PAPER SESSION 4: ERRORS AND DISTORTION
4-3 Time Domain Analysis of Open Loop Distortion
in Class D Amplifier Output Stages
Nyboe(1,2) Lars Risbo(1), Pietro Andreant(2).
(1) Texas Instruments, Lyngby, Denmark.
(2) Technical University of Denmark, Lyngby, Denmark.
During the long history of class AB amplifiers, many
topology improvements have been developed with the
aim of reducing open-loop THD. As class D amplifiers
become widely used, a new learning of such improvements
is needed, since the basic distortion mechanisms
are very different from those of class AB amplifiers.
This is even more important with class D
designs because the very high feedback loop gains
seen in class AB designs is not always achievable in
class D designs. In some cases no feedback is used
at all, because it cannot easily be applied to digital
input systems at low cost. This paper analyzes the
nature of different contributors to THD in class D output
stages. It is shown how large-signal transfer
characteristic analysis can be applied to individual
parts of a PWM output signal, to help identify problems
and optimize a design for minimum THD.