't cause added dissipation in the FETs, because each FET is either on or off, and so cannot dissipate much power at all!

The situation is very different in a class AB linear amp that has moderate feedback. Due to this feedback, the FETs behave like signal generators with some internal resistance. So the reflected harmonics will indeed cause some dissipation, which depends on their phase, among other things. If the filter is on the other side of a broadband transformer, the situation is quite bad, because the transformer adds a phase shift that varies a lot according to the band.

This shows once again that diplexer filters can be a good idea in some situations, but not in others.

The phase distortion arising from voltage-variable MOSFET capacitances indeed is something that worries me time and again. I don't have yet a good way to simulate or calculate that, and when measuring IMD it's hard to tell what comes from amplitude distortion and what from phase distortion. The good news is that LDMOSFETs capable of VHF and even UHF have capacitances that are comparatively low at HF, so the problem is reduced. I would really like to know how much distortion comes from this...

I would expect that additional phase distortion coming from MOSFET capacitances and crooked waveforms when not using a diplexer, should be low. The basic phase distortion coming from variable amplitudes is probably much larger, and anyway, during operation on a given frequency, the phase of the reflected harmonics stays constant. And putting a real resonant tank circuit directly on the drains, without a transformer in between, assuring a clean voltage waveform while the transistor conducts distorted current, should be best in reducing this form of distortion.

It's wrong to think that transistors are naturally broadbanded and tubes are naturally narrow-banded. The truth instead is that any device can be used broadbanded only over the range where its internal capacitances can still be absorbed into low Q lowpass networks. With transistors that's anywhere between a few MHz and probably 100MHz or so. With tubes, the upper limit is far lower. When operating above this capacitance-dictated limit, it's necessary to use a higher Q load circuit to absorb this capacitance, and it's here where things start getting narrow banded. In a typical HF tube amplifier, on 10 meters the tube's capacitance is so high that the Pi tank operates almost without any added tuning capacitance, and often needs a Q of 14 or even higher to be able to absorb the comparatively very high capacitance of the tube! This prevents making broadband HF tube amps, while it's easy to make broadband transistor amps - but not mandatory! In fact there are quite a few advantages to making narrow-band solid state amps.

There is another reason that makes tube broadband amps impractical: Broadband transformers usually need magnetic cores. The best material currently available for them are certain ferrite mixes. But the characteristics of this material are such that to work at the high voltage tubes need, the transformers either need very large cores, or a lot of turns. Both approaches result in excessive winding length, so that the transformer has a much too low frequency limit, given simply by the outphasing along those long wires!

Pure transmission-line transformers get around this restriction, but winding transformers from high impedance transmission lines isn't very practical either. And a pure transmission line transformer matching roughly 3000 ohm to 50 ohm needs eight separate, big ferrite cores wound with 400 ohm line. Or a three-stage transmission line transformer, using six cores, the first stage being wound with 1500 ohm transmission line...

Pi tanks are indeed impractical at low impedances, but there are enough other tank configurations that work fine at low impedances. So that's not a problem. Take a Pi tank, remove the tuning cap, and add it in series either with the input or the output. There you have two tank configurations that result in practical values when designed for low impedance transistors and 50 ohm loads.

Transistor amps using a resonant tank circuit directly from drain to antenna work very well, but only on one band. The problem is that it's very hard to do any bandswitching at impedance levels of a few ohm. So practical solid state amps first use a broadband transformer to raise the impedance to 50 ohm or anything in that order, and only then do the bandswitching - be it switching low pass filters, or bandpass ones. But this broadband transformer is the biggest factor causing poor efficiency and linearity! It's enlightening to use a circuit simulator to try a basic push pull RF amplifier using either a life-like transformer having a coupling factor of 0.97 or so, and then a perfect transformer having a coupling factor of 1. This simple change often makes the efficiency change from 45% to 68%, the drain waveform to change from a horrible and funny thing to a clean sine wave, and the IMD go down by 10 or 15dB - at the same power level. Too bad that we don't have perfect transformers available, to build real amplifiers! In practice the performance of a real-world push pull solid state amp stands and falls with the quality of the coupling between the two drains. The usual bifiliar chokes are very often not good enough on the higher bands, the usual transformers with two brass tubes as primary do not provide any coupling between the drains (!), and the amplifiers often built nowadays that don't even have bifiliar chokes are even worse. No wonder many of them barely make 45% efficiency at full power, and have poor IMD.

