I try to design a class D op amp. The input sin wave has the amplitude of 1 V and 20K frequency with the 1V dc. The sawtooth waveform has the amplitude of 2V and 200k frequency. The mosfet I choose is IRF540. When I set the Vdd of the high side mosfet is 40V and low side is 0V, the system works well. But if I change it to symmetric system with Vdd of the high side mosfet is 20V and low side is -20V, the system does not work well. Does anyone know how I can fix this problem? I appreciate your help.