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Class A, A/B and D Amplification

Updated: Sep 24, 2023

The choice between Class A, A/B, and D amplifiers often hinges on the application, desired sound quality, efficiency needs, size, and cost. While Class A is loved by many audiophiles for its pure sound quality, Class A/B offers a balanced approach suitable for many consumer electronics. On the other hand, Class D's high efficiency and compact size make it a popular choice for portable devices and certain high-power applications.


Accuphase Class A amplifier
Credit: Accuphase

Class A amplifiers

Class A amplifiers are the most linear and distortion-free amplifiers, but they are also the least efficient. This is because they have a constant current flowing through the output transistors, even when there is no signal. This means that they are always dissipating heat, even when they are not amplifying anything. Class A amplifiers are typically used in high-end audio applications where sound quality is the most important factor.


The output transistors of a Class A amplifier are biased so that they are always conducting a small current, even when there is no input signal. This means that the amplifier is always amplifying the signal, even if it is very quiet. This results in very low distortion and excellent sound quality. However, the constant current also means that Class A amplifiers are very inefficient. They typically only convert about 20% of the power supplied to them into sound output. The remaining 80% of the power is dissipated as heat.


Class A/B amplifiers

Class A/B amplifiers are a compromise between efficiency and distortion. They have a lower current flowing through the output transistors when there is no signal, which makes them more efficient than Class A amplifiers. However, they also have a higher distortion than Class A amplifiers. Class A/B amplifiers are often used in home theater and car audio systems.


Class A/B amplifiers use a technique called push-pull amplification to achieve higher efficiency than Class A amplifiers. In push-pull amplification, two output transistors are used to amplify the signal. One transistor amplifies the positive half of the signal, while the other transistor amplifies the negative half of the signal. This cancels out the even-order harmonics that are produced by Class A amplifiers, resulting in lower distortion.


However, Class A/B amplifiers still produce some odd-order harmonics, which can cause distortion. The amount of distortion produced by a Class A/B amplifier depends on the bias current of the output transistors. If the bias current is too low, the distortion will be high. If the bias current is too high, the efficiency will be low.


Class D amplifiers

Class D amplifiers are the most efficient of all the amplifier classes. They work by switching the output transistors on and off very quickly, which allows them to amplify the signal without dissipating much heat. Class D amplifiers have a higher distortion than Class A or Class A/B amplifiers, but they are still considered to be high-fidelity amplifiers. Class D amplifiers are typically used in portable audio devices and subwoofers.


Class D amplifiers use a technique called pulse-width modulation (PWM) to amplify the signal. In PWM, the output transistors are switched on and off very quickly, with the duty cycle of the pulses corresponding to the amplitude of the signal. This allows the amplifier to amplify the signal without having to pass any current through the output transistors. This results in very high efficiency, with some Class D amplifiers converting up to 90% of the power supplied to them into sound output.


However, Class D amplifiers also produce some distortion. The amount of distortion produced by a Class D amplifier depends on the frequency of the PWM pulses. If the frequency of the pulses is too low, the distortion will be high. If the frequency of the pulses is too high, the efficiency will be low.


Ultimately, the best amplifier class for you will depend on your specific needs and budget. If you are looking for the highest possible sound quality, then a Class A amplifier is the way to go. However, if you are looking for an amplifier that is both efficient and affordable, then a Class D amplifier is a good choice.

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