The circuit contains an input signal unit, an amplifier unit, and a comparator unit, cascaded in the foregoing order. The circuit further contains a ramp generator unit providing a ramp signal to the comparator unit. In one embodiment of the present invention, at least one of the two links between the input signal unit and the amplifier unit is connected to a voltage unit. By providing an appropriate voltage through the voltage unit, the signals fed into the amplifier unit have different DC bias voltages. As such, the ramp signal crosses these signals at different points of time respectively, and the zero-crossing times of the first and second Class-D signals output by the comparator unit are thereby separated so as to achieve low switching noise and electromagnetic interference.