| 摘要 |
The first control circuit controls the gain of the variable gain amplifier circuit and the cut-off frequency of the first low-pass filter so as to keep constant a product of a transfer function of the variable gain amplifier circuit and a transfer function of the first low-pass filter. The second control circuit compares a level of the second feedback signal and a preset threshold value after completion of gain control of the variable gain amplifier circuit, the second control circuit inputting the signal outputted from the first low-pass filter, as the first feedback signal to the first terminal as long as the level of the second feedback signal is not lower than the threshold value, the second control circuit inputting the signal outputted from the second low-pass filter, as the first feedback signal to the first terminal.
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