Hi,
   I am simulating a H-bridge class d amplifier (bridge-tied-load). I find the second harmonic distortion (HD2) dominates the THD. I am thinking what is the main reason to produce this HD2.
   One possible reason is that architecture of the H-bridge. For class d amplifier, the H-bridge is not differential, hence the HD2 could be high.
   Another possible reason is that the non-linear on-resistance of the PMOS and NMOS transistors. The HD2 could be produced by the non-linear on-resistance.
   But an extra consideration is we may take the output signal from the class d amplifier output as the digital signal, the distortion shall be lower for the digital signal.
   Currently, most of the reported Works just mention THD is an issue for class d amplifier, and just add the local feedback loop to improve it. However, nobody introduces where the distortion comes from and the analysis.
   Also, the HD2 can be improved by the local feedback loop (integrator + comparator) under the condition of quiet supply. After i add the noise supply, in fact the SNDR and HD2 are dropped a lot, I couldn't figure out why.
   Can anybody gives me some hints or points me to some good articles?
   Thank you.