Audio amplifiers are devices or circuits that amplify and buffer audio signals to drive a low impedance, inductive speaker load. They are used in applications like high-powered audio systems, television sets, headphones and microphones. Some devices incorporate a serial interface for direct control of parameters by a processor. They are also available in different input signal types, including analog, pulse width modulation (PWM) and a number of digital industry standard formats. Examples of digital formats typically supported are: I²S, time division multiplex (TDM), left-justified (LJ) and right-justified (RJ).

 

An audio amplifier has a number of characteristics to consider. Amplifiers with digital inputs will support ‘sample rates’ that can range from 8 kHz to 192 kHz. They may be designed to incorporate the digital audio flow from a DSP and have sophisticated configurations for Bluetooth speakers, sound bars, docking stations and even sub-woofers. Analog inputs may be single-ended or differential and include a preamp, clip detection and tweeter detect. Power amplifiers may be closed or open loop designs, incorporating internal feedback and current limiting. Some amplifiers will have fault, over temperature and clipping detection. These may be available as digital output signals for a connected processor. Most audio amplifiers will incorporate a MUTE circuit.

 

Many audio amplifiers will support multiple channels. These usually have a rated output power for contiguous continuous operation into speaker impedance (for example 4 Ohms). They will also be rated for a specific maximum total harmonic distortion and noise (THD+N) under different load and power conditions. Power Supply Rejection Ratio (PSRR) is also a very important consideration in reduction of hum and noise coupled from the power supply. Multiple Channel Audio Amplifiers may support “bridge tied load” configurations where outputs are combined to drive a single load and increase output power. This is important in sub-woofer amplifier design.

 

Most audio amplifiers are designed for efficiency to reduce both power consumption and heat sink size requirements. Analog input signals are turned into PWM waveforms and sent to a ‘gate driver’ that is capable of driving the high capacitive load of the MOSFET amplifier stage. The high frequency switching is filtered by the output circuit, which also exploits the natural low pass characteristics of the speaker.