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APPLICATION INFORMATION

BRIDGE-TIED LOAD

Power +

V

(rms)

2

R

L

V

(rms) +

V

O(PP)

2 2

(1)

fc +

1

2p R

L

C

C

(2)

TPA741

SLOS316C – JUNE 2000 – REVISED JUNE 2004

Figure 23 shows a linear audio power amplifier (APA) in a BTL configuration. The TPA741 BTL amplifier consists

of two linear amplifiers driving both ends of the load. There are several potential benefits to this differential drive

configuration, but initially consider power to the load. The differential drive to the speaker means that as one side

is slewing up, the other side is slewing down, and vice versa. This, in effect, doubles the voltage swing on the

load as compared to a ground referenced load. Plugging 2

× V

squared, yields 4

× the output power from the same supply rail and load impedance (see Equation 1).

Figure 23. Bridge-Tied Load Configuration

In a typical portable handheld equipment sound channel operating at 3.3 V, bridging raises the power into an 8-

Ω

speaker from a singled-ended (SE, ground reference) limit of 62.5 mW to 250 mW. In sound power that is a 6-dB

improvement, which is loudness that can be heard. In addition to increased power, there are frequency response

concerns. Consider the single-supply, SE configuration shown in Figure 24. A coupling capacitor is required to

block the dc offset voltage from reaching the load. These capacitors can be quite large (approximately 33 µF to

1000 µF), so they tend to be expensive, heavy, occupy valuable PCB area, and have the additional drawback of

limiting low-frequency performance of the system. This frequency-limiting effect is due to the high-pass filter

network created with the speaker impedance and the coupling capacitance and is calculated with Equation 2.

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.

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