TPA721

700-mW MONO LOW-VOLTAGE AUDIO POWER AMPLIFIER

SLOS231B – NOVEMBER 1998 – REVISED MARCH 2000

13

POST OFFICE BOX 655303

APPLICATION INFORMATION

bridged-tied load (continued)

For example, a 68-

µF capacitor with an 8-Ω speaker would attenuate low frequencies below 293 Hz. The BTL

configuration cancels the dc offsets, which eliminates the need for the blocking capacitors. Low-frequency

performance is then limited only by the input network and speaker response. Cost and PCB space are also

minimized by eliminating the bulky coupling capacitor.

RL

CC

VO(PP)

VO(PP)

VDD

–3 dB

fc

Figure 24. Single-Ended Configuration and Frequency Response

Increasing power to the load does carry a penalty of increased internal power dissipation. The increased

dissipation is understandable considering that the BTL configuration produces 4

× the output power of a SE

configuration. Internal dissipation versus output power is discussed further in the

thermal considerations

section.

BTL amplifier efficiency

Linear amplifiers are notoriously inefficient. The primary cause of these inefficiencies is voltage drop across the

output stage transistors. There are two components of the internal voltage drop. One is the headroom or dc

voltage drop that varies inversely to output power. The second component is due to the sinewave nature of the

power dissipation of the amplifier.

An easy-to-use equation to calculate efficiency starts out being equal to the ratio of power from the power supply

to the power delivered to the load. To accurately calculate the RMS values of power in the load and in the

amplifier, the current and voltage waveform shapes must first be understood (see Figure 25).

V(LRMS)

VO

IDD

IDD(RMS)

Figure 25. Voltage and Current Waveforms for BTL Amplifiers