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f

1

2 p R

(7)

−3 dB

Single-Ended Operation

BTL AMPLIFIER EFFICIENCY

TPA0252

SLOS288B – JUNE 2000 – REVISED SEPTEMBER 2004

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, eliminating the need for 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.

Figure 37. 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, since the BTL configuration produces 4

× the output power of the SE configuration.

Internal dissipation versus output power is discussed further in the Crest Factor and Thermal Considerations

section.

In SE mode (see Figure 37), the load is driven from the primary amplifier output for each channel (LOUT+ and

ROUT+).

The amplifier switches to single-ended operation when the SE/BTL terminal is held high. This puts the negative

outputs in a high-impedance state, and reduces the amplifier's gain by 6 dB.

Class-AB amplifiers are inefficient, primarily because of voltage drop across the output-stage transistors. The two

components of the internal voltage drop are the headroom or dc voltage drop that varies inversely to output

power, and the sine wave nature of the output. The total voltage drop can be calculated by subtracting the RMS

An easy-to-use equation to calculate efficiency begins as the ratio of power from the power supply to the power

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

amplifier, the current and voltage waveforms must be understood (see Figure 38).

Figure 38. Voltage and Current Waveforms for BTL Amplifiers

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