Classes of Operation: A, AB, B, C, D

When you open up a ham radio transceiver and trace the signal path, you will find amplifier stages everywhere — in the receiver, the driver chain, and the final power stage. But not all of these amplifiers work the same way. The final power amplifier in a Class C CW transmitter is built and behaves completely differently from the audio amplifier driving the loudspeaker, which in turn is different from the preamplifier at the antenna input. The reason for these differences comes down to a single fundamental concept: the class of operation, which describes what fraction of each signal cycle the transistor is actually conducting current.

Understanding amplifier classes is not just theory. It tells you immediately why your transceiver's final stage runs far too hot to touch, why your receiver front end draws only a tiny amount of current even at full volume, and why the SSB final amplifier must be more linear than the CW one. Every design decision in a radio — efficiency, heat, distortion, power output — flows directly from the choice of amplifier class.

Conduction angle comparison for Classes A, B, AB, and C. Class A conducts for the full 360°; Class B for 180°; Class AB for slightly more than 180°; Class C for less than 180°.

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The Conduction Angle Concept

Imagine a transistor connected in an amplifier circuit driven by a sine wave. During the positive half of the sine wave the input voltage rises above a threshold, and during the negative half it falls below it. Whether the transistor is conducting — passing collector or drain current — at any given instant depends on where the transistor's bias point is set relative to that input waveform.

The conduction angle is the portion of each 360° cycle during which the transistor is actively conducting. If the transistor conducts for the entire cycle, the conduction angle is 360°. If it conducts for only half the cycle, the conduction angle is 180°. If it conducts for only a narrow pulse at the peak of each cycle, the conduction angle might be 90° or less. This one number — the conduction angle — determines almost everything about how the amplifier behaves: its efficiency, its linearity, the distortion it produces, and the amount of heat it generates.

The conduction angle is set by the quiescent bias: the DC operating point established at the transistor's input before any signal is applied. Position the bias high on the transistor's transfer curve, inside the active region, and the transistor conducts for nearly all of each cycle (Class A). Lower the bias toward the transistor's cut-off region and you progressively reduce the conduction angle. Push the bias right at cut-off and you get Class B (conduction angle ≈ 180°). Push it below cut-off so the transistor is normally off and only turns on briefly at signal peaks, and you get Class C (conduction angle < 180°).

Class A: Maximum Linearity

A Class A amplifier biases the transistor at the centre of its active operating region. The transistor is always conducting — even when no input signal is present — because the bias current holds it in a state of partial conduction. When the input signal swings positive, the transistor conducts harder; when the signal swings negative, it conducts less, but it never cuts off entirely. The result is that the output is a faithful, amplified copy of the input for the entire 360° of the signal cycle.

This continuous conduction is what makes Class A so linear. The transistor's output follows its input through the whole waveform without ever hitting the cut-off region (where it would clip the negative peaks) or the saturation region (where it would clip the positive peaks). The relationship between input and output is as straight as the transistor's transfer characteristic allows, which means harmonic distortion is very low.

The price for this linearity is efficiency. Because the transistor is always conducting, it always draws current from the power supply. Even with no signal present, the transistor is burning power — turning it into heat. The theoretical maximum efficiency of a Class A amplifier is only 50%, and in practice it is often worse, perhaps 25–35%. For every watt of RF output, a Class A stage may dissipate 2–4 watts as heat. In a low-level stage — a preamp drawing 20 mA from a 12 V supply — this is manageable. In a 100 W power amplifier, it would require an enormous heatsink and a substantial power supply just to throw most of the energy away as heat. For this reason, Class A is used in low-power stages where linearity matters more than efficiency: receive preamplifiers, driver stages, and audio stages in receivers.

Class B: Push One, Pull the Other

A Class B amplifier biases the transistor precisely at the cut-off point — the boundary between conducting and non-conducting. In this state the transistor draws virtually no quiescent current when no signal is present. When the input signal swings in the direction that forward-biases the transistor, it conducts; when the signal swings the other way, the transistor cuts off completely. The conduction angle is approximately 180°.

The consequence is that a single Class B transistor can only reproduce one half of the input waveform. The other half produces zero output. The resulting output waveform is a half-wave rectified version of the input — which is obviously useless for amplification because it is severely distorted.

