Contacter l'auteur / Contact the author

Recherche dans ce site / Search in this site

 

 

 

 

 

 

Technical review

Class AB (IV)

In Class A we have discovered that the ouput power is quite limited and we cannot reach high signals while in Class B the ouput signal is more powerful but yield no linearity, and both classes show a too low efficiency.

As amateur radio we need power, 100W in input and up to 2 kW ouput (that depends of your national regulation), we need a linear signal and efficiency. To get these results, electronicians invented the Class AB and its derivated Classes AB1 and AB2. Today all linear amplifiers used by hams belong to Class AB which efficiency does not exceed 50%, there are tubes or solid-state amplifiers.

These amplifiers use power devices that are are slightly polarized so that they work in the linear region of the load line for weak signals (Class A) and in Class B for stronger signals. In this way we avoid cross-over distortions as the working point of transistors or tubes can be placed a bit higher than the cut off at the lower end of the active region.

This polarization is less than in Class A, allowing the transistor or tube to emit very few current when there is no signal in input. But as they are somewhat, the linearity is modified. Therefore in all amateurs amplifiers the output circuit is completed with a filtering function to cut all harmonics produced in this process.

The amplification factor of AB2 triodes

The popular 8802/3-500Z triode (mainly found in Kenwood TL-922 and Heathkit SB-220 amplifiers) has an average amplification factor of 130 (Eimac) to 200 (Amperex). The Amperex version appears to be electrically equivalent to the 8163/3-400Z with the exception of the anode dissipation rating. The maximum-input rating of the Svetlana 3-500Z is 110 MHz. 3-500Zs work well above 110 MHz if the power is de-rated (usually to 60%) as frequency increases. Other types of modern tubes amplifiers commonly used in HF amplifiers have an even higher amplification factor and a frequency rating of up to 500 MHz. The discontinuing 8874 is a good example of a high gain, 500 MHz triode. It has an average amplification factor of 240 ! This is definitely a high-m triode.

Class AB is subdivided in Class AB1 and AB2 depending the power stage characteristics :

Class AB1 concerns a power stage that does not absorb current from the previous stage; this is usually the case with vacuum tubes without grid current and the Field Effect Transistors, the famous FETs. This stage yield usually a high impedance.

Class AB2 concerns a power stage that absorbs some current from the previous stage; some vaccuum tubes are used in this manner and all the classic bipolar transistors.

In the field it is easy to recognize a ham using a Class AB amplifier so-called linear. When speaking some of his sentences are grinded and works lack in the conversation. This effect occurs when the polarization is too low, with for consequence to transmit only the peaks of modulation; the current is transmitted during less than half of a wave of the output signal. To solve this problem this ham has to increase the quiescent current.

Classe C

A Class C amplification is requested each time that we need a powerful signal. With a short pulse at the input, the amplifier yield a high power. The signal being biased with negative voltages, the working point is far beyond the cut off point as displayed in the graph at right. The resulting output signal is strongly non-linear and the its missing part has to be recovered with a resonant circuit that converts the collector current pulses into a continuous sine wave. The collector current pulses contain also many harmonics which are filtered out by the tuned output circuit.

Due to these drawbacks, such amplifiers are not used in SSB transmitters. They are mainly use to increase the RF power level in AM transmitters, the final stage being modulated. It is also use to multiply frequencies by connecting a resonant circuit tuned to some integer multiple of the input frequency in the output. 

The efficiency of a Class C amplifier is very higher, reaching 75% in best cases, but this mode generates strong cross-over distortions too.

Other classes

To be complete there are several other amplifications classes, Class D, E, G and H. They are mainly use for audio amplifications by manufacturers as well-kown as Sony, Hitachi and other Soundcraftsmen.

In Class D used by Sony, the power stage works with square waves which width varies according to the signal. This modulation is obtained with a high frequency triangle wave (>500 kHz) mixed with the audio signal. At every intersection, a wing of the square wave is obtained. Strong filtering is necessary to suppress the high frequency before it reaches the loudspeakers and destroys them ...

Although the efficiency is this kind of amplification is absolutely exceptional and does not heat much, it presents so much problems mainly with the filtering that Sony has practically gave up the project; they only produce a few amplifiers of very high power for professionals (up to 1.2 kW).

Class D and Class E concern power amplifiers. They use transistors as switches to produce a square-wave output at the carrier frequency. As previously tuned circuits are used to eliminate harmonics and match impedances. Switching power amplifiers are the most efficient because they can achieve efficiencies of 90 to 98%.

Sine wave

Square wave

Triangle sweep

TIP31A BJT transistor

Class G is a variant of a Class B combined to a Class C where an additional stage of tubes/transistors with its own PSU work only during music peaks. This idea came after having noticed that the peaks in a musical signal are very short in duration, hence the saving of the power supply. Hitachi provides such amplifiers that know the same success as the Class D.

At last the Class H developed by Soundcraftsmen, is a class G where the voltage of the additional stage power supply varies with the signal.

Efficiency

There is always a trade-off between amplifier efficiency (the ratio of RF output power to DC input power) and linearity. HF amplifiers are generally operated Class AB (180 < conduction angle < 360). This is a compromise between Class A (360, most linear, least efficient) and Class B (180, most efficient, least linear owing to crossover distortion). The devices are biased on to a standing current sufficient to minimise crossover distortion. It is possible to adjust the bias for minimum intermodulation distortion of 3d order (IMD3) by performing a 2-tone test at full rated PEP, and observing IMD3 on a spectrum analyser during the adjustment procedure.

Do I need or not of an amplifier ?

