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Technical review

How to select a solid-state HF amplifier ? (I)

If you like digital gears and if complain about performances of your antenna system, in this case the purchase of a solid-state HF amplifier can be a good solution to work DX stations in better conditions. 

I do no mean that your amplifier might replace a defective antenna. Far from me this idea. Do never use a high power to compense the poor performances of an antenna. This is indeed the best way to create QRM. Build or purchase first a better antenna !

If fundamentally a solid-state amplifier ensures the same functionalities as a tube amplifier, its architecture is quite different as well as the intensity of voltages and currents flowing in its circuits.

Tube and a solid-state amplifier : differences

Not only currents and voltages are differents in a tube and a solid-state amplifier but the load resistance differs also in both designs. So if a tube amplifier requires a RF tank circuit (Pi-L network) to transform the load resistance in 50-ohm, due to the low load resistance of a solid-state PA module, a broadband architecture is requested using ferrite-core transformers. As the input-transformer primary presents a 50-ohm resistive load, tuned input networks are no more necessary because the exciter always sees a 50 ohm resistive input; on other words, an auto-tuner is not required to drive a solid-state amplifier.

In the hearth of the Yaesu VL-1000 Quadra amplifier : four pairs of MRF-150 MOSFET power transistors to develep 1 kW PEP out.

To get 1 kW PEP output, you need first twice that power in DC input (see below), then  several pairs of RF power devices, usually MRF-150 MOSFET power transistors, each of them being a larger version of the PA stage that we find usually in HF transceivers. If a tube amplifier uses only one PA module, the tube(s), a solid-state amplifier is always push-pull to reduce even harmonics (due to low Q).

In a kW-class amplifier we find an hybrid-transformer power-splitter. The splitter and combiner can for example be 5-ports circuits, 1 input and 4 outputs, the combiner having 4 inputs and 1 output.

Contrarily to the tube amplifier, in which a tuned circuit like a Pi-L network helps reducing harmonics, a solid-state amplifier uses low-pass filters (LPF) behind the combiner output to remove harmonics and spurious emissions to a level less than 46 dBc (up to -60 dBc). Its output is routed via the output T/R relay to a reflectometer for security purposes, and then to the antenna connector. This last entry is wired via the input T/R relay to the input port of the splitter. All this building makes a broadband, self-tuning solid-state HF amplifier.

For short, a solid-state amplifier is constituing of the following modules : a PA stage, a controller board, an ALC, low-pass filters, an optional (automatic) antenna tuner, a band-switcher, input interfaces, and the power supply unit. Let's review each of these modules.

Characteristic

Tube amplifier

Solid-state amplifier

PA module :

RF power devices :

RF power voltage :

RF power currents :

Load resistance :

Matching network :

RF input circuit :

Match to 50-ohm load :

Tube(s)

1 to 4 tubes

High (4 kV)

Moderate

2000 ohms on anode

Q = 12

Broadband or broadly-tuned

Pi-network

Transistors

4 or 8 pairs of RF power transitors

Low (50 V)

High (40 A)

3 ohms on collector

Q 1

Broadband

Ferrite-core transformers

PA stage

Linearity and efficiency

As explained in pages dealing with amplification classes, the main limiting factor of power devices is the constancy of power gain over the entire power-output excursion. Generally, MOSFET transistors will exhibit superior linearity as compared to older bipolar junction transistors (BJTs).

Linearity vs. temperature.

Due to the higher voltage excursion, amplifiers powered between 40-50V DC (thus desktop models like Yaesu VL-1000 Quadra) exhibit also considerably better linearity, and thus lower IMD, than units powered on 13.8V DC, usually portable, because the longest is the output/drive power curve, the better is the linear portion. In addition, it is more difficult to design a power supply  for a 13.8V amplifier due to high current requirements (e.g. typically 80A peak for a 500W PEP amplifier like Ameritron ALS-500MX). So usually a tube amplifier is more linear than any solid-state model.  

At last the power output is determined by the maximum ratings of RF power transistors, and by the linearity of these devices. Usually these latter are arranged in push-pull pairs, each module being able to sustain 150W or 250W output.

Due to these specifications, you will find 4 push-pull pairs of 150W power transistors in the Yaesu VL-1000 Quadra or Tokyo Hy-Power HL-2KFX amplifier to get 1.2 kW PEP output (for 2.1 kW DC input).

Power gain

The power gain of a RF transistor, defined as the ratio of RF output to RF drive power, decreases as frequency increases, MOSFETs offering a power gain a bit higher than BJTs (at 14 MHz, respectively 12 dB vs 10 dB). That means that to drive a 1 kW amplifier, BJT's requests input power levels of 100W against 65W only using MOSFETs. If the second model is more efficient, in all cases any solid-state transceiver will drive a kW-class amplifier to full rated power.

Cooling system

A solid-state class AB amplifier yields an efficiency of 45-50%. In a model rated at 1200 W PEP there are only 600 W output, the "missing" watts being transformed in heat (like a car engine). Due to the high power of RF devices, in emission this heat is dissipated in the amplifier case. Therefore a quality PA stage requires a very efficient cooling system, able to maintain the case at nominal temperature.

