Amplifier circuit and A/D converter

An amplifier circuit that amplifies differential input signals and outputs the amplified differential signals from a first and a second output node, respectively, the amplifier circuit comprising:

• a transistor differential pair having gate electrodes to which the differential input signals are respectively provided;
• a first cascode stage that is connected between a first reference voltage line and the first and the second output node, includes a first boost amplifier, and uses conductive transistors that are different from the transistor differential pair;
• a second cascode stage that is connected between a second reference voltage line and the first and the second output node, includes a second boost amplifier, and uses conductive transistors that are same as the transistor differential pair;
• a current source transistor that is connected between a common source electrode of the transistor differential pair and the second reference voltage line, and that is turned on and operates as a current source when a switching control signal takes a first value and is turned off when the switching control signal takes a second value;
• a first switch that is connected between the first output node and the first reference voltage line, and that is turned off when the switching control signal takes the first value and is turned on when the switching control signal takes the second value;
• a second switch that is connected between the second output node and the first reference voltage line, and that is turned off when the switching control signal takes the first value and is turned on when the switching control signal takes the second value;
• a third switch that is connected between an output terminal of the first boost amplifier and a first voltage line having a first predetermined voltage, and that is turned on for a predetermined period of time when a value of the switching control signal is switched from the second value to the first value and is turned off after the predetermined period of time has elapsed;
• and
• a fourth switch that is connected between an output terminal of the second boost amplifier and a second voltage line having a second predetermined voltage, and that is turned on for a predetermined period of time when the value of the switching control signal is switched from the second value to the first value and is turned off after the predetermined period of time has elapsed.