Patent ID: 7554393

Claim:
A signal amplifier comprising: a discrete-variable-gain amplifying unit, a gain of which takes a discrete value and changes stepwise; a linear-variable-gain amplifying unit connected in series to the discrete-variable-gain amplifying unit, a gain of which changes continuously; control-signal outputting means for detecting a level of an output signal from a series connection circuit of the discrete-variable-gain amplifying unit and the linear-variable-gain amplifying unit and outputting a control signal corresponding to a difference between the level of the output signal and a reference voltage set as a comparative level to control the gain of the linear-variable-gain amplifying unit; and gain-switching control means for controlling to switch the gain of the discrete-variable-gain amplifying unit when the control signal deviates from a setting range for the control signal that is set according to a variable gain range of the discrete-variable-gain amplifying unit.