The class D amplifier or switching amplifier uses the switching mode of operation of transistors to control the out power rather than using the linear operation of transistors (class AB).The cause of the high power efficiency is that the switching element (transistor or MOSFET) is never operated in the active region.They are operated in the switching mode only (either ON or OFF).So the out put has also two states OFF or ON.The result is greater power efficiency and low weight.

The amplifier uses pulse width modulation or sigma delta modulation for controlling the output power.The input signal is converted int o train of puses whose pulse width is proportional to the frequency and voltage of input signal.A power transistor switches the output to drive the speaker.In addition to the
switching transistors filter circuits have to be also included to filter off unwanted frequencies and harmonics.

The PWM frequency is generated by comparing the input signal with a high frequency comparator.The out put of the comparator will be a sequence of pulses whose pulse width is proportional to the frequency and voltage of the input signal.The PWM output of the comparator drives a switching controller made up of transistors or MOSFETS which generates a high power copy of the PWM signal.The output of the switching controller is filtered using a low pass filter and fed to the speaker.Some Class D designs incorporate a negative feed back too to reduce oscillations and improve overall gain.

Advantages.

1. Low weight because heat sinks are smaller due to high power efficiency.Less power is wasted as heat.
2. Overall power efficiency is very high (90%).

Disadvantages.

1. Complicated circuitry
2. Complex design.

Block Diagram of Class D Amplifier.

Authors view.

Class D Amplifiers stand above the conventional Transistor , MOSFET or even Valve amplifiers only on the parameters like power efficiency and low weight. I feel there is no advantage for Class D Amplifiers over conventional ones on parameters like fidelity, linearity,THD etc.Therefore Class D Amplifiers till now cannot cause much threat to our conventional transistor or MOSFET amplifiers.