Datasheet
SLOS336C − DECEMBER 2000 − REVISED OCTOBER 2002
14
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
BTL amplifier efficiency (continued)
Although the voltages and currents for SE and BTL are sinusoidal in the load, currents from the supply are very
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 transistors 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.
Efficiency of a BTL amplifier +
P
L
P
SUP
(3
)
where
(4
)
P
L
+
V
L
rms
2
R
L
, and V
LRMS
+
V
P
2
Ǹ
, therefore, P
L
+
V
P
2
2R
L
P
L
= Power delivered to load
P
SUP
= Power drawn from power supply
V
LRMS
= RMS voltage on BTL load
R
L
= Load resistance
V
P
= Peak voltage on BTL load
I
DD
avg = Average current drawn from the
power supply
V
DD
= Power supply voltage
η
BTL
= Efficiency of a BTL amplifier
and
P
SUP
+ V
DD
I
DD
avg
and
I
DD
avg +
1
p
ŕ
p
0
V
P
R
L
sin(t) dt
+
1
p
V
P
R
L
[cos(t)]
p
0
+
2V
P
p R
L
therefore,
P
SUP
+
2V
DD
V
P
p R
L
substituting P
L
and P
SUP
into equation 7,
Efficiency of a BTL amplifier +
V
P
2
2R
L
2V
DD
V
P
p R
L
+
p V
P
4V
DD
V
P
+ 2P
L
R
L
Ǹ
h
BTL
+
p 2P
L
R
L
Ǹ
4V
DD
where
therefore,