Datasheet

FULLY DIFFERENTIAL AMPLIFIER
V
(LRMS)
V
O
I
DD
I
DD(avg)
Efficiency of a BTL amplifier +
P
L
P
SUP
where:
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
+
2 V
DD
V
P
p R
L
substituting P
L
and P
SUP
into equation 6,
Efficiency of a BTL amplifier +
V
P
2
2 R
L
2 V
DD
V
P
p R
L
+
p V
P
4 V
DD
V
P
+ 2 P
L
R
L
Ǹ
where:
(6)
TPA6203A1
www.ti.com .......................................................................................................................................................... SLOS364F – MARCH 2002 – REVISED JUNE 2008
EFFICIENCY AND THERMAL INFORMATION
Class-AB amplifiers are inefficient. The primary cause
of these 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 average value of the supply current, I
DD
(avg),
determines the internal power dissipation of the
amplifier.
Figure 33. Voltage and Current Waveforms for
BTL Amplifiers
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 Although the voltages and currents for SE and BTL
accurately calculate the RMS and average values of are sinusoidal in the load, currents from the supply
power in the load and in the amplifier, the current and are very different between SE and BTL
voltage waveform shapes must first be understood configurations. In an SE application the current
(see Figure 33 ). 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.
Copyright © 2002 – 2008, Texas Instruments Incorporated Submit Documentation Feedback 13
Product Folder Link(s): TPA6203A1