Datasheet
SLOS315B − JUNE 2000 − REVISED OCTOBER 2002
13
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
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.
R
L
C
C
V
O(PP)
V
O(PP)
V
DD
−3 dB
f
c
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
The primary cause of linear amplifiers 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 output. The total voltage drop can
be calculated by subtracting the RMS value of the output voltage from V
DD
. The internal voltage drop multiplied
by the RMS value of the supply current, I
DD
rms, determines the internal 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)
V
O
I
DD
I
DD(RMS)
Figure 25. Voltage and Current Waveforms for BTL Amplifiers