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.
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.