This selection of power amplifier classes is used to strengthen AC signals. The letter symbols differentiate between amplifier types and indicate the expected performance and characteristics. Basic amplifier classes such as A, B, AB, and C relate to the time that the amplifier is operating, or conducting, expressed as a fraction of the period of a signal applied to the input: class A amplifiers conduct through all the period of the signal; class B only for one-half the signal period, class AB is in between A and B; and class C conducts for much less than half the signal period (Figure 1). Other classes indicate voltage rail switching or rail modulation or pulse width modulation techniques. Specifiers need to be alert; amplifier makers sometimes create a new letter symbol for a proprietary design.

Amplifier basics

Class A amplifiers can be single-ended or push-pull topologies. In both cases, the switches in a class A amplifier conduct through the full 360 degrees of the signal cycle. In the case of push-pull class A amplifiers, both devices conduct through the full signal cycle, but one handles the bulk of the work during the positive part of the cycle, and the other handles it during the negative part of the cycle. Compared with single-ended designs, push-pull amplifiers have lower distortion and are less susceptible to hum. Class A amplifiers are considered to deliver the highest level of sound quality. The tradeoff for high quality is low efficiency, with practical designs delivering efficiencies between 15 and 35 percent.

Class B amplifiers use a push-pull topology in a manner that only one device is conducting at a time, with one covering the +180 degrees portion of the signal, and the other covering the -180 degrees part. Class B amplifiers can deliver over 75% efficiencies, but they suffer from high levels of crossover distortion. Crossover distortion is an artifact of the delay in switching between the switches handling the positive and negative portions of the signal. The problem with crossover distortion has limited the use of Class B amplifiers.

Class AB seeks to combine the benefits and minimize the drawbacks of both A and B amplifiers. Class AB amplifiers increase the period of the conduction cycles from 180 degrees to between 181 and 200 degrees, depending on the implementation. That effectively eliminates the problem of crossover distortion while delivering efficiencies up to 70%. Some AB implementations operate in class A mode for low power levels, improving sound quality without sacrificing power dissipation.

Class C amplifiers are the most efficient, with a maximum of 80% when used to amplify radio frequency signals. Still, they have high levels of distortion and are not generally used in audio applications. There are two modes of class C amplifier operation; tuned and untuned. Tuned operation is less efficient, but has lower distortion compared with untuned operation. In class C designs, the conduction angle is less than 180 degrees.

Classes G & H are unofficial

Amplifier classes G & H embody variations on class AB operation and are not generally recognized as separate amplifier classes. Class G amplifiers use voltage rail switching to improve class AB performance (Figure 2). Under low power operation, the system uses a lower voltage rail than a comparably rated AB amplifier to reduce power consumption. As the need for output power rises, the amplifier switches to a higher voltage rail.

Class H amplifiers take the concept of optimizing the voltage rail one step further and modulate the voltage rail to match the instantaneous power envelope needed (Figure 3). The efficiency improvements in class G and H amplifiers come at the cost of significantly increased complexity. As a result, these variations on class AB are only used with high power amplifiers where the increase in efficiency has meaningful impacts on thermal dissipation and energy costs.

Classes D and DG deliver highest efficiency

Class D amplifiers are designed using either a half-bridge or a full-bridge output topology and can deliver 90% or higher efficiencies. Class D’s most common modulation technique is pulse width modulation (PWM) with a sawtooth (or triangle) oscillator. A basic half-bridge class D amplifier includes a pulse-width modulator, two output MOSFETs with driver circuits, and an external lowpass filter to recover the amplified audio signal. A full-bridge class D amplifier combines two half-bridge stages to drive the load differentially. Class D amplifiers generally use switching frequencies between 250 kHz and 1.5 MHz. The output is a square wave that is PWM modulated using the input audio signal.

The class DG amplifier uses a multilevel output stage (similar to a class G amplifier) and switches between supply rails as needed to boost efficiency higher than a basic class D amplifier (Figure 4). In other respects, class DG operates like a class D amplifier using PWM to produce a digital output signal with a variable duty cycle modulated with the input audio signal.

Summary

Not all power amplifier types are suitable for audio applications. The wide range of amplifier classes is a reflection of the wide range of application requirements for power amplifiers. A, B, AB, and C classes relate to the time that the amplifier output is conducting, expressed as a fraction of the period of a signal applied to the input, and deliver widely different levels of efficiency and output quality. Classes G and H are higher efficiency variations on class AB. Class D amplifiers use PWM techniques to enhance efficiency further, and class DG combines the attributes of classes D and G to deliver the highest levels of efficiency.

References

Classes of Audio Amplifiers, Analog Devices
Go to School on RF Power Amplifier Classes, Mini-Circuits
Power amplifier classes, Wikipedia