Audio amplifier basics: Select the best topology for your design
Introduction
An audio amplifier increases the amplitude of a small signal to a useful level, all the while maintaining the smaller signal’s detail. This is known as linearity. The greater the amplifier linearity, the more the output signal is a true representation of the input.
With the ever-changing performance requirements for amplifiers in the audio market, there have been many advances in audio amplifier topologies. Consequently, designers must know the types of audio amplifiers available and the characteristics associated with each. This is the only way to ensure that you select the best audio amp for an application. We shall examine the most important characteristics of each class of audio amp available today.
Class A Amplifiers
The simplest type of audio amp is Class A. Class A amps have output transistors (Figure 1) which conduct (i.e., do not fully turn off), irrespective of the output signal waveform. Class A is the most linear type of audio amp, but it has low efficiency. Consequently, these amps are used in applications that require high linearity and have ample power available.
Figure 1: A Class A audio amp is typically associated with high linearity but low efficiency.
Class B Amplifiers
Class B amps use a push-pull amplifier topology. The output of a Class B amp incorporates a positive and negative transistor. To replicate the input, each transistor only conducts during half (180°) of the signal waveform (Figure 2). This allows the amp to idle with zero current, thereby increasing efficiency compared to a Class A amp.
Figure 2: With a Class B audio amp the output transistors only conduct during half (180 degrees) of the signal waveform. To amplify the entire signal two transistors are used, one conducting for positive output signals and the other conducting for negative outputs.
There is a trade-off that comes with a Class B amp: the increased efficiency degrades audio quality. This happens because there is a crossover point at which the two transistors transition from the on state to the off state. Class B audio amps are also known to have crossover distortion when handling low-level signals. They are not a good choice for low-power applications.
Class AB Amplifiers
A compromise between Class A and Class B amplifier topologies is the Class AB audio amp. A Class AB amp provides the sound quality of the Class A topology with the efficiency of Class B. This performance is achieved by biasing both transistors to conduct a near zero signal output, i.e., the point where Class B amps introduce nonlinearities (Figure 3).
Figure 3: A Class AB amp biases both transistors so they conduct when the signal is close to zero. Thus these amps provide more efficiency than Class A with lower distortion than Class B.
For small signals, both transistors are active, thus functioning like a Class A amp. For large-signal excursions, only one transistor is active for each half of the waveform, thereby operating like a Class B amp.
Class AB speaker amps offer high SNR, low THD+N, and typically up to 65% efficiency. This makes them ideal choices as high-fidelity speaker drivers. Class AB amps like the MAX98309 and MAX98310 are used in portable media players, digital cameras, tablets, and e-readers where high fidelity is a must.
Some headphone amplifiers use a Class AB topology in a bridge-tied-load configuration. As an example, the MAX97220 headphone amp offers exceptionally low THD+N throughout the audio band while delivering up to 125mW of power.
Class D Amplifiers
The popularity of handheld mobile audio devices, such as smartphones, MP3 players, and portable docking stations, brings power consumption into greater focus. Now it is necessary to reduce power consumption to increase battery life.
Class D amplifiers use pulse-width modulation (PWM) to produce a rail-to-rail digital output signal with a variable duty cycle to approximate the analog input signal (Figure 4). These amps are highly efficient (often up to 90% or higher) because the output transistors are either fully turned on or fully turned off during operation. This approach completely eliminates the use of the linear region of the transistor that is responsible for the inefficiency of other amplifier types. Modern Class D amps also achieve fidelity comparable to Class AB amps.
Figure 4: A Class D audio amp outputs a switching waveform at a frequency far higher than the highest audio signal that needs to be reproduced. These amps are highly efficient because the output transistors are either fully turned on or off during operation.
With its high efficiency, Class D switching amplifiers such as the MAX98304 and MAX98400 are widely used in portable applications.
Class G Amplifiers
Class G amps are similar to Class AB, except that they use two or more supply voltages. When operating at low signal levels, the amp selects a low supply voltage. As the signal level increases, the amp automatically selects the appropriate supply voltage (Figure 5). Class G amps are more efficient than Class AB amps because they use the maximum supply voltage only when required; a Class AB amp always uses the maximum supply voltage.
Figure 5: A Class G amp is more efficient than Class AB amps because it uses the maximum supply voltage only when required.
There is a common problem in portable audio applications: the limited supply voltage available to the speaker amp. Class G power amps solve this supply-voltage problem by using a charge pump to boost the supply voltage. As examples, the MAX9730 speaker amp is optimized for traditional dynamic speakers, while the MAX9788 speaker amp is designed for ceramic speakers.
Class DG Amplifiers
The Class DG amp uses PWM to produce a rail-to-rail digital output signal with a variable duty cycle. In this respect, a Class DG amp is the same as a Class D amp. The Class DG amp, however, also uses a multilevel output stage to sense the magnitude of the output signal (Figure 6). It then switches the supply rails, as needed, to supply the required signal power more efficiently. Class DG amps, such as the MAX98308, use the same dual-power concept with a switching Class D topology for even higher efficiency.
Figure 6: A Class DG amplifier senses the magnitude of the output signal and then switches the supply rails, as needed, to supply the required signal power more efficiently.
Class H Amplifiers
Class H amps modulate their supply voltage to minimize the voltage drop across the output stage. Implementations range from using multiple discrete voltages to an infinitely adjustable supply. Though similar to the Class G technique of reducing dissipation across output devices, the Class H topology does not require multiple power supplies (Figure 7).
Figure 7: Class H audio amps reduce dissipation across the output devices connected to that supply. This allows the amp to operate with an optimized Class AB efficiency, regardless of output power level.
Class H amps are generally more complex than other audio amplifier designs. These amps require extra control circuitry to predict and control the supply voltage. Audio codec ICs such as the MAX98090 integrate a Class AB headphone amp with a Class H power structure to provide a very low-power, complete audio solution.
Summary
This has been a brief survey of the many types of audio amplifiers commonly used in designs today. Clearly, when designing an audio circuit for any type of device, care should be taken in determining the audio amplifier topology best suited to the application. A good understanding of these different classes of audio amps will help you choose the best audio amp for your design.
About the Author:
Robert Nicolette joined Maxim Integrated in 2001 and is now the company’s Engineering Manager for Audio Applications. He has a BSEE degree from San Jose State University.
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