Abstract
This paper presents a time-domain analysis of the intermodulation distortion (IMD) of a closed-loop Class-D amplifier (amp) with either first- or second-order loop filter. The derived expression for the IMD indicates that there exist significant third-order intermodulation products (3rd-IMPs) within the output spectrum, which may lead to even greater distortion than the intrinsic harmonic components. In addition, the output expressions are compact, precise, and suitable for hand calculation so that the parametric relationships between the IMD and the magnitude and frequency of the input signals, as well as the effect of the loop filter design are straightforwardly investigated. In order to accurately represent the IMD performance of class-D amp, a modified testing setup is introduced to account for the dominantly large 3rd-IMPs when the International Telecommunication Union Radiocommunication Sector (ITU-R) standard is applied.
Highlights
Recently, there have been increasing demands on the analysis of the multi-tone response of a Class D amplifier, which is quantified by the intermodulation distortion (IMD), as IMD makes music sound harsh and unpleasant [1]
The most common IMD measurement standard in the professional, broadcast, and consumer audio fields is set by the Society of Motion Picture and Television Engineers (SMPTE) and is referred as SMPTE method, in which a two-tone test signal consisting of a low-frequency high-amplitude tone (60 Hz) is linearly combined with a high-frequency tone (7 kHz) at 1/4 the amplitude (–12 dB) of the low-frequency tone [15]
In the rest of this section, all the verification is based on the 2nd-order loop filter with Design II parameters, which manages to achieve a maximized attenuation of the non-ideality of the power stage and supply noise, yielding more accurate results on the intrinsic IMD of the closedloop Class D amp
Summary
There have been increasing demands on the analysis of the multi-tone response of a Class D amplifier (amp), which is quantified by the intermodulation distortion (IMD), as IMD makes music sound harsh and unpleasant [1]. We have demonstrated in [10, 11] that time-domain modeling technology, coupled with asymptotic analysis, provides an accurate mathematical prediction of the intrinsic total harmonic distortion (THD) of PWM-based Class D amps. The residual carrier ripples inside the output signal of the loop filter cause timing errors in the switching times of the modulated high frequency pulse signal (PWM signal) These timing errors are input signal dependent and cause intrinsic distortion in the form of both harmonic distortion and intermodulation distortion on the demodulated output signal. A more systematic, and easier to understand, time-domain analysis was reported in our previous work [11] to model the output signal of a 2nd-order loop filter Class D amp; in addition, an explicit stability criterion was introduced to avoid the pulse skipping problem. For a 1st-order loop filter design, R3 will be removed and the serial connected C1 and C2 can be represented by a single capacitor C0 with value equal to C1/2
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