About the Demodulation of Pwm-signals with Applications to Audio Amplifiers

In this paper, the reason for the relatively high distortion level of conventional class-D audio amplifiers is discussed. It is pointed out that the classical approach is insufficient for high performance applications. Therefore the importance of the matching between modulation and demodulation is demonstrated by simulation. 1. THE CLASS-D-AUDIO AMPLIFIER The concept of class-D amplifiers was invented by Baxandall [ l ] in 1959 for applications in oscillator circuits. In contrast to these tuned amplifiers first ideas for broadband applications were presented in the early sixties. In the following several papers were published about this subject (for further references see e.g. [2], [3], [4], [ 5 ] ) . Although classD amplifiers are very attractive because of their high power efficiency several disadvantages occur. Especially the switching behaviour of the power transistors and the distortion performance wen: responsible that class-D amplifiers were not very successful in audio applications. With the invention of power MOS transistors the class-D amplifiers became interesting again [6]. For example Sony developed its TA-N88 around 197'7 where this amplifier principle is used. More recently class-D amplifiers are discussed in conjunction with digital audio concepts. New commercial realisations of amplifiers were presented by Harris Semiconductor and Peavey as well as research papers of this subject were published by Lau [8] and Smedley [9]. The main idea behind the class-D concept is the conversion of an audio signal into a pulse width modulated signal (pwm signals) with constant frequency. Since pwm signals have only two levels a simple switched mode amplifier can be used to obtain high output power. It is well known that the good power efficiency is due to the switched mode. The pwm signal must be demodulated in order to reconstruct the (amplified) audio signal. Usually a filter has to be lossless. The structure of a class-I) amplifier is shown in figure 1. low pass filter is used for this purpose. Clearly this properties of these modulators. On the other hand it is not obvious in which sense low pass filters work as a pwm demodulator. Especially since several pwm modulators are available we discuss the advantages and disadvantages of known demodulation techniques. audio pulse pulse width switched mode LCtype audio load inpul width modulated power amlrlifier low pass output signal moduiator puls train carrier frequency DC-power Fig. 1: Structure of a Class-D-Amplifier