RF Amplifier Classes A, AB, B and C: What Hams Need to Know
When you read the specification of a linear amplifier or a transceiver’s final stage, you meet RF amplifier classes: class A, class AB, class B, class C and, in the more advanced material, classes D, E and F. The letters look like a ranking from good to bad, but they are really a record of a compromise. Every class trades linearity, which means a faithful copy of the input, against efficiency, which means how much of the supply power reaches the antenna instead of the heat sink. Knowing the difference explains why an SSB amplifier is biased in a particular way, why a CW or FM transmitter can use a more efficient stage, why a splattering signal appears when you overdrive a linear and why every stage is followed by a filter.
The Idea Behind the Classes
The Wikipedia article on power amplifier classes says that the class gives a broad indication of an amplifier’s efficiency, linearity and other characteristics, and that, broadly, as you go through the alphabet, the amplifiers become more efficient but less linear, while the reduced linearity is dealt with by other means. The first classes, A, AB, B and C, are defined by the conduction angle, that is, the fraction of the signal cycle during which the active device passes current. A class-A amplifier conducts during the entire cycle, or 360 degrees. A class-B stage conducts for half of the cycle, 180 degrees, and a class-C stage for much less than half. Class AB lies between A and B.
The reasoning is easy to follow. A device that is always conducting, as in class A, reproduces the whole waveform but also draws current all the time, even when there is no signal, and that current becomes heat. A device that conducts only when it is needed wastes less energy but cuts parts of the waveform, and cutting a sine wave creates harmonics and distortion. In RF equipment the missing part can often be restored by a tuned circuit, which is the reason why classes B and C are used so widely in radio, and much less in audio.
The Classes Compared
| Class | Conduction angle | Maximum theoretical efficiency | Linearity | Typical amateur use |
|---|---|---|---|---|
| A | 360 degrees | 25% in the usual configuration, 50% with a transformer or inductively coupled load | Best | Low-level stages, receiver preamplifiers, drivers, QRP rigs for clean output |
| AB | Between 180 and 360 degrees | Below 78.5% for full sine waves in transistor amplifiers | Good | SSB linear amplifiers and the final stages of most HF transceivers |
| B | 180 degrees | About 78.5% (pi over four) | Fair, often as a push-pull pair | Linear amplifiers with push-pull transistors or tubes |
| C | Well below 180 degrees | Can reach about 80% in RF applications | Poor, needs a tuned load | CW and FM transmitters and constant-amplitude signals |
| D, E and others | Switching operation | Above 90% is achievable in class D with MOSFETs | Needs special methods for linear signals | Switching RF stages, class E in efficient transmitters |
The efficiency figures come from the article and are theoretical limits. Real stages fall below them, and the actual numbers depend on the power level, the supply, the load and the design. They give a fair idea of the order of magnitude, and the order matters: at 25% efficiency, an amplifier delivering 100 watts of RF dissipates about 300 watts of heat, whereas at 60% it dissipates about 67 watts. That difference decides the size of the heat sink and the fan.
A Quick Heat Calculation
To see why efficiency is not an academic number, work out the heat for a 100 watt output at different efficiencies. The supply power is the output divided by the efficiency, and the heat is the difference. At 25% efficiency, a class-A stage draws 400 watts and dissipates 300 watts. At 50%, which is a reasonable figure for a practical class-AB transmitter stage at full power, it draws 200 watts and dissipates 100 watts. At 70%, a good class-C or class-E stage draws about 143 watts and dissipates about 43 watts. A small fan can remove 43 watts, while 300 watts needs a massive cooling system. The same arithmetic explains why battery-powered portable rigs use efficient final stages: the energy that is turned into heat comes out of the battery, and it shortens the time you can stay on the air.
Class A: Clean but Hot
The article notes that class-A amplifiers are good for amplifying the weak signals received by radio receivers because of their low distortion, and that their efficiency is poor because the device conducts all the time and generates heat. In a ham shack, class A is found in receiver front ends, in the first drivers of a transmitter chain and in some low-power transmitters where a clean signal is more important than efficiency. For a QRP transmitter that produces a watt or two, the waste of power is acceptable, and the stage is very simple to design. For a 100 watt amplifier, class A would need a huge power supply and a large heat sink.
Class AB and Class B: The Workhorses for SSB
A single sideband signal has a varying amplitude, and the amplifier must follow the envelope faithfully, or the distortion produces intermodulation products that splatter on the neighbouring frequencies. For this reason the final amplifiers of SSB transceivers and external linear amplifiers are normally biased in class AB, which the article describes as sacrificing some efficiency in favour of linearity relative to class B, and as much more efficient than class A. In class B, two devices work in a push-pull arrangement, each for half of the cycle. The push-pull output recombines the halves, and good transistors with the right bias remove the small distortion at the crossover. At radio frequencies, a single device in class B can also work if the load is a tuned circuit, because the stored energy in the circuit supplies the missing half of the waveform, which is the point made in the article about RF linear amplifiers.
The article adds an observation that applies to our amplifiers: devices operating in class B are used in linear amplifiers, which are called linear because the RF output power is proportional to the square of the input excitation. This is the property that preserves the shape of an SSB envelope. Our guide on choosing an HF linear amplifier covers the practical choices, and the article about peak envelope power and the average power of SSB explains why the amplifier must be able to handle the peaks.
