Patent ID: 7345546

Claim:
An amplifier circuit capable of changing a gain to amplify an input signal according to a control signal, comprising: a differential amplifier circuit adapted to receive and amplify the input signal to output the amplified input signal as an output signal; a first resistance and one or more second resistances adapted to set the gain, wherein a first end of the one or more second resistances is connected to the differential amplifier circuit; and one or more gain control circuits connected to a second end of the one or more second resistances, each gain control circuit receiving a control signal and a reference voltage, each gain control circuit comprising: a first transistor configured to receive the reference voltage and to output a reference voltage to the second end of the second resistances; a second transistor configured to be controlled on or off by the control signal and connected between the second end of the second resistances and one end of a third resistance having another end connected to a ground, wherein, when the second transistor is turned off, the first transistor supplies the reference voltage to the second end of the second resistances, wherein, when the second transistor is turned on, the first transistor connects the reference voltage to the ground via the third resistance such that a predetermined voltage that is determined based on the reference voltage and the third resistance is provided to the second end of the second resistances such that the gain is controllable based on the voltage level of the reference voltage.