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.