Given that transformers with perfect coupling are impossible in practice, and at the low impedances resulting from 50V high power amps we cannot built transformers that have even decent coupling, it would be best to avoid those transformers altogether.

To make a transformer-less, band-switched solid state amplifier, it would be really great to have transistors that can operate at a load impedance that falls in a switch-friendly and relay-friendly range. Something around 50 ohm is fine. But for a legal limit amp this requires a supply voltage around 300V. Yes, I'm aware of the ARF1505, but its capacitances are very high, much higher than acceptable for broadband use to 30MHz, and still pretty high for use with tuned tanks. This is also bad news regarding phase distortion from the voltage-induced variability of these capacitances. And I have read that this transistor isn't really suited to linear applications at 300V, due to hotspotting issues. It's only intended for CW applications in the industry, like certain welding machines and such. If anybody here has tried that transistor in linear mode, I would be very interested in the results.

At this time my best bet to obtain high-voltage RF transistors is to use many small cheap switchmode MOSFETs in parallel, with individual source and gate resistors added. But even this approach works well only to about 150V supply voltage. Above that the capacitances kill the performance. In my experience, at this voltage level one can make legal limit amplifiers in which the transformer issues are reduced to an acceptable level, but not really transformerless ones. But the switchmode transistors, even if optimally selected and applied, are always a bit marginal in an RF power application. They weren't made for it, of course. The best side about them is that their low cost makes them very suitable for experimenters: 50 dollars will buy you the active devices for a legal limit amp! And 150V power supplies are easier to make than either 50V ones or 3000V ones, at the 3kW power level.

About LDMOSFET reliability: The new "extra rugged" parts should be impossible to destroy from load mismatch, as long as their maximum safe temperature isn't exceeded. Since heating them up takes time, at least several milliseconds, the combination of these LDMOSFETs with comprehensive protection circuitry should result in a pretty indestructible amplifier. Instead older technology MOSFETs were prone to failure from drain voltage spikes. If an antenna gets disconnected during transmission, for example due to a worn coax connector or a faulty relay, the voltage can go up far too quickly for any protection circuit to respond. Old solid state devices were prone to fail under such conditions, no matter how much protections were built in. Modern extra rugged parts survive this. That's their main ruggedness advantage, in my opinion.

But these modern LDMOSFETs have "don't-touch-me" gates. They are very prone to failure from gate overvoltage. It's essential in the design of any amp using them to make sure that the gate voltage will never be exceeded - neither from overdriving, nor from self-oscillation, nor from switch-on or switch-off phenomena.

Going through the reports I have from other experimenters who blew LDMOSFETs, it's obvious that the prime cause of death in the hands of experimenters is overheating, when they didn't understand how to properly calculate the temperature rise of the junctions under the selected operating conditions, and the second one is gate overvoltage, often arising from self-oscillation. The gates need to be resistively loaded down over a wide frequency spectrum, wider than the one over which the amplifier has gain! And those loads must go from gate to source (ground), not from gate to gate! A push pull amplifier with gate-to-gate load is unloaded in common-mode, and will happily oscillate.

And even with proper gate loads and strictly limited drive power it's a good idea to add voltage clamps to the gates, just in case...

So, solid state amp design does have its pitfalls. But so does tube amp design! Only that tube amps have been used for so long, that all these pitfalls were worked out long ago, and properly published in a time when mainstream ham literature was still of a technical nature... Just think about glitch resistors, plate choke resonance isssues, hum modulation from AC in the filaments, crooked unstable drive impedance of grounded grid amps, and so on. Today we know them all, and the solutions to them. With high performance solid state amps, there is still a thing or two to learn.

Manfred

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