[Top] [All Lists]

Re: [Amps] Solid State Amps

Subject: Re: [Amps] Solid State Amps
From: Manfred Mornhinweg <>
Date: Sat, 18 Oct 2014 15:49:47 +0000
List-post: <">>

> What turns me off to Solid State is the fact when you need replacement
> transistors, they most likely have become obsolete. I'm tired of it.

For that exact reason I hesitate to design amplifiers around specific transistors for which there are no equivalents, like the ARF1500 or the IXZ210N50L. If the company making them decides to stop production, there's nothing that can directly replace them. So, I prefer using transistors that are made by several different companies. It could be a specific type second-sourced by other companies, or it could be a type of transistor for which many equivalents exist. For example in an amplifier design I'm using some TO-220 cased switching transistors which work OK to 30MHz, and I have tried some from 4 different companies (IR, Ixys, ON, AOT), all in the same circuit, with results similar enough to just pull out a FET and insert another one. So this design is likely to be maintainable in the future.

Note that with tubes, the problem of obsolescence is dramatic. Many tube types are no longer manufactured, forcing even the established amplifier manufacturers to abandon a design and make a new one. The only reason why we hams manage to keep our old amplifiers running, is that stockpiles of antique tubes still exist, and that companies in Russia and China are making tubes discontinued in the US. And not always are these equivalents really equivalent!

My NCL-2000 amplifier uses 8122 tubes. The day they fail, I will junk that amplifier, because NOS 8122 tubes are scarce and expensive. I just see them on eBay, offered for a starting bid of $750 for one pair! That's the starting bid, not the final selling price... Sometimes there are lower prices, but there is always some doubt whether the tubes sold are truly NOS, or are pulls in unknown condition sold as NOS. Some years ago I wanted to buy a set of spares, and got a quotation for some made by Burle, not RCA. The quoted price threw me off my chair! I could have bought a complete new amplifier for that much.

Solid state amplifiers do have one advantage, at least: The transistors shouldn't wear out, while tubes do wear out, even if slowly. Of course that's only slim comfort, knowing that anything can fail at any time, for any reason.

> Same
> with displays.  You need to throw the radio away because the display that
> went out is no longer available.

Here I agree with you. My FT-736's display is old, tired and very dark. The radio has about 130,000 hours on it. Almost all of the time the display was kept dimmed, but still it wore out and now I can't read it when dimmed, and only barely when undimmed. Guess what? That display is a vacuum tube, a fluorescent display. Instead my TS-450's display, which is even older and has seen roughly the same amount of use, is still perfect. That one is an LCD. But it's special too, and the day it fails, I either find a parts donor, or I can operate the radio only via computer. I would certainly prefer radios that use industry-standard parallel input dot matrix displays, which are made by many companies, and will remain available for a long time.

> I'm off this "buy a new rig every year" Merry Go round.

So am I. My factory-made radios are all over 20 years old. The oldest piece of ham gear I have is 68 years old. Some accessories are newer than 20 years, though. My homebuilt gear spans my whole life as ham, and I keep building stuff.


> Has someone patented this who might block production?

Sure! Probably not the exact complete concept I outlined, but there must be thousands of patents flexible enough that a good lawyer can make them cover every detail of an amplifier like the one I described. In fact, when making patent searches, I wonder how anyone can bring to market any product, without infringing thousands of patents! It seems that everything is covered, and usually by many overlapping patents.

So I prefer not caring about patents. This is ham radio, a hobby, not a business. Anyone of us can build his own amplifier, without worrying about any patents. And if we develop such an amplifier collectively and publish it, for others to copy, I understand that's still 100% legal. And selling kits of parts to hams probably still is. But when it comes to installing a little factory producing these things and selling them in stores, probably a herd of lawyers would have to be engaged to sort out all the patent issues, and another herd of engineers to redesign every detail in such ways that all those patents are evaded, and the herd of lawyers would have to work hand-in-hand with the herd of engineers to get patents on all those new circuit details. And that, my friends, is not an activity I would like to be involved in! So I will _not_ set up an amplifier factory, but I will keep building my own stuff, and possibly collaborating with some open projects.

> Any concerns about potential RFI from a switching-amp in a rf-amp circuit?

No concerns. We just can't make the FETs fast enough to create lots of hard, difficult to filter, high harmonics. A practical class D RF amplifier has a third harmonic voltage at the FETs that is at most 10dB higher than the one a class AB amplifier has. So, having 10dB more attenuation at the third harmonic, in the circuit between the FETs and the antenna, is all you need to reach the same spectral purity. Given that a class D amplifier would use a resonant network with a moderate Q (like 3 to 5), instead of a lowpass filter with unitary Q, it would be easy to meet this level of harmonic attenuation.

> Has anyone tried this before?

I have seen application notes where this is done for ISM applications, and I have also seen it done in AM broadcast transmitters. I have built a few test amplifiers in class D, also in class E, with wrap-around linearization circuits, but so far only at low power levels, with the aim of getting a reasonable practical knowledge of these techniques. I would not be surprised if some ham has built high power amps in this way. And I have been in contact with a guy using wrap-around linearization of a class-C amplifier, by bias modulation, in a near-kilowatt amplifier using cheap switching MOSFETs - and he runs it on CB! He sent me screenshots that demonstrate good amplitude linearity, although he hasn't done IMD tests, and I fear that he could have lots of IMD due to phase distortion.

