Class D Amplifier Tutorial!

A Class D amplifier converts an input wave (like a sine wave) into a high-frequency pulse width modulated (PWM) square wave. This square wave is then filtered to produce an amplified original signal. The video demonstrates this process using a 2Hz sine wave, showing how the input signal changes the pulse width of a constant 300 kHz square wave to control power to a load like a speaker.

To prepare the signal for a speaker, an LC low-pass filter is added to remove high-frequency noise from the PWM square wave. While this produces an amplified sine wave, it still has a DC offset. A high-pass filter is then introduced to remove this DC component, ensuring only AC current reaches the speaker, which is crucial for proper audio reproduction.

The tutorial then guides viewers through building a Class D amplifier using a TPA3122 chip, a 12-volt power supply, and various capacitors. Key components include 1000uF capacitors for power filtering, additional capacitors for chip stability and reference voltages, and bootstrap capacitors for driving internal transistors. Gain pins are configured for desired amplification (e.g., a gain of 63), and shutdown and mute pins are appropriately connected.

An LC low-pass filter (33uH inductors and 50V ceramic capacitors with X7R dielectrics) is added to convert the 300 kHz square wave back into an audio signal. To prevent DC from reaching the speaker, 1000uF electrolytic capacitors are placed in series with the amplifier's output. Finally, 1uF capacitors are used for input audio signal high-pass filtering and to protect the input device.

The video explains the high efficiency of Class D amplifiers by comparing their output stage to Class AB amplifiers. While Class AB designs use transistors in the forward active region, generating significant heat due to several ohms of resistance, Class D amplifiers utilize two MOSFETs that act like switches with very low resistance (0.2 ohms each). Only one MOSFET is on at a time, leading to less heat generation and greater efficiency.

Class D amplifiers are cheap and highly efficient, making them common in modern consumer electronics. However, they rely on filtering a square wave, which can introduce a small amount of distortion, preventing a perfect reproduction of the input signal. While this distortion is often unnoticeable on most speakers, professional musicians or owners of expensive audio equipment might prefer less efficient, higher-quality Class A or Class AB designs. For most, Class D provides a suitable and cost-effective solution for applications like home stereo systems or boomboxes.