Class C: Efficient When the Amplitude Is Constant
Class C uses a conduction angle well below half the cycle. The article says that distortion is high and that practical use requires a tuned circuit as the load, and that efficiency can reach 80% in radio-frequency applications. It states the typical application: RF transmitters operating at a single fixed carrier frequency, where the distortion is controlled by a tuned load on the amplifier. The input signal switches the active device, pulses of current flow through the tuned circuit and the tuned circuit restores the sine wave. For amateur radio, this fits modes with a constant or keyed carrier: CW, FM and some digital modes with a constant envelope. It does not fit SSB, AM or digital modes that carry information in the amplitude, since a class-C stage does not reproduce the amplitude changes in proportion to the input.
The same article observes that the signal bandwidth of the amplifier is limited by the Q factor of the tuned circuit, and that any residual harmonics can be removed by a further filter. This is why a transmitter that uses a class-C or class-E final needs a good low-pass filter, discussed in our guide to the low pass filter for harmonics and TVI.
Classes D and E: Switching Amplifiers
In class D and E, the transistors act as switches that are either fully on or fully off, instead of linear devices. The article says that the main advantage of class D is power efficiency, with more than 90% achievable using MOSFETs and above 80% fairly common, because the pulses have a fixed amplitude and the switching device is either off or on. Class E, according to the article, is a highly efficient tuned switching power amplifier used at radio frequencies, with a single-pole switching element and a tuned reactive network between the switch and the load, and it gets its efficiency by operating the switch only at points of zero voltage or zero current. Class-E stages appear in modern, compact transmitters and in some QRP designs for CW or FM, where the output has a constant envelope. To carry an SSB signal with these stages, designers use special methods such as separating the amplitude and phase, which is a topic for a more advanced article.
What the Class Means for Your Signal
- Overdriving a linear. A class-AB or class-B linear is linear only up to a certain input level. If you push the drive beyond that, the output saturates and distortion appears in the form of splatter. The ALC and the correct drive setting matter, as explained in our guide to SSB audio, compression and ALC.
- Keying a class-C stage. A hard, fast keying of a stage that switches on and off creates key clicks, which are discussed in the article on key clicks and splatter. The cure is shaping of the rise and fall time of the keyed signal.
- Harmonics. The less linear the stage, the more harmonics it produces and the more filtering it needs. A transceiver with a class-AB final has a low-pass filter bank and a good design shows low harmonics, but the filter is never optional.
- Heat. Efficiency and dissipation are two sides of the same number. A stage that is run at a low drive in class AB can be quite inefficient, and the heat sink and the fan work harder in a long digital mode transmission.
- Operating mode. A CW or FM amplifier can use a more efficient bias, and an amplifier that is advertised as suitable for all modes has to be linear enough for SSB.
Choosing and Using an Amplifier Sensibly
If you are buying a linear amplifier, look at how it is specified. A claim of 1 kW on SSB means the peak envelope power, and the class tells you which modes it can cope with. A class-AB amplifier with a specified IMD performance is the safe choice for SSB and digital modes. For FM repeaters and CW, a simpler amplifier with high efficiency may be adequate. Use the drive that the maker recommends, watch the ALC and the reflected power, and do not trust the amplifier’s own meter more than a good external one. The station is as clean as its worst stage, and a poor setting of an excellent amplifier gives a worse signal than a correct setting of an average one.
The Bottom Line
Amplifier classes describe how a device is biased, and each class is a compromise between linearity and efficiency. Class A is linear but wasteful, with a theoretical limit of 25% or 50% in special arrangements. Class AB and class B, with up to about 78.5% efficiency in class B, are the standard for SSB linears. Class C reaches about 80% in RF use but needs a tuned load and works only for constant-amplitude signals such as CW and FM. Classes D and E switch the device on and off for efficiencies above 80% to 90%. Whatever class is used, the amplifier needs a filter on its output, a correct drive level and good cooling to give you a clean signal.
Frequently Asked Questions
What is the difference between class A and class AB?
Class A conducts for the whole cycle and is linear but inefficient. Class AB conducts for more than half but less than the whole cycle, giving much better efficiency with good linearity.
Which class is used for SSB linear amplifiers?
Normally class AB, sometimes class B in push-pull. These classes reproduce the amplitude variations of an SSB signal with acceptable distortion and reasonable efficiency.
Can I use a class-C amplifier for SSB?
No. A class-C stage does not follow amplitude changes, so it distorts and splatters on SSB and AM. It is suited to CW, FM and other constant-envelope signals.
Why does a class-C amplifier need a tuned circuit?
The stage conducts for only a small part of the cycle, so the output is a series of pulses. The tuned circuit restores a sine wave and suppresses harmonics.
Which amplifier class is the most efficient?
The switching classes D and E, which can exceed 90% in class D with MOSFETs, are the most efficient, but they need special methods for signals that vary in amplitude.
Why does a linear amplifier splatter when I overdrive it?
Beyond its linear range the output saturates, creating intermodulation products. Reduce the drive so that the ALC shows little or no action.