TPA6204A1

SLOS429B − MAY 2004 − REVISED SEPTEMBER 2009

www.ti.com

11

FULLY DIFFERENTIAL AMPLIFIER

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

drop multiplied by the average value of the supply current,

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 and average values of power in the load and in

the amplifier, the current and voltage waveform shapes

must first be understood (see Figure 18).

Figure 18. 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

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

Where:

P

L +

V

L

R

L

, and V

LRMS +

V

P

2

, therefore, P

L +

V

P

2

2R

L

power supply

1

p

p

0

V

P

R

L

sin(t) dt

+* 1p

V

P

R

L

p

0

+

2V

P

p R

L

Therefore,

P

SUP +

2V

DD

V

P

p R

L

Efficiency of a BTL amplifier

+

V

P

2

2R

L

2V

p R

L

+

p V

P

4V

DD

V

P +

2P

L

R

L

Where:

h

p 2P

L

R

L

4V

DD

Therefore,

(4)

(5)