The solution is to use two Class B transistors: one handles the positive half-cycles, the other handles the negative half-cycles, and their outputs are combined to reconstruct the complete waveform. This is the push-pull arrangement. One transistor "pushes" (sources current into the load), while the other "pulls" (sinks current from the load), alternating each half-cycle. When matched and combined correctly, the full sine wave is reconstructed. Push-pull amplifiers are covered in detail in the M09H lesson.

The efficiency improvement over Class A is dramatic. Because neither transistor conducts in the absence of a signal, and each only conducts for half the cycle when a signal is present, the quiescent power drain is negligible. The theoretical maximum efficiency of Class B is 78.5%, a vast improvement over Class A's 50%. For large power outputs — audio finals, RF power stages — this efficiency advantage is significant. A 100 W amplifier that is 75% efficient dissipates only 33 W as heat; the same stage at 33% efficiency would dissipate 200 W as heat. The heatsink and power supply requirements are correspondingly different.

Class B has one important flaw: crossover distortion. At the transition point where one transistor stops conducting and the other starts — the "crossover" from positive to negative half-cycles — both transistors are momentarily off, and there is a brief period of zero output. This creates a notch in the output waveform right at the zero crossing, which produces audible distortion in audio applications and spectral splatter in RF applications. Class AB was developed specifically to address this problem.

Crossover distortion in a Class B push-pull amplifier. The dead zone near zero volts, where neither transistor is conducting, creates a notch in the output waveform.

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Class AB: The Practical Compromise

Class AB is the workhorse of practical amplifier design. The bias is set slightly above the cut-off point — far enough above cut-off that both transistors conduct a small overlap current around the zero crossing of the signal, but nowhere near the centre bias of Class A. The conduction angle is slightly more than 180°.

The overlap current near zero crossing eliminates the notch that causes crossover distortion. Each transistor takes over from the other smoothly because there is always at least one of them conducting at any instant. The distortion level is dramatically lower than Class B while the efficiency remains much better than Class A.

In practice, the efficiency of a Class AB amplifier falls between Class A and Class B — typically in the range of 50–70% depending on the exact bias point and signal level. At low signal levels (where the signal rarely drives the transistors into high conduction) efficiency drops toward the Class A end of the range. At high signal levels (where both transistors are frequently in high conduction) efficiency rises toward the Class B end.

Class AB is the standard for linear power amplifiers in ham radio. Your SSB transceiver's final amplifier stage is almost certainly Class AB. It is also universally used in audio power amplifiers, from cheap portable radios to high-end hi-fi equipment. The entire market for audio ICs such as the LM386, TDA2030, and TPA3116 produces Class AB outputs. When you see a transistor audio amplifier with a diode (or two diodes) connected between the bases of a complementary pair, that diode is providing the small forward bias current that places the circuit in Class AB rather than Class B — this small detail is what eliminates crossover distortion.

Class C: Maximum Efficiency for RF

Class C is the opposite extreme from Class A. The transistor is biased below cut-off — it is normally turned off, and it only conducts for a brief pulse at the peak of each positive input half-cycle. The conduction angle is less than 180°, often as low as 90° in high-efficiency designs. Because the transistor is spending most of each cycle completely off, drawing no current at all, the efficiency is very high — theoretically 100% for a conduction angle approaching zero, and practically 70–85% in real circuits.

The distortion produced by a Class C stage is enormous. The output is not a sine wave; it is a series of narrow current pulses, one per RF cycle. These pulses contain the fundamental frequency plus rich harmonic content — second harmonic, third harmonic, fourth, and beyond. If you were to connect a loudspeaker to a Class C amplifier's output and feed it audio, the result would be unintelligible. So why is Class C used at all?

The answer is the resonant tank circuit. In an RF power amplifier, the output of the Class C transistor is not connected directly to the antenna; it is connected to a parallel LC tank circuit tuned to the desired operating frequency. The tank circuit is a flywheel of energy — each current pulse from the transistor delivers energy to the tank, which then oscillates sinusoidally between its capacitor and inductor, delivering a smooth, clean sine wave to the antenna. The tank's Q factor (selectivity) determines how much of the harmonic content is filtered away. A well-designed tank circuit presents very low impedance at harmonic frequencies and very high impedance at the fundamental, so essentially all harmonic energy is filtered out and only the clean fundamental frequency reaches the antenna.

