What do the different classes of amplification mean? Part 1

A work in progress.

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  • What do the different classes of amplification mean?

    You may have seen terms such as “Class D” or “Class H” or “Class AB” and wondered what they mean and whether they have any effect on sound quality. Most of these classes are defined by the Institute of Electrical and Electronic Engineers (IEEE), although for marketing purposes some manufacturers come up with their own letters for particular proprietary variations. We’ll take a look at each of the common amp classes.

    An amplifier circuit requires one or more active devices to be able to boost a signal. An active device is a component that uses a small signal voltage or current to control a larger current flow, like an electronic valve. It could be a bipolar transistor, a field-effect transistor (FET), or a vacuum tube. Audio amplifier circuits commonly use multiple active devices, depending on the nature of the circuit’s function. The amplifier classes’ definitions are based primarily on how the active devices operate on the signal waveform. For the sake of simplicity, we’ll use only bipolar transistors in the circuit examples.

    Class A
    In a Class A amp circuit, the transistor or transistors each conduct current throughout the signal waveform. Because of this, they must be biased (turned partially on) at quiescence (at rest, with no signal) so that their output voltage is about midway between the positive and negative limits.

    Class A circuits can be great for audio fidelity, as there is no need to make transitions between transistors for different parts of the waveform, so there is nothing to cause a discontinuity. The downside is the circuit’s poor electrical efficiency. Because of this, Class A is used mostly in small-signal circuits and very, very rarely in power amp output circuits, with the exception of some boutique “high-end” audio amplifiers. A class A power amp of even a moderate power rating would emit a great amount of heat and would require a very large power supply and huge heat sinks. It could help keep your house warm in winter, though!

    This circuit is a single-transistor class A amplifier stage in a common-emitter arrangement, which provides voltage gain but also inverts the polarity of the signal. (The arrows show how the signal causes variations in the current flow through the circuit.)

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    This circuit is a class A stage in a common-collector arrangement (often called an emitter follower), which does not provide voltage gain but does increase the available output current. Thus, it is useful as a buffer, or voltage follower.

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    Class B
    This topology improves efficiency because the transistor is biased at cutoff. That means that at quiescence—at rest—the transistor conducts no current. As a result, it responds only to signal voltage swings in one direction, either positive or negative.

    In order to reproduce the entire signal waveform, then, we need at least two transistors: one for the positive side of the signal and one for the negative. Typically, designs that use bipolar transistors employ a complementary arrangement of NPN for one signal polarity and PNP for the other. (A few decades ago, when PNP power transistor technology and quality lagged behind that of NPN, it was common for design engineers to use a “quasi-complementary” arrangement of NPN devices for both the positive and negative parts of the signal.) The current drawn from the power supply by each transistor is then proportional to the signal voltage. No signal, no current; small signal, small current; large signal, large current. High efficiency. All is great … except that it generally doesn’t quite work as well as that.

    The problem lies in the transitions between the positive and negative parts of the audio signal. Bipolar transistors arranged this way require a bit of signal voltage just to start to turn on—about 0.6 to 0.7 volt in either direction. Below this threshold, there won’t be enough signal level to turn either transistor on, and even signals that exceed 0.6 volt will be distorted by the discontinuity. This phenomenon is called crossover distortion (named for the crossing over between negative and positive, not for circuits that divide the audio spectrum), and it is particularly nasty sounding; unlike clipping distortion, it affects all signals, but the smaller ones it affects proportionally more severely.

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    There is a solution to this quandary, and it’s called Class AB because it combines some of the characteristics of both Classes A and B. It uses the basic Class B circuit but biases the transistors slightly on at quiescence. The transistors thus conduct a little bit of current with no signal present, but the crossover transitions are much smoother. The proper amount of bias is a balance between keeping the quiescent current low on the one hand and minimizing crossover non-linearity on the other.

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    Class C
    In Class C, the transistor is biased “below cutoff” so that it conducts only during a smaller portion of the signal waveform. This is used in single-carrier radio-frequency amplification, and the short pulse of conduction is usually used to excite a resonant circuit. The efficiency can be extremely high, but it has no practical application in audio.

    Class D
    Class D amplification has been getting much attention in recent years as manufacturers seek ways of achieving ever-higher power and/or efficiency. Unlike Classes A, B, and AB, Class D is completely non-linear. It uses pulse-width modulation to actuate switching transistors and create a high-power pulse train that alternates between a pair of positive and negative rail voltages. The pulse width is proportional to the audio signal voltage. A passive low-pass filter—the "reconstruction" filter—on the output smoothes the pulse train back into an audio signal.

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    Because the switching transistors alternate between being completely on (the transistor has high current flow through it, but nearly zero voltage across it) and completely off (high voltage across it, but zero current through it), the power lost in them is extremely low. The heat generated in normal operation, then, is quite low, and the electrical efficiency is very high.

    Class D does not mean or stand for “digital.” In fact, most class D amps are completely analog, with analog modulation. The PL380 was QSC’s first stand-alone class D amplifier, and now it is joined by the CXD and PLD Series amplifiers. The amp modules in the K Family (K, KW, and KLA Series) powered loudspeakers are class D.

    Classes G and H will be covered in Part 2.
    Bob Lee
    Technical Communications Developer
    QSC, LLC
    Fellow, Audio Engineering Society
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    "If it sounds good, it is good." —Duke Ellington
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