Types of Amplifiers

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Types of Amplifiers - Class A Class B Class AB Class D

Those persons serious about their audio go to great lengths to purify whatever sound comes forth from their speakers. They obsess over seemingly small details like cables, location, and even the quality of the electricity feeding their equipment. Some of these obsessions have good reason, some don't. 

One area gaining in importance to may audiophiles and those aspiring to be, is the concept of amplifier operational class, and what that means to the musical reproduction sought by the listener. There are four common classes of amplifier in the high-fidelity reproduction of audio: 

1. Class A
2. Class B
3. Class AB
4. Class D

You may have seen these designations on equipment and not known exactly what was meant by them. Well, we're going to go a ways to demystify the meanings of these classes. 

Let's recap what transistors (and vacuum tubes) do when used as amplifiers in stereo equipment. Speech and music can be represented by complex sinewaves. The object of an amplifier circuit is to increase the amplitude (the peaks of the waveform) of a signal, that is they take a small signal's waveform, and make it bigger, while maintaining the smaller signal's detail (this is called linearity). 

How does a transistor do this? Well, technically, a transistor has three terminals. A current or voltage applied through two terminals creates and controls a larger current or voltage produced through the third terminal. This allows a small input from say, a line level source, to be amplified to power loudspeakers and create the sound reaching your ears. Vacuum tubes are similar in that after the tube warms up, a signal is usually applied to the "grid" of the tube and the resultant output of the same frequency is at a much higher amplitude. 

The key question is "how well do these devices create these larger signals?" 

Classes??
By classes of operation, we mean there are several broad types of amplifier that exist, each with a slightly different operational profile, that impacts how it sounds to the ear of most listeners. Of course, some listeners may disagree with how an amp is characterized sonically, but we are talking about general cases here. 

Class A
This is the most linear of the classes, meaning the output signal is a truer representation of what was imputed. Here are the characteristics of the class: 

1. The output device (transistor) conducts electricity for the entire cycle of input signal. In other words, they reproduce the entire waveform in its entirety.
2. These amps run hot, as the transistors in the power amp are on and running at full power all the time.
3. There is no condition where the transistor(s) is/are turned off. That doesn't mean that the amplifier is never or can never be turned off; it means the transistors doing the work inside the amplifier have a constant flow of electricity through them. This constant signal is called "bias".
4. Class A is the most inefficient of all power amplifier designs, averaging only around 20.

Because of these factors, Class A amplifiers are very inefficient: for every watt of output power, they usually waste at least 4-5 watts as heat. They are usually very large, heavy and because of the 4-5 watts of heat energy released per watt of output, they run very hot, needing lots of ventilation (not at all ideal for a car, and rarely acceptable in a home). All this is due to the amplifier constantly operating at full power. The upside is that these amps are the most enjoyed of all amplifiers. These amps dig out musical detail, since the transistor reproduces the entire audio waveform without ever cutting off. As a result the sound is cleaner and more linear; that is, it contains much lower levels of distortion. 

They are the most accurate of all amps available, but at significant cost to manufacture, because of tight tolerances, and the additional components for cooling and heat regulation.

Class B
In this amp, the positive and negative halves of the signal are dealt with by different parts of the circuit. The output devices continually switch on and off. Class B operation has the following characteristics: 

1. The input signal has to be a lot larger in order to drive the transistor appropriately.
2. This is almost the opposite of Class A operation
3. There have to be at least two output devices with this type of amp. This output stage employs two output devices so that each side amplifies each half of the waveform. [li Either both output devices are never allowed to be on at the same time, or the bias (remember, that trickle of electricity?) for each device is set so that current flow in one output device is zero when not presented with an input signal.
4. Each output device is on for exactly one half of a complete signal cycle.

These amps run cooler than Class A amps, but the sound quality is not as pure, as there is a lot of "crossover" distortion, as one output device turns off and the other turns on over each signal cycle.

This type of amplifier design, or topology, gives us the term "push-pull," as this describes the tandem of output devices that deliver the audio signal to your speakers: one device pushes the signal, the other pulls the signal. They can be less expensive, because one can use two cheap output devices instead of one high-quality one in the design. 

