Audio systems are now heading towards development of power-saving systems with low distortion, high efficiency and excellent sound quality at desired volumes. To meet these requirements, Class D amplifiers have become quite popular to develop audio systems in recent years. Compared with other types of amplifier designs, the power dissipation is small in Class D amplifiers. This, in turn, produces less heat, saves circuit board space and cost, and extends battery life in portable systems.
A Class D amplifier, usually called a switching amplifier, amplifies an analog audio signal by using transistors (usually MOSFETs) as switches. Pulse width modulation (PWM), pulse density modulation or other method can be used to convert the analog signal to be amplified to a series of pulses before being applied to the amplifier. After amplification, a passive low pass filter with LC elements again converts the output pulse train back to the analog signal.
Though low-power dissipation brings strong motivation to use Class D for audio applications, there are many important challenges presented to the designer. Some of these include choice of modulation scheme, output transistor’s size, output-stage protection, sound quality, EMI, LC filter design and system cost. An ideal Class D amplifier has no distortion and 100% efficiency. But practically, there are many sources of distortion — power supply voltage fluctuations, non-linearity in the output LPF, parasitic components causing ringing, unwanted characteristics of switching devices, to name a few.
The above challenges are looked after by the Class-D amplifier in many ways. A temperature-monitoring and control circuit keeps the temperature within limits by controlling the volume level. An under-voltage condition is handled by an under-voltage lockout circuit, which allows the output stages to operate only when the power supply voltage is above a threshold. Sound quality can be hampered by regular clicks and pops due to amplifier turning on or off. Signal-to-noise ratio (SNR) is kept quite high to avoid audible noise. Power-supply rejection (PSR) is quite needed as power-supply noise couples to the speaker with very little rejection. Class D amplifiers use Pulse-width modulation (PWM), the most common modulation technique, to encode the audio signal into a stream of pulses. This is because it uses carrier frequencies of few hundred kilohertz to allow 100-dB or better SNR. System cost and space are minimized to an extent by elimination of cooling apparatus like heat sinks or fans due to Class D operation. Another approach to reduce space is to minimize the number of LC filter components required per audio channel. Electromagnetic Interference (EMI) is troublesome like in other switching applications. PCB layout is crucial for both ruggedness of the design and reduction of EMI. Modern Class D audio amplifiers use integrated single-chip solutions combining programmable-gain amplifiers, sigma-delta modulators, protection against overheating and overcurrent, special timing control, soft start, and much more to handle above mentioned challenges.
The following reference designs highlight the use of Class D amplifiers in designing high sound quality audio systems with complete schematics, layout and application notes.
- 2.1-Channel Audio amplifier for MP3 Player: The reference design is a compact stereo audio dock intended to be used with a portable audio player as its main music source. The design features active equalization, power-supply monitoring, and dynamic equalization for the subwoofer. Two MAX98400 ICs are used to drive a 3-channel speaker system consisting of two 2in satellite speakers and one 5in subwoofer. A MAX98400A is configured in stereo mode to drive the left and right speakers. For the third channel, a MAX98400A is configured as a mono subwoofer amplifier. Use of Class D amplifier achieves high sound quality. More on this Reference Design
- Class D Audio Power Amplifier with Scalable output: Here is a reference design of a two-channel Class D audio power amplifier that features output power scalability from 25W – 500W. The design finds applications in home theatre systems, automobiles, musical instrument amplifiers, and more. An optimum PCB layout is achieved using the IRS2092 Class D audio driver IC along with digital audio dual MOSFETs from International Rectifier. A small heatsink is required for normal operation. The design comes equipped with all the required power supplies and protections.More on this Reference Design
- 2-channel Class D Audio Amplifier: This reference design discusses a high-performance 2-channel Class D audio amplifier for MP3 players. The design uses the Si8241 audio driver to reduce PCB size and BOM cost. This audio amplifier delivers 120 W per channel into 8 ohm, while giving high signal-to-noise ratio and less Total Harmonic Distortion (THD). The design comprises over-current protection and under-voltage protection circuits. More on this Reference Design
- 2.1 Class-D Stereo Audio Dock with Signal Processing: This reference design is a 2.1 Audio Amplifier solution which can output up to 10 watts output power with excellent sound quality. A stereo TAS5756M Class-D IC amplifier is used for the left and right audio channels and a TAS5760M Class-D amplifier provides a mono bass channel. The design can be easily integrated into any audio system and comes equipped with biquad filters, dynamic bass enhancement and other miniDSP processing blocks. The design has small footprint by using inductor free (ferrite bead) filter for TAS5756 stereo channels, making it suitable for Bluetooth speakers where small solution size and cost are critical. More on this Reference Design