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R
L
C
C
V
O(PP)
V
O(PP)
V
DD
BTL AMPLIFIER EFFICIENCY
V
L(RMS)
V
O
I
DD
I
DD(RMS)
TPA4861
SLOS163C – SEPTEMBER 1996 – REVISED JUNE 2004
APPLICATION INFORMATION (continued)
Figure 35. Single-Ended Configuration
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 times the output power of the SE
configuration. Internal dissipation versus output power is discussed further in the thermal considerations section.
Linear amplifiers are notoriously inefficient. The primary cause of these inefficiencies is voltage drop across the
output stage transistors. The internal voltage drop has two components. One is the headroom or dc voltage drop
that varies inversely to output power. The second component is due to the sine-wave 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 as 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 36 ).
Figure 36. Voltage and Current Waveforms for BTL Amplifiers
Although the voltages and currents for SE and BTL are sinusoidal in the load, currents from the supply are
different between SE and BTL configurations. In an SE application, the current waveform is a half-wave rectified
shape, whereas in BTL it is a full-wave rectified waveform. This means RMS conversion factors are different.
Keep in mind that for most of the waveform, both the push and pull transistor are not on at the same time, which
supports the fact that each amplifier in the BTL device only draws current from the supply for half the waveform.
The following equations are the basis for calculating amplifier efficiency.
14