Audio characteristics

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Audio characteristics

Several parameters are used to characterize amplifier circuits. Among them are gain, total

harmonic distortion, power-supply rejection ratio and output power. The following is a brief

primer on these topics.

2.1

Harmonic distortion

Ideally, the output of an audio amplifier is a linear reproduction of its input; the output differs

only in amplitude and is identical in all other respects.

Figure 3.

Ideal and non-ideal waveforms

In reality, several things combine to distort the waveform resulting in the addition of

harmonics in the output, as depicted in Figure 4. The amplifiers are not perfectly linear, so

some reshaping of the waveform will occur. This happens in a symmetric fashion resulting in

the addition of odd harmonics.

The crossover point at which one drive transistor turns on and the other turns off is imperfect

and results in crossover distortion as shown in Figure 3 (exaggerated for effect). This too

adds odd harmonics.

The lack of perfect symmetry in the complementary output transistors will cause the top and

bottom halves of the waveforms to be asymmetric resulting in even harmonics.

The result is that the amplifier output will contain several artifacts not present in the input.

Ideal output is simply a linear scale

of the input.

Asymmetric nature of waveforms

due to imperfect symmetry of

complementary P and N transistors

Reshaping of waveforms due to

non-linear nature of transistors

Crossover distortion because

switchover from P to N channel

drivers does not occur perfectly

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DISTORTION

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