In addition to all that, I know that it's used in some military equipment.

> Are you considering a crowd-funding proposal to raise funds for the project?

No. This isn't an expensive project. The parts involved in development can easily be afforded by most hams, without any help. What it takes is lots of brain time, not lots of money. So I'm considering the possibility of an open project, with several people developing the circuit, experimenting, and interchanging information. In fact when I joined this forum some time ago, I hoped to find something like this here. But what I found is mostly a group of tube amplifier users, with a few tube amplifier builders in between, and just a very few people who know anything about modern technologies. And those mostly don't have the time to put into such a project!


Wonder if you could elaborate or
provide a referene on "... we need to place our RF power transistors in half
bridge or full bridge configurations, with effective antiparallel diodes."
I'm unfamiliar with that design.

It's the same layout used in almost every PC power supply, almost every electronic fluorescent lamp ballast like those used in CFL's, and which is known in the RF world as a class-D amplifier.

The half-bridge: Put one FET with its source at ground, the otehr FET with its drain at the power supply. Join the free drain and source. That's the output node. Place a series resonant circuit from that output node to the antenna output, and a parallel resonant circuit from the antenna output to ground. The desired Q factor, and the impedance ratio, determine the values of the 4 reactive components. Drive the two FETs from two independent secondary windings (usually one turn each) on the drive transformer, in anti-phase. The drive signal can either be the exciter's signal as a sine wave, but at high amplitude, or a buffer/limiter can be used to square it up. The latter results in slightly better efficiency.

At low frequencies this circuit can be used as described. But at RF, the body diodes inside the FETs are far too slow. So each FET needs to be paralleled by an external RF-capable diode, cathode to drain, anode to source. This might suffice in some cases, but often the circuit inductances will be enough to make that these diodes don't fully prevent body-diode turn-on. In that case you need to place another diode (a low voltage Schottky is fine) in series with the drain, and then place the RF diode in parallel with the FET-Schottky combination.

Some manufacturers sell FETs that have these two diodes already built in. I have a few of those in my treasure chest. Check the datasheet of the IXKF40N60SCD1, although this particular device is too large and too slow for HF use.

A full bridge is just a combination of two such half bridges of FETs, and the output is taken in balanced mode from the two output nodes. Typically you would use a series resonant circuit in series with one of those nodes (or for visual symmetry, place the coil on one node and the capacitor on teh other!), followed by a parallel resonant circuit between the two lines, and then a transmission line balun (AKA common mode choke) from there to the antenna output and ground.

Current sensing is typically done with a shunt resistor in the ground leg of the circuit. In the full bridge, a single resistor is used for both sides. Very simple.

Note that the series resonant circuit of moderate Q doesn't allow current spikes to occur in the FETs. So, since voltage spikes are fully clamped by the diodes, all that's needed to make the circuit foolproof is something that detects the average current over an RF cycle or two, and shuts down the circuit if this is excessive. Nanosecond reaction time isn't needed. Instead in a traditional solid state amplifier, very narrow current pulses can occur, and cause damage.

If the FETs are very fast and the frequency isn't too high, it's possible to run them with square wave drive and no bias. In fact that's the normal way to work in switchmode power supplies and switching audio amplifiers. But when the frequency is high, the delay times of the FETs would prevent operation. That's when a bias needs to be used, along with controlled drive. That allows to eliminate the delay times. Bridge circuits will require the bias source for the "high" FET(s) to be floating, but this is easy to do, because no significant bias current is needed, just a voltage.

> Also, I wonder if a CW-only amplifier using
switch mode techniques would be significantly easier to implement than a
purely linear design?

Certainly! If the requirement for linearity is dropped, things become easy. Little more than the circuit described above, and a power supply, would be needed. In the simplest case, the power supply can be a rectifier bridge across the 240V line, followed by a filter capacitor. For good measure you can throw in an inrush limiter and an RFI filter before the rectifier. And if you want to be good, not cheap, then instead of the inrush limiter you can include a power factor correction circuit. It's not terribly hard to make, and it will provide a roughly regulated output voltage, along with near perfect power factor, meaning that you don't need to oversize fuses, breakers, wires, etc. Such an amplifier can produce 1500W output, while being powered from a 2000VA gasoline generator. Amplifiers without power factor correction can't do that, let alone class AB amplifiers!

Such an amplifier would be usable for CW, FM, RTTY and a few more modes, but not for SSB, AM, nor PSK31.

I don't think there is a huge market for such some-modes-only amplifiers. Most hams want to have equipment that covers all modes, even if they operate mostly in a single mode. To add all-mode capability to the amp just described, wrap-around linearization of some sort is needed, and there things get somewhat complex, even if not expensive.


Visit my hobby homepage!
Amps mailing list

<Prev in Thread] Current Thread [Next in Thread>