There are two basic types of power amplifiers: linear and switch-mode. The Class-D amp is a type of switch-mode amplifier that consists of three stages: the input switching stage, the power amplification stage, and the output filter stage. A common misconception is that a switching power amplifier is an amplifier with a switching power supply. In actuality, a switching power amplifier utilizes switching technology as a means to amplify the signal, whereas a switching power supply is using switching to provide AC mains power. In linear amp designs, the output power transistors amplify the incoming signal analogous to the input signal (same signal only louder). In a switching amp, the incoming audio signal modulates the width of an ultrasonic square wave in the output transistors. This modulated square wave is then low-pass filtered. The result is the amplified analog signal sent to the speakers. This signal processing is called Pulse Width Modulation or PWM.
In operation, a switching power amp uses several pairs of power output transistors with each pair operating as switches (transistors can operate either as amplifiers or switches) in a push-pull arrangement. One transistor of each pair turns fully on (saturation) while the other is off, then they alternate. One transistor always produces a positive voltage when it is on; the other always produces a negative voltage when it is on. When a transistor acts as an open switch, the current through it is ideally zero. When the same transistor acts as a closed switch, the voltage across it is also ideally zero. Whether the switch is open or closed, the power dissipated as heat – current times voltage – would ideally be zero. The result is less power wasted, which means improved efficiency – theoretically 100%.
Since the output transistors develop almost no current (when off) or no voltage (when on), they produce almost no power wasted as heat. Increased efficiency requires less power from a power supply and smaller or no heat sinks for the amplifier. These are important advantages in portable battery-powered equipment. Since the class-D amplifier works in an on/off mode, people mistakenly think that the “D” in class-D means “digital“. This is not the case. There is no digital coding of the signal. The function of class-D is based on analog principles. Class-D amplifier applications range from battery-powered portable products such as mobile phones, high-end professional amplifiers, musical instruments, and home multimedia systems.