| Class D amplifiers are designed to provide high efficiency, typically above 90%. It is this ability that makes Class D amplifiers popular in high power and battery-powered devices. Unfortunately, Class D amplifiers suffer from high frequency harmonics produced in the modulation process. In order to reduce the distortion, a simple passive LC filter is commonly used. While this improves the output fidelity, there are still high amounts of distortion present at the output. Additional LC filters can be added to improve the fidelity of the amplifier. However, bulky inductors and capacitors make the design less compact and impractical in small electronic devices.;In this thesis, negative feedback is used to effectively increase the order of a simple LC filter. A circuit is developed to monitor the current flowing through the filtering capacitor. This quantity is amplified and fed to a Class AB current source. The current source acts to cancel the harmonic effects present at the output of the amplifier. An analytical solution for the active filter is developed and LTSPICE simulations are given to validate the design. The simulations show that at a signal frequency of 10 kHz, the passive filter results in a THD of approximately 0.3% while the active filter exhibits only 0.08% THD. The efficiency of the composite filter (the combination of the LC filter and the active network) has an efficiency of above 80% for frequencies below 10 kHz. |
