TPA721

700-mW MONO LOW-VOLTAGE AUDIO POWER AMPLIFIER

SLOS231B – NOVEMBER 1998 – REVISED MARCH 2000

14

POST OFFICE BOX 655303

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.

I

DD

rms

+

2V

P

p R

L

P

V

DD

I

DD

rms

+

V

p R

L

Efficiency

+

P

L

P

SUP

Efficiency of a BTL Configuration

+

p V

P

2V

DD

+

p

P

L

R

L

2

1 2

2V

DD

(3)

Where:

(4)

P

L +

V

L

rms2

R

L

+

Vp

2

2R

L

V

L

rms

+

V

P

2

Table 1 employs equation 4 to calculate efficiencies for three different output power levels. The efficiency of the

amplifier is quite low for lower power levels and rises sharply as power to the load is increased, resulting in a

nearly flat internal power dissipation over the normal operating range. The internal dissipation at full output

power is less than in the half power range. Calculating the efficiency for a specific system is the key to proper

power supply design.

Table 1. Efficiency vs Output Power in 3.3-V 8-

Ω BTL Systems

OUTPUT POWER

(W)

EFFICIENCY

(%)

PEAK-to-PEAK

VOLTAGE

(V)

INTERNAL

DISSIPATION

(W)

0.125

33.6

1.41

0.26

0.25

47.6

2.00

0.29

0.375

58.3

2.45†

0.28

† High-peak voltage values cause the THD to increase.

A final point to remember about linear amplifiers (either SE or BTL) is how to manipulate the terms in the