As I mentioned before, the input signal has to be lot larger, meaning that from the amplifier input, it needs to be "stepped up" in a gain stage, so that the signal will allow the output transistors to operate more efficiently within their designed specifications. This means more circuitry in the path of your signal, degrading sound even before it gets to the output stage. 

Class AB
This is the compromise of the bunch. Class AB operation has some of the best advantages of both Class A and Class B built-in. Its main benefits are sound quality comparable to that of Class A and efficiency similar to that of Class B. Most modern amp designs employ this topology. 

Its main characteristics are: 

1. In fact, many Class AB amps operate in Class A at lower output levels, again giving the best of both worlds
2. The output bias is set so that current flows in a specific output device for more than a half the signal cycle but less than the entire cycle.
3. There is enough current flowing through each device to keep it operating so they respond instantly to input voltage demands.
4. In the push-pull output stage, there is some overlap as each output device assists the other during the short transition, or crossover period from the positive to the negative half of the signal.

There are many implementations of the Class AB design. A benefit is that the inherent non-linearity of Class B designs is almost totally eliminated, while avoiding the heat-generating and wasteful inefficiencies of the Class A design. And as stated before, at some output levels, Class AB amps operate in Class A. It is this combination of good efficiency (around 50) with excellent linearity that makes class AB the most popular audio amplifier design. 

There are quite a few excellent Class AB amps available. This is the design I recommended for most general-use applications in home and car. Usually, parts choice rivals that of Class A amps, and dollar for dollar these are some of the best values in stereo amplification. There can be some variation in design principle, but generally these are well-designed amps since their function is very well-understood by audio designers. 

Class D
These amplifiers are erroneously called "digital" amplifiers by the press and many audio "experts." Here's the skinny on Class D: 

1. While some Class D amps do run in true digital mode, using coherent binary data, most do not.
2. They are better termed "switching" amplifiers, because here the output devices are rapidly switched on and off at least twice for each cycle.
3. Depending on their switching frequency, they may be "switched on" or "off" millions of times a second.
4. Class D operation is theoretically 100% efficient, but in practice, they are closer to 80-90% efficiency.
5. This efficiency gain is at the cost of high-fidelity.

Think of Class D amps as being similar to a switchable power supply, but with audio signals controlling, or modulating, the switching action. To do this, you use a technology called Pulse Width Modulation (or PWM, a technology found in many CD players). 

According to experts, audio signals can be used to modulate a PWM system to create a high power audio amplifier at fairly low voltages using very small components. Class D audio uses a fixed, high frequency signal having pulses that vary in width based on input signal amplitude. So, for example, a deep bass note creates a large pulse in the carrier signal. This can be translated into a musical signal by the on/off nature of the output devices. 

Class D amplifiers are generally used for non-high-fidelity, or subwoofer applications.

There is a fifth (and, nominally, a sixth) class of amplifier, but they are rarely seen in practice in the consumer market. One is the Class G and the other Class H. These are similar in design to Class AB topologies, but both feature two power supplies that switch on or off, depending on the musical signal imputed. Using two power supplies improves efficiency enough to allow significantly more power for a given size and weight. Class G is becoming common for pro audio designs. Class H amps are designed to use the same topology as Class G, but it provides just enough voltage for optimum operation of the output devices. Again, its an attempt to increase efficiency, but at the expense of fidelity ultimately.

In summary: 

• Class G and H amplifiers add complexity to the signal and degrade it because of the need for switching depending on the input signal
• Class D amplifiers are models of efficiency, but with a loss of detail and fidelity
• Class B amplifiers generally introduce some crossover distortion, but move away from Class D, G, and H's extreme non-linearity.
• Class AB amplifiers may introduce some crossover distortion, but they get closer to the ideal of Class A for most of its operating regime.
• They are indeed the best compromise of performance versus cost.
• Class A amplifiers introduce no crossover distortion and are the most desirable amps to own, but they are expensive, run hot, and have to be very well-built.