Imagine that you live near a wide open field on a very conductive soil. You own a good high-end 100W transceiver equipped with all necessary filtering options to suppress QRM; you own a good antenna system, probably a beam or a performing wire antenna (long dipole or large vertical with radials). All is fine until you try to work some far DX stations located in the middle of nowhere, to say over 10000 km away. Sometimes you are unable to work them if they not bear their antenna in your direction.

Kenwood TL-922, one of the best kW linear, efficient, very robust, it is quasi silent. It is no more available except on the second-hand market for a price ranging between 500-1600.

Idem with pileups or during days of low propagation : you have sometimes difficulties to work DX stations or to get good reports over 53-55. That problem occurs sometimes with near stations, located 3000 km away.

Sometimes indeed you call, and you wait, wait, and wait to work an interesting station. In fact your signal is probably sunk and lost among the ones of hundreds more powerful stations (nearest or more powerful) trying to work that DX too. When you work in bad propagation conditions, your signal has difficulties to reach the other side of the Earth and probably vanishes at some distance of its objective. How to improve this situation ?

This problem is not due to your antenna farm that can be directive enough to work that station if you were alone to work it on the frequency or when bands are wide open. In such conditions your correspondent, whatever his location, receive you, but too weakly to get a clear and loud signal. On your side you hear him, not always loudly, but at least 53 or with a strong QRK.

So the sole parameter you can still change is either to work in CW or increase your emitting power in SSB. Beside your 100W PEP barefoot transceiver you can add a good amplifier, up to your class limit, 400W, 1 kW or more, depending your national regulation. Good news, you will tell me !

Indeed, when all your ham shack is already invaded with radio gears, SWR and accessories, that your antenna farm is performing, proved by your hundreds QSL received from far DX stations, we can say that you have a big gun. Only in this case the last thing to buy is well an amplifier. Thanks to it you will not be in a position to increase your correspondent's signal, that you usually hear 53 or louder with your beam, but at least you could reach DX stations with ease with the hope to be heard and to work these stations. But at the condition to use your amplifier properly. What we are going to discover.

Good practices

If your RTX is not equipped with performing filters to reduce QRM, if your antenna yield a low gain or is omnidirectional, it will be more useful to improve their performance that using an amplifier. In all cases adding an amplifier to your current installation will probably not help you much in working far DX stations that arrive 53 or not at all to you. Improve first your antenna system, then add some filters (Collins, etc), change maybe your transceiver for a more complete model and at last, after one year of experimentation, see if an amplifier is really necessary.

Many experimented amateurs will tell you too that with a beam and a 200W PEP high-end transceiver you do no more need of a kW amplifier to work the world. In this case the amplifier has to be seen as a useful option to improve the quality of your signal, to get a stronger QRK to your correspondent, but not really to be heard by a far DX station as usually they all hear you with your actual installation, the beam ensuring the most of that work.

Going up from 100W to 1 kW PEP you increase your signal of only 10 dB. And practically you cannot get higher powers (10 kW or so). So to gain some dozen or hundreds watts or a few S-units and be able to work some far DX stations, the best thing to do is first to optimize your installation to get the higher output as possible is reducing all loses : use a low loss coaxial line, work with an antenna well tuned, and able to sustain high power, get the lowest VSWR as possible, use an antenna offering low take-off angles, etc. These are as much features that will increase performances of your installation for a given power. These advice are wiser than using an amplifier at all costs and be unable to hear your correspondent.

Using an amplifier requests to understand how work such a device, you need to read some books about that subject and about accessories (antennas, line, SWR, etc), and try to find a near friend or a radio club to whom you can find anwers to your questions.

When the power does not help to work DX stations. This screendump displays the waterfall spectrum of two kw-stations (EA then RA above the center) calling VK3MO, Ian, on 20 meter on June 2015 compared to the spectrum of Ian (below) using a 20 element stacked beam and 100W. Clic on the image to listen the two kw-stations calling and not taken and clic here to listen VK3MO in QSO with W3WC some minutes earlier, supporting QRM with much calm and patience.

In a few words, if you badly tune your system, you will make QRM in broaden your working frequency, you will overheat both your transceiver and amplifier, and you will probably saturate your tubes without working more stations. This does not respect the ham spirit and is not very useful.

Overdriving your amplifier, you can also damage it, dissipate much heat and, worst, due to the high power generate QRM that your neighborhood could detect while watching TV and not appreciate at all.  

You have also to know that a HF amplifier is an expensive accessory, usually almost as expensive as your transceiver if not more if you want an auto-switching and automatic model. High-ends are also cumbersome (45x40x10cm) and very heavy (over 30 kg). Know also that for the price of a good kW amplifier you can buy a rotor and a small beam or even stacked beams.

So before deciding to buy an amp, check twice if all your equipment cannot be improved in a way or in another and if you own all accessories to check the output power (an external SWR-meter up to 1 or 2 kW, a dummy load, etc), and begin in reading technical documentation about amplifiers and how shortwaves propagate in lines and antennas. Last but not least, do never use your amplifier to run over the others amateurs.

For more detail I suggest you to read my two others pages dealing with the tube amplifier and the solid-state amplifier.

It's QRO !

For more information

How a Vacuum Tube Works, Svetlana

ND2X's power amplifiers pages, Paul S. Goble, III

Transistors, by Williamson Labs

Semiconductor amplifiers, by Deutsche Welle

Frank's Electron Tube Data Sheets

Eimac power tubes catalog

How amplifiers work, by How Stuff Works

Amplifier basics, by Rod Elliott - ESP

Amplifiers, by GSU

RFParts

Back to Menu

Page 1 - 2 - 3 - 4 -


Back to:

HOME

Copyright & FAQ