3 Post Filter Feedback

TI’s Post filter feedback allows the user to add the ferrite bead inside the class-D loop as shown in Figure 3-1. This change in loop configuration makes sure that the errors added by ferrite beads are added inside the class-D loop, and are corrected by the loop gain. This coupled with the high loop gain that comes with TI’s 4^th order class-D amplifiers in TAS 2764, 2780, 2781, and TAS2783 makes sure that THD degradation due to the ferrite bead addition are minimized, allowing users to use the more cost optimum ferrite beads without worrying about the adverse effects of these beads. Also, since the loop is closed after the ferrite beads filters, there is no dip in the frequency response of the amplifier, giving the amplifier a flat amplitude response across the audio band of frequencies. Figure 3-2 compares the THD+N vs Frequency for TAS2560 with 2506036017Y2 ferrite bead filter coupled with a 100pF parallel cap. As shown in Figure 3-2, the THD degrades significantly at higher frequencies when the Ferrite bead is kept outside the class-D loop like in legacy class-D amplifiers. But with the new PFFB configuration, the performance degradation due to ferrite bead is greatly reduced and the performance is almost identical to that of the Class D output without the presence of bead.