This is an important insight: the harmonic distortion created by Class C operation is not a problem in an RF stage because the tank circuit removes it. The distortion that does remain is controlled by the Q of the tank. High-Q tank circuits produce extremely clean output. This is why Class C is the preferred choice for CW and FM transmitters, where constant-envelope signals (signals where the amplitude does not carry information) are the norm. The transmitted signal's amplitude never changes (it is always at full power), so nonlinearity in the amplifier causes no harm to the information content.

Class C cannot be used for SSB or AM signals, where the amplitude of the output must faithfully follow the amplitude of the modulating signal. A Class C stage would clip and compress the amplitude variations, destroying the information. This is why SSB transmitters use Class AB: they sacrifice some efficiency to maintain the linearity that SSB requires.

Class C amplifier with output tank circuit. The transistor conducts brief pulses (top waveform), but the tuned tank circuit oscillates at the fundamental frequency, delivering a clean sine wave to the load (bottom waveform).

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Class D: Switching Amplifiers

Class D takes the concept of efficiency one step further by making the transistor operate as a pure switch — either fully on (saturated, very low voltage drop) or fully off (no current). There is no linear operating region at all. The input signal is converted to a high-frequency pulse-width modulated (PWM) square wave: at high input amplitudes the pulse is wide; at low amplitudes the pulse is narrow. The transistor switches at high speed (typically 200 kHz to several MHz) between fully on and fully off, following the PWM pattern.

The output of a Class D stage is then low-pass filtered to remove the switching frequency, recovering a reconstructed version of the original audio signal. Because the transistor is never operating in the linear region — where it is both conducting current and sustaining a voltage across it simultaneously — it dissipates very little power. Theoretical efficiency approaches 100%; practical Class D amplifiers achieve 90–95%, which is why they are used in portable equipment, high-power audio amplifiers, and increasingly in digital modes radio equipment.

Class D is now the technology behind most modern audio power amplifiers, from the tiny ICs in portable Bluetooth speakers to the high-power amplifiers in PA systems. In ham radio you will encounter Class D in switching power supplies (where the concept originated), in some digital-mode power stages, and in linear amplifier control circuits. Class D audio amplifiers are also used in modern transceivers for driving loudspeakers, where they dramatically reduce power consumption compared to Class AB audio stages.

A variant, Class E, is used in high-efficiency RF power amplifiers. Class E uses a carefully designed switching waveform and reactive network that ensures the transistor switches with zero voltage across it (zero voltage switching, or ZVS), eliminating the switching losses that reduce Class D efficiency in RF applications. Class E amplifiers can achieve efficiencies of 90% or more at RF frequencies and are used in QRP (low-power) transmitter designs and experimental high-efficiency power stages.

Comparison Table and Ham Radio Applications

Worked Example: Efficiency Calculation

Understanding efficiency calculations is essential for practical radio work. Here is a worked example comparing Class A and Class C stages producing the same RF output power.

A Practical Note: Reading Transceiver Schematics

When you look at the block diagram of a modern HF transceiver, you will typically see the signal chain labeled with the class of operation at each stage. The front end (LNA) is usually Class A. Driver amplifiers are often Class A or Class AB. The final amplifier stage for SSB/CW operation is Class AB (linear). The same final for FM operation may switch to Class C (nonlinear, but the constant-envelope FM signal is not harmed). Some transceivers have separate driver and final stages for SSB/CW versus FM, while others use switchable bias to change the operating class. Recognising these class designations on a schematic tells you immediately what kind of stage you are looking at and what problems you might expect if it fails.

A common fault in ham radio power amplifiers is a shift in bias point that moves the stage out of its intended class. If the bias resistor or bias diode in a Class AB final fails open, the stage drops to Class B or below — crossover distortion becomes audible as a harsh, raspy quality on your SSB signal. Reports of "bad audio" from other operators often trace back to a bias fault. Conversely, if the bias is set too high, the stage moves toward Class A: the quiescent current rises, the transistor runs hot, and the efficiency drops. Both faults are diagnosable by measuring quiescent current with no RF drive applied.