[Amps] Solid State Amps

[Manfred Mornhinweg]

Lee,

 > 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.


David,

 > 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!


Jim,

> 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.

Manfred

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