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Analyzer

20Hz-20kHz

(a)BasicClass-AB

APA

Signal

Generator

PowerSupply

Analyzer

20Hz-20kHz

R

L

(b)TraditionalClass-D

Class-D APA

Signal

Generator

PowerSupply

L

filt

C

filt

R

L

TAS5601

SLAS585 – FEBRUARY 2008

• EVM or other complete audio circuit

Figure 19 shows the block diagrams of basic measurement systems for class-AB and class-D amplifiers. A sine

wave is normally used as the input signal because it consists of the fundamental frequency only (no other

harmonics are present). An analyzer is then connected to the audio power amplifier (APA) output to measure the

voltage output. The analyzer must be capable of measuring the entire audio bandwidth. A regulated dc power

supply is used to reduce the noise and distortion injected into the APA through the power pins. A System Two™

audio measurement system (AP-II) by Audio Precision™ includes the signal generator and analyzer in one

package.

The generator output and amplifier input must be ac-coupled. However, the EVMs already have the ac-coupling

attenuating the test signal, and is important because the input resistance of APAs is not high. Conversely, the

milliohms and can be ignored for all but the power-related calculations.

Figure 19(a) shows a class-AB amplifier system. It takes an analog signal input and produces an analog signal

output. This amplifier circuit can be directly connected to the AP-II or other analyzer input.

This is not true of the class-D amplifier system shown in Figure 19(b), which requires low-pass filters in most

cases in order to measure the audio output waveforms. This is because it takes an analog input signal and

converts it into a pulse-width modulated (PWM) output signal that is not accurately processed by some

analyzers.

Figure 19. Audio Measurement Systems

Copyright © 2008, Texas Instruments Incorporated

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