Datasheet

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APPLICATION INFORMATION
BRIDGED-TIED LOAD VERSUS SINGLE-ENDED MODE
Power 
V
(rms)
2
R
L
V
(rms)

V
O(PP)
2 2

(1)
R
L
2x V
O(PP)
V
O(PP)
–V
O(PP)
V
DD
V
DD
f
(corner)

1
2 R
L
C
C
(2)
TPA4861
SLOS163C – SEPTEMBER 1996 – REVISED JUNE 2004
Figure 34 shows a linear audio power amplifier (APA) in a bridge-tied load (BTL) configuration. A BTL amplifier
actually consists of two linear amplifiers driving both ends of the load. There are several potential benefits to this
differential drive configuration, but initially, let us 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 twice the voltage into the power
equation, where voltage is squared, yields 4 times the output power from the same supply rail and load
impedance (see Equation 1 ).
Figure 34. Bridge-Tied Load Configuration
In a typical computer sound channel operating at 5 V, bridging raises the power into an 8-Ω speaker from a
singled-ended (SE) limit of 250 mW to 1 W. In sound power that is a 6-dB improvement, which is loudness that
can be heard. In addition to increased power, frequency response is a concern; consider the single-supply SE
configuration shown in Figure 35 . A coupling capacitor is required to block the dc offset voltage from reaching the
load. These capacitors can be quite large (approximately 40 µF to 1000 µF) so they tend to be expensive,
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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