Abstract
There are many different designs for audio amplifiers. Class-D, or switching, amplifiers generate their output signal in the form of a high-frequency square wave of variable duty cycle (ratio of on time to off time). The square-wave nature of the output allows a particularly efficient output stage, with minimal losses. The output is ultimately filtered to remove components of the spectrum above the audio range. Mathematical models are derived here for a variety of related class-D amplifier designs that use negative feedback. These models use an asymptotic expansion in powers of a small parameter related to the ratio of typical audio frequencies to the switching frequency to develop a power series for the output component in the audio spectrum. These models confirm that there is a form of distortion intrinsic to such amplifier designs. The models also explain why two approaches used commercially succeed in largely eliminating this distortion; a new means of overcoming the intrinsic distortion is revealed by the analysis.
Highlights
Class-D audio amplifiers are becoming increasingly popular, at the high end of the hi-fi audio amplification market
For sufficiently large m, when |ωc ∓ mω| lies in the audio range, terms with n = ±1 must be included in the sum (2.14), rendering inappropriate our assumption that only terms with n = 0 contribute to the output audio spectrum
We describe a third modification to the standard negative-feedback class-D amplifier, which eliminates the intrinsic distortion at O( 2)
Summary
Class-D audio amplifiers are becoming increasingly popular, at the high end of the hi-fi audio amplification market. The key feature of their design is that they switch their output between two voltage levels at a very high frequency (typically 500kHz), well above the audio range. The audio signal is essentially encoded in the relative durations of the pulses at the two output voltage levels. The discrete nature of the switching allows the output stage to be highly efficient; the audio signal is recovered by low-pass filtering of the output. The concept of class-D amplifiers using this pulse-width modulation (PWM) technique has been known for at least fifty years [1], it is only much more recently that electronic components have become available that make their practical implementation feasible. Several commercial amplifiers at the high end of the audio market use class-D amplifier technology
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