Amplifier Classes

October 20, 2013 in AV Design Tips, The Basics by Sam Davisson

6v6_top_sideAmplifier Classes

When I first started this article I fully intended to finish up Impedance by discussing amplifiers, speakers and the effects of impedance on audio quality. That plan changed. I think amplifier classes deserve to be a post of their own and need to be discussed before any meaningful continuation of the impedance discussion. Besides, thinking about all this along with trying to research to find out exactly what a Class G and Class H amplifier was has given me a splitting headache. 😉

This picture looks eerily like one of my early Heath Kit assemblies and if I remember correctly it taught me the value of keeping a fire extinguisher nearby when working with electronic components you were not familiar with. For those who may not know about Heath Kit, they were the originator of DIY electronic projects. You could by a box of parts and instructions from them to build just about any electronic device of the time.

Amplifier Classes

There are complete books on amplifiers and the differing classes of amplifiers. Im not going into that depth here this is the basics after all. Also, don’t expect a discussion of every class of amplifier in existence. Classes C, E and F are more typically found in RF applications than audio applications.

Class A
In a Class A amplifier, the output devices are continuously conducting for the entire cycle, or in other words there is always bias current flowing in the output devices. This topology has the least distortion and is the most linear, but at the same time is the least efficient at about 20%

Class B
This type of amplifier operates in the opposite way to Class A amplifiers. The output devices only conduct for half the sinusoidal cycle (one conducts in the positive region, and one conducts in the negative region), or in other words, if there is no input signal then there is no current flow in the output devices. This class of amplifier is obviously more efficient than Class A, at about 50%, but has some issue with linearity at the crossover point, due to the time it takes to turn one device off and turn the other device on. In other words, it is not distortion free.

Class AB
This type of amplifier is a combination of the above two types, and is the most common type of power amplifier in use. Here both devices are allowed to conduct at the same time, but just a small amount near the crossover point. Hence each device is conducting for more than half a cycle
but less than the whole cycle, so the inherent non-linearity of Class B designs is overcome, without the inefficiencies of a Class A design. Efficiencies for Class AB amplifiers is about 50%. This amplifier remains the amplifier of choice for the true audiophile.

The above amplifiers are known as linear amplifiers.

Class D
This class of amplifier is a switching or PWM amplifier. In this type of amplifier, the switches are either fully on or fully off, significantly reducing the power losses in the output devices. Efficiencies of 90-95% are possible. The audio signal is used to modulate a PWM carrier signal which drives the output devices, with the last stage being a low pass filter to remove the high frequency PWM carrier frequency.

The primary differences between linear (Class A and Class AB) amplifiers and switching (Class D) digital amplifiers is the efficiency. This is the whole reason for the invention of Class D amplifiers. Linear amplifiers are inherently very linear in terms of performance (therefore very distortion free), but are also very inefficient when it comes to power, whereas a Class D amplifier is much more efficient from a power standpoint but suffer from added noise and distortions due to the switching nature and additional filters.

Classes G & H
Honestly, before I started writing this I had heard of these classes and understood they were "improvements" to the Class AB amp in that they are supposed to provide distortion free audio with much greater power efficiencies. In doing the research for this article I found that I probably had about as clear of an understanding as anyone else.

The general consensus seems to be that Class-G runs from a low voltage rail until the signal goes beyond a certain voltage, and then a higher rail (or rails) is switched in. Class-H refines this to use a variable higher voltage rail (or rails), also known as a modulated rail or use a ‘bootstrap’ capacitor that lifts the rails as needed, but cannot maintain them at the full voltage for more than a few cycles.

Multiple rail amplifiers usually allow significantly higher efficiency than single-rail Class-AB design. The more rail levels are used, the higher the efficiency, assuming they’re spaced properly. While in all reality it is impossible, efficiencies can reach 100 percent with an infinite number of voltage rails.

Multiple rail amplifiers typically use only two voltage rails, which amounts to more than 80 percent theoretical efficiency at maximum power. Using more rails becomes impractical due to added power supply complexity, but with four rail voltages, efficiency at maximum power can reach 90 percent theoretically.

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