A read-channel module including a VGA module to amplify a signal based on a variable gain, an ADC module to generate a sample based on the amplified signal, and an AGC module to control the variable gain of the VGA module. A gain adjusting module generates (i) a first gain and (ii) a second gain when an amplitude of the sample is (i) less than or equal to a first predetermined threshold and (ii) greater than or equal to a second predetermined threshold, respectively. The AGC module (i) increases and (ii) decreases the variable gain of the VGA module based on (i) the first gain and (ii) the second gain, respectively. The gain adjusting module generates the first gain and the second gain by (i) multiplying a present gain of the AGC module by a predetermined multiplier or (ii) adding a predetermined offset to the present gain.