AN-1497滤波器D类放大器

　　Introduction

　　One area where manufacturers look to decrease power consumption is the audio portion of the system. Inthe past， system designers were offered only one option， the tried and true， but inefficient linear audio amplifier. Steps were taken to improve the linear amplifier efficiency while preserving audio fidelity resulting in the topology that provides the best balance between performance and efficiency， Class AB. Class AB exhibits a theoretical peak efficiency of 78%， and is only 30% - 40% efficient at normal operating levels. Efficiency is where the switching， or Class D， audio amplifier has large advantages over linear classes. Although Class D amplifiers offer increased battery life and lower operating temperatures， early generations were met with guarded interest; cost， size and performance were sub-par compared to linear　amplifiers and limited their adoption. However， recent advances in Class D technology have made these　amplifiers more appealing. Improved audio performance and the introduction of filterless devices have　contributed to the increasing acceptance of Class D.　The major advantage of using Class D is efficiency. The more efficient a device is， the less power it　wastes; a 90% efficient amplifier at 1W output dissipates 100mW， while a 60% efficient amplifier with the　same 1W output consumes 400mW. Lower power dissipation， or power consumed by the device， leads to　longer battery life and less heat generation.

　　Figure 1 compares the efficiency of the LM4673， a mono 2.65W Class D amplifier to a similar Class AB　amplifier. At peak output power （1.5W） the class D amplifier has 88% efficiency while the Class AB　amplifier only achieves 75%. At a mere 13%， the difference does not appear siginificant. However， a　comparison at nominal output levels reveals a dramatic difference between the two architectures. At 500mW into an 8Ω load， a typical class D amplifier exhibits 85% efficiency while the Class AB amplifier is　only 44% efficient. The difference in power dissipation is a very significant 550mW. At a 500mW output　level the increased power consumption shortens the life of an 800mAh single cell lithium-ion battery by　197 minutes， well over three hours. Figure 2 shows the theoretical operating time of an 800mAh lithiumion　battery powering a LM4673 and a comparable Class AB amplifier.