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18W Hi-Fi AMPLIFIER AND 35W DRIVER

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Fig.21 High power active box for musical

instrument

20 to 40W

Amplifier

20 to 40W

Amplifier

LINEAR INTEGRATED CIRCUIT

Fig.22 Overshoot phenomenon in

feedback amplifiers

FEEDBACK

PATH

βV4

INPUT

PRE

V1 AMPLIFIER V2

POWER

OUTPUT

V3 AMPLIFIER V4

V1

20 to 40W

Amplifier

V2

20 to 40W

Amplifier

V3

V4

method for the measurement of TIM consicts of feeding sine waves superimposed onto square wavers,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 tather delicate

operation and an expensive spectrum analyser is essential.A new approach (see Technical Note 143(Applied by

SGS to monolithic amplifiers measurement is fast cheap,it requires nothing more sophisticated than an

oscilloscope-and sensitive-and it can be used down to the values as low as 0.002% in high power amplifiers.

The "inverting-sawtooth" method of measurement is based on the response of an amplifier to a 20KHz sawtooth

waveform.The amplifier has no difficulty following the slow ramp but it cannot follow the fast edge.The output will

follow the upper line in Fig.23 cutting of the shade area and thus increasing the mean level.If this output signal is

filtered to remove the sawtooth,direct voltage remains which indicates the amount of tIM distortion, although it is

difficult to measure because it is indistingishable from the DC offset of the amplifier.This problem os neatly avoided

in the IS-TIM method by periodically inverting the sawtooth waveform at a low audio frequency as shown in

Fig.24.Inthe case of the sawtooth in Fig. 25 the means level was increased by the TIM distortion, for a sawtooth in

the other direction the opposite is ture.

SR(V/µs)

m2

m1

Fig.23 20kHz sawtooth waveform

Input

Signal

Filtered

Output

Siganal

Fig.24 Inverting sawtooth waveform

YOUWANG ELECTRONICS CO.LTD

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