This selection of power amplifier classes is used to amplify AC signals. Letter symbols distinguish the types of amplifiers and indicate the expected performance and characteristics. The main classes of amplifiers, such as A, B, AB and C, refer to the time when the amplifier operates or conducts, expressed as a fraction of the period of the signal supplied to the input: Class A amplifiers conduct the signal throughout the period; class B only for half of the signal period, class AB is between A and B; and class C spends much less than half of the signal period (Figure 1). Other classes show methods of switching voltages in rails or rail modulation or pulse-width modulation. Specifiers should be on hand; amp manufacturers sometimes create a new letter symbol for their own design.
Amplifier basics
Class A amplifiers can be one-stroke or two-stroke. In both cases, the Class A amplifier switches conduct the entire 360-degree signal cycle. In the case of two-stroke Class A amplifiers, both devices conduct a full signal cycle, but one handles the bulk of the work during the positive part of the cycle and the other during the negative part of the cycle. Compared to single-stroke designs, two-stroke amplifiers have less distortion and are less susceptible to hum. Class A amplifiers are believed to provide the highest level of sound quality. The trade-off for high quality is low efficiency, and practical designs provide efficiencies of 15 to 35 percent.
Class B amplifiers use a two-stroke topology so that only one device conducts simultaneously, with one covering part of the signal +180 degrees and the other -180 degrees. Class B amplifiers can provide efficiencies of more than 75%, but they suffer from high levels of cross-distortion. Distortion interference is an artifact of delay switching between switches that process the positive and negative parts of a signal. The problem of cross-distortion has limited the use of Class B amplifiers.
Class AB seeks to combine the advantages and minimize the disadvantages of amplifiers A and B. Class A amplifiers increase the period of conduction cycles from 180 degrees to 181-200 degrees, depending on the implementation. This effectively eliminates the problem of cross-distortion, providing an efficiency of up to 70%. Some AB implementations operate in Class A mode for low power levels, improving sound quality without compromising power dissipation.
Class C amplifiers are the most efficient, a maximum of 80% when used to amplify radio frequency signals. However, they have a high level of distortion and are not commonly used in audio applications. There are two modes of operation of the Class C amplifier; tuned and not tuned. Well-adjusted work is less efficient, but has less distortion compared to unsettled work. In Class C designs the conductivity angle is less than 180 degrees.
The G&H classes are informal
G&H amplifier classes embody variations in class AB performance and are not generally recognized as separate amplifier classes. Class G amplifiers use voltage switching in the rails to improve class AB performance (Figure 2). When operating at low power, the system uses lower voltage rails than the comparator rated AB amplifier to reduce power consumption. When the need for output power increases, the amplifier switches to a higher voltage bus.
Class H amplifiers take the concept of voltage rail optimization one step further and modulate the voltage rail to match the required instantaneous converter power (Figure 3). Improving the efficiency of Class G and H amplifiers is due to a significant increase in complexity. As a result, these AB class variants are only used with high-power amplifiers, where efficiency gains significantly affect thermal dissipation and energy costs.
Classes D and DG provide the highest efficiency
Class D amplifiers are designed using either an output topology with a half-bridge or full bridge and can provide efficiencies of 90% and above. The most common method of Class D modulation is pulse-width modulation (PWM) using a sawtooth (or triangular) oscillator. The base Class D half-bridge amplifier includes a pulse-width modulator, two MOSFET outputs with driver circuits, and an external low-pass filter to recover the amplified audio signal. The Class D full-bridge amplifier combines two half-bridge stages for differential load control. Class D amplifiers typically use switching frequencies between 250 kHz and 1.5 MHz. The output is a square wave that is modulated by the PWM using an input audio signal.
The DG Class Amplifier uses a multi-level output stage (similar to the Class G amplifier) ​​and switches between the power rails as needed to increase efficiency higher than the Class D base amplifier (Figure 4). In other respects, the DG class acts as a Class D amplifier, using PWM to produce a digital output signal with a variable duty cycle modulated by the input audio signal.
Summary
Not all types of power amplifiers are suitable for audio applications. The wide range of amplifier classes is a reflection of the wide range of application requirements for power amplifiers. Classes A, B, AB and C refer to the time spent by the amplifier output, expressed as a fraction of the period of the signal supplied to the input, and provide very different levels of efficiency and output quality. Classes G and H are higher efficiency options for class AB. Class D amplifiers use PWM techniques to further improve efficiency, while Class DG combines Class D and G attributes to provide the highest level of efficiency.
Literature
Amplifier classesanalog devices
Go to school for RF power amplifier classesMini-schemes
Power amplifier classesWikipedia
Where do you use circuits A, B, AB, D, G, DG and H?
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