To respect good RF practices and to prevent an automatic shutdown of the amplifier, the PA cooling system must maintain the amplifier case at nominal temperature, around 80C (176F) for at least 30 min in SSB or 10 minutes in key-down CW transmission at rated output. This is accomplished using a heatsink or a heat-dissipator system, both blowing forced-air to maintain transistors at low temperature. In a tube amplifier the heat generated by tubes is simply dissipated in the air and blow out by a fan.

If you mainly use your amplifier for contests and hunting DX stations, or at high ambient temperatures (tropical climate) you might need a more powerful air circulation, able to dissipate much more heat. In this purpose, some manufacturers like Yaesu specify the power-output rating as ICAS, what can mislead to de-rating the duty cycle of power transistors. 

CCS vs ICAS 

Continuous Commercial Service (CCS) covers applications involving continuous operation in which maximum dependability and long life are the primary considerations.

Intermittent Commercial and Amateur Service (ICAS) is defined as a service including the many applications where the transmitter design factors of minimum size, light weight and considerably increased power output are more important than long tube life. In this service, life expectancy may be one-half that obtained in Continuous Commercial Service. 

However we should all known that above the "maximum ratings", it usually result in catastrophic failure. Therefore power devices running all the time should refer to the more conservative CCS ratings. In the same way amateurs should never force the power stage of their amplifier too high to preserve its lifetime. Keep in mind and we will repeat it again, that the nominal power of your transceiver (usually 60-80W) is far enough to drive your amplifier at the rated power without over-heating the system.

Inside this Yaesu VL-1000 Quadra amplifier we discover in front of the fans the large heatsink that preserves the case to exceed a temperature of 80C (176F) nominal.

Cooling systems are various, all depending of the amount of heat to dissipate, the technology used and the space available. Most cooling systems use the forced-air method that doesn't require much place and is easy to install. No hamradio product uses liquid cooling, excepting home-made models cooled by oil circulating in a large heatsink. The disadvantage of the liquid cooling are size and technical skills required to install the kit (incorporating the impeller, the fluid reservoir, the tubing, fan and power supplies). To reserve to the few over-clocked computers.

The simplest colling system is constituted of fans blowing air across the heatsink. We can also find cylindrical heatsink used in combination with a fan to move large volumes of air. At last high-ends amplifiers take advantage of thermodynamic heat-exchangers : the power transistors are cooled thanks to the circulation of a refrigerant. Its evaporation cools the devices while the heat generated is transfered outside the case.

A kW-class amplifier requires two or more fans of 500W each, DC-powered. Two should be placed just near the power devices, and another one near inductors and fixed capacitors that dissipate much heat. Two more fans can be installed in the power supply unit. Like in emitters, most fans run only when the temperature reaches a threshold (50C, 122F) or when the amplifier is keyed. 

Knowing that a fan speed can exceed 7000 rpm (like fans used in computers), some low-ends models generate over 60 dB of noise. Hopefully, thanks to ball bearings that noise can drastically be reduced while the fan blows the air at full speed. On others the speed can be adjusted from half speed to full power or the fan is two speeds and is sensed by the increase in airflow temperature.

Thermal protection

A solid-state amplifier must be protected from over-temperature, which affects performances of the PA stage. The least over-temperature reduces RF drive or triggers off shutdown if the transistor case temperature exceeds the operating limit (about 80C). In addition to adequate fan capacity, air intakes and outlets must offer a sufficient area to ensure proper airflow. These specifications must be engineered into the mechanical packaging of the amplifier itself. 

Like in a tube amplifier, during operations no dust can be found on components, and to meet this requirement dust filters must be installed and be easily accessible for cleaning. Sometimes an airflow detector is included in the amplifiers protection system.

Sealed reed relays.

High-speed sealed relays used for fast QSK.

To avoid a premature death of your amplifier, remind you as a thumb rule that in decreasing the temperature of your active components by 10 you can double the lifetime of the power stage. Even if this rule is empiric do never push your amplifier too far, in both power and temperature; nominal working conditions are far better to the maximum. Of course this rule is also valid for tubes amplifiers.

T/R switching

In old amplifiers the T/R (Transmit/Receive) switch is a mechanical relay working at about 15 ms that can not switch fast enough to allow amateurs to operate fast break-in (QSK) in modes like CW, SSB-VOX or AMTOR. Indeed in these applications is it requested that the switching reaches or is below 3 ms. 

To get this speed solid-state amplifiers use circuits made of high power PIN diodes or still better, miniature high-speed sealed relays, which are rated for millions of operations in a life. To prevent "hot-switching" carrier-on timing in inserted in the exciter what delays the application of drive until relays have switched. This method extend the relay life. Some amplifiers offer a feature that "drills" all relays in the amplifier, low-pass filters and optional automatic antenna tuner by operating and releasing them periodically when the amplifier is switched on in idle mode.

2d part

ALC and linearity

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