SLOS406A − FEBRUARY 2003 − REVISED MARCH 2003
26
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APPLICATION INFORMATION
Figure 29 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 since it consists of the fundamental frequency only (no other harmonics
are present). An analyzer is then connected to the 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)
(Reference 1) 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
capacitors, (C
IN
), so no additional coupling is required. The generator output impedance should be low to avoid
attenuating the test signal, and is important since the input resistance of APAs is not very high (about 10 k
Ω
).
Conversely the analyzer-input impedance should be high. The output impedance, R
OUT
, of the APA is normally
in the hundreds of milliohms and can be ignored for all but the power-related calculations.
Figure 29(a) shows a class-AB amplifier system, which is relatively simple because these ampli
fiers are
linear
their output signal is a linear representation of the input signal. They take analog signal input and produce analog
signal output. These amplifier circuits 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 29(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.
Summary of Contents for TPA3003D2
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