Patent ID: 7471147

Claim:
An automatic gain controller comprising: a variable gain amplifier which amplifies an input signal having a specified direct current (DC) level, and outputs a first differential signal having a specified gain, a second differential signal having a 180° phase difference from the first differential signal, and a main output signal obtained by subtracting the second differential signal from the first differential signal; a full-wave-rectifying unit which receives and full-wave-rectifies the first differential signal and the second differential signal without using an operational amplifier; a low-pass filter which extracts DC component from an output signal of the full-wave-rectifying unit and outputs the extracted DC component; a reference voltage level adjustment unit which receives an input reference voltage and adjusts a DC level of the input reference voltage corresponding to a DC level produced when the first differential signal and the second differential signal are full-wave-rectified; and a comparison unit which compares an output signal of the low-pass filter with an output signal of the reference voltage level adjustment unit to adjust the specified gain of the variable gain amplifier.