Whether it’s for mixing tracks in the studio or providing live sound reinforcement for touring or installed PA systems, amplifiers are an important part of an audio signal chain. While studying the specifications for a powered studio monitor, active loudspeaker, or power amplifier, you may have noticed that the amplifier is designated by a class followed by a letter or combination of letters. What are the different classes of amplifiers, how do they work, and what are their respective benefits, tradeoffs, and the intended applications of each class?
What is an Amplifier?
In simple terms, an amplifier is an electronic device that is used to increase (or amplify) the current, voltage, or power of an analog electrical signal. In the recording studio, preamplifiers are used to increase the signal captured by a microphone to a line-level signal that can be routed through a mixer or digitized with an analog-to-digital converter.
On stage, a power amplifier (as a stand-alone unit, as a module in a studio monitor, or as a module in a PA speaker) is used to increase a line level signal to a sufficient level so that it drives the speaker elements (often referred to as drivers) to the intended SPL output level and frequency response.
The design and implementation of these devices can vary dramatically, and the art of amplifier design is always evolving.
A Class-A amplifier is one in which both output stages of the device are constantly on at full power. Because both stages are constantly on, Class A is considered to be the least efficient of power amplifier designs, with an average efficiency of about 20% (50% at best, theoretically).
Class A amplifiers are the most linear design with the least amount of distortion, but at the expense of efficiency. On the other side of the spectrum, the efficiency of Class D amplifiers can be above 90% but at the expense of linearity and distortion.*
If you consider that the conversion efficiency of electrical to acoustic energy in many speaker designs (excluding horns, which transform the acoustic impedance of the air) is only around 1%, then a 20%-efficient amplifier connected to a typical speaker is going to use and lose a tremendous amount of electricity in the form of heat. Because of this, Class-A amplifiers often have to be very large and heavy to fulfill the requirements of their applications.
The advantage of a Class-A design is that because both the input and output stages are constantly on at full power, they exhibit the most linear response with the lowest distortion of the various designs.
Class A is found most often in applications that require low power and low distortion, such as for radio or guitar amplifiers.
In contrast, a Class-B amplifier is designed so that only one of the output stages is allowed to be on at a time. The current bias of the device is set to zero in the output stage that doesn’t see the input signal. After one half sinusoidal cycle of the signal, the current bias switches to the other output device. This approach improves the electrical efficiency of the device at the expense of linearity at the crossover point, because of the time required to switch one output on and the other off. Class-B amplifiers are found in applications that require efficient power consumption, such as when powered by a battery.
A hybrid of the aforementioned Class-A and Class-B designs, the Class-A/B approach allows for both of the output stages to be on at the same time, but with one output receiving the current flow for between one half and one full cycle, while the other output receives just enough current flow to remain on so it can respond instantly to the input signal. This approach leverages the strengths of both designs with greater efficiency than Class A, and without the non-linear crossover distortion of class B. For this reason, it is the most commonly found form of amplifier in studio monitors, hi-fi systems, guitar amps, and many powered speakers.
Typically only found in radio frequency transmission applications, a Class-C amplifier turns on one output device at a time for some percentage of half a cycle. This generates a series of pulses that represent the signal for a highly efficient but distorted design. Using tuned radio-frequency circuits, the distortion can be managed and the high efficiency utilized for high output transmission of radio signals.
Also known as a switching power amplifier, a Class-D amplifier uses active transistors to function as electronic switches that can be either on or off. The amplifier implements a technique such as pulse-width modulation, delta-sigma modulation, or pulse-density modulation to convert the input signal into a stream of pulses wherein the time average power of the pulses is proportional to the original analog signal. The frequency of the pulses is many times higher than the highest frequency of the input signal so that a passive low-pass filter can be used to smooth the pulses and convert them back to an analog signal.
Class-D amplifiers are very efficient in comparison to Class-AB, which allows for smaller and lighter form factors, as well as much cooler operation. They excel at band limited high power applications, such as for driving subwoofers.
Class E, F, G, H, I and Variations (AB1, AB2, AB+B, BD)
In the course of reading amplifier specifications or conducting general research, you may encounter other designations such as Class E, F, G, H, I, as well as variations of the above such as AB1, AB2, AB+B, BD. Most of these devices build upon the concepts already described but for specialized applications, or employ the designation (e.g. G or H) to describe proprietary improvements on existing designs. For example, a Class-G amplifier improves upon the efficiency of a Class-AB design by using multiple rails at different voltages to amplify the input voltage more efficiently. A Class-H design goes further by using a rail of infinitely variable voltage.
There are inherent strengths and weaknesses in each design, which are taken into consideration and then optimized by manufacturers for the intended application. A wealth of white papers and other technical resources is available online through various manufacturers and the Audio Engineering Society (AES), should you be interested in studying some of the more specialized amplifier designs in greater detail.
I hope you enjoyed this brief overview of amplifier designs and encourage you to check out other fun and informative articles on B&H Explora.
*Amplifier designers have developed ways of addressing distortion in Class-D designs such as by building full-bridge (as opposed to half-bridge) circuit topologies which cancel even order harmonic distortion and the circuit’s DC offset. The more complex full-bridge topology also enables a quantized less erroneous three level pulse width modulation (PWM) of the waveform.