Understanding output filters for Class-D amplifiers

Single-ended amplifiers also have a few disadvantages compared to amplifiers with bridge-tied load (BTL) outputs. First, single-ended amplifiers require either split positive and negative power supplies or DC blocking capacitors. If DC blocking caps are used, they need to be large in order to prevent them from affecting the low-frequency performance of the amplifier. For example, an amplifier with an 8Ω speaker needs a 1000µf cap in order to achieve a -3dB point of 20Hz.

DC blocking caps can also cause audible pops as they charge up to Vcc/2 when the amplifier is turned on. A resistor divider from Vcc to ground can be used to charge the capacitor up to Vcc/2 at a relatively slow rate when the power is turned on, minimizing or eliminating the pop.

If the amplifier does not have feedback then PSRR might be a problem. Two DC blocking capacitors can be used to create a low-impedance AC voltage divider to improve the PSRR. If two DC blocking capacitors are used then each cap only needs to have ½ of the capacitance because the circuit sees the parallel impedance of the two capacitors.

Figure 2a: Single-ended amplifier with a DC blocking capacitor

Figure 2b: Single-ended amplifier with a resistor divider to minimize pop at turn on

Figure 2c: Single-ended amplifier with two DC blocking caps to improve PSRR

BTL Outputs
Amplifiers with BTL outputs are popular because they do not require DC blocking caps even when operating with a single positive power supply. DC blocking caps limit the low-frequency response of the amplifier and can be quite large.

BTL amplifiers have another advantage over amplifiers with single-ended outputs - the maximum peak-to-peak voltage that a amplifier with BTL outputs can apply to the speaker is twice the power supply voltage, which in turn means that up to four times as much output power can be delivered to the load compared to a single-ended amplifier. This can be a big advantage in applications where the power supply voltage is limited, especially in portable applications where the amplifier is operating off of a battery.