Low quiescent current (IQ) is critical for Class-D audio amplifiers in mobile devices to extend battery usage time , since typical audio signals have a high crest factor of 10 to 20dB. In addition, low distortion is also important for audio fidelity. Distortion sources in closed-loop Class-D amplifiers can be classified into two types. One is attributed to the nonlinearities of PWM modulators and power stages, while the other is due to the aliasing of fed-back PWM high-frequency residuals, the latter of which comprises phase-error and duty-cycle-error distortions . Figure 3.6.1 shows 2nd-order closed-loop amplifiers and existing techniques for enhancing an amplifier's linearity. Increasing the loop filter order to obtain a higher in-band loop gain by using more integrators  or the single-amplifier-biquad  suppresses all aforementioned distortions except for the phase-error distortion, which can be suppressed by adding a phase-error-free PWM modulator . However, these techniques increase IQ and/or die area due to the additional active circuits and/or several resistors and capacitors. Since phase-error distortion, as well as duty-cycle-error distortion, is caused by the fed-back PWM high-frequency residuals aliasing with the reference triangular wave VTRI, a uniform PWM  with a sample-and-hold circuit implemented before the PWM modulation reduces the PWM residuals via an equivalent notch filtering. However, loop stability is affected by the notch filtering unless the PWM switching frequency fSW is increased, but doing so increases power consumption . Though the technique in  uses a feed-forward path with a replicated loop filter to eliminate the PWM residuals without affecting loop stability, the replicated loop filter increases both IQ and area.