User Manual

The TPA005D02 Audio Power Amplifier Evaluation Module

3-15

Details

3.2.4 Efficiency of Class D vs. Linear Operation

Amplifier efficiency is defined as the ratio of output power delivered to the load

to power drawn from the supply. In the efficiency equation below, P

is power

across the load and P

SUP

is the supply power.

Efficiency

SUP

A high-efficiency amplifier has a number of advantages over one with lower

efficiency. One of these advantages is a lower power requirement for a given

output, which translates into less waste heat that must be removed from the

device, smaller power supply required, and increased battery life.

Audio power amplifier systems have traditionally used linear amplifiers, which

are well known for being inefficient. Class D amplifiers were developed as a

means to increase the efficiency of audio power amplifier systems.

A linear amplifier is designed to act as a variable resistor network between the

power supply and the load. The transistors operate in their linear region and

voltage that is dropped across the transistors (in their role as variable

resistors) is lost as heat, particularly in the output transistors.

The output transistors of a class D amplifier switch from full OFF to full ON

(saturated) and then back again, spending very little time in the linear region

in between. As a result, very little power is lost to heat because the transistors

are not operated in their linear region. If the transistors have a low ON

resistance, little voltage is dropped across them, further reducing losses. The

ideal class D amplifier is 100% efficient, which assumes that both the ON

resistance (R

DS(ON)

) and the switching times of the output transistors are zero.

3.2.4.1 The Ideal Class D Amplifier

To illustrate how the output transistors of a class D amplifier operate, a

half-bridge application is examined first (Figure 3–9).

Figure 3–9. Half-Bridge Class D Output Stage

OUT

–