Transient intermodulation distortion (TIM)

TDA2030A

14/23

Doc ID 1459 Rev 2

4

Transient intermodulation distortion (TIM)

Transient intermodulation distortion is an unfortunate phenomen associated with negative-

feedback amplifiers. When a feedback amplifier receives an input signal which rises very

steeply, i.e. contains high-frequency components, the feedback can arrive too late so that

the amplifiers overloads and a burst of intermodulation distortion will be produced as in

Figure 25. Since transients occur frequently in music this obviously a problem for the

designer of audio amplifiers. Unfortunately, heavy negative feedback is frequency used to

reduce the total harmonic distortion of an amplifier, which tends to aggravate the transient

intermodulation (TIM situation). The best known method for the measurement of TIM

consists of feeding sine waves superimposed onto square waves, into the amplifier under

test. The output spectrum is then examined using a spectrum analyser and compared to the

input. This method suffers from serious disadvantages : the accuracy is limited, the

measurement is a rather delicate operation and an expensive spectrum analyser is

essential. A new approach applied by STMicroelectronics to monolithic amplifiers

measurement is fast, cheap (it requires nothing more sophisticated than an oscilloscope)

and sensitive - and it can be used for values as low as 0.002% in high-power amplifiers.

Figure 25.

Overshoot phenomenon in feedback amplifiers

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