AMPLIFIER

PURPOSE:To obtain the amplifier with a good S/N which switches operation conditions of a differential amplifier. CONSTITUTION:The amplifier has a 1st input terminal e1 and a 2nd input terminal e2, and further has 1st and 2nd output terminals eo1 and eo2, and the 1st input terminal e1 and 2st output terminal eo1 are compared and detected by a 1st differential amplifier Ad1 for comparison and detection and added through an in-phase feeding resistor RD1; and the other 2nd input terminal e2 and 2nd output terminal eo2 are compared and detected by a 2nd differential amplifier Ad2 for comparison and detection and added through an opposite-phase feeding resistor RD2, and the 2nd input terminal e1 is grounded by a 1st switch SW1. Then the 2nd output terminal eo2 is switched by a 2nd switch SW2 to the difference signal of the 1st and 2nd output terminals eo1 and eo2 to switch the input of the 2nd differential amplifier Ad2 for comparison and detection to a differential input or unbalanced input; and the amplifier has no noise generation source due to a resistance in an input circuit, so the amplifier with the good S/N is obtained.