Patent ID: 8538363
Filing Date: 2013-09-17
Classification: H03F,H03G

Abstract:
1. A semiconductor integrated circuit comprising: a first adder that outputs a first addition signal obtained by adding an input signal inputted to an input terminal and a first inverted signal obtained by inverting a first feedback signal outputted from a first terminal; a variable gain amplifier circuit that outputs an output signal obtained by amplifying the first addition signal to an output terminal, the variable gain amplifier circuit having a variable gain; a first low-pass filter that is fed with the output signal and outputs a signal obtained by filtering the output signal, the first low-pass filter having a variable cut-off frequency; a second low-pass filter that is fed with the output signal and outputs a signal obtained by filtering the output signal, the second low-pass filter having a lower cut-off frequency than that of the first low-pass filter; a second adder that outputs a second addition signal obtained by adding the input signal and a second inverted signal obtained by inverting a second feedback signal inputted to a second terminal; a third low-pass filter that is fed with the second addition signal and outputs a signal obtained by filtering the second addition signal, as the second feedback signal to the second terminal; a first control circuit that 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; and a second control circuit that 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 when the level of the second feedback signal falls below the threshold value.