AMPLIFIER

PURPOSE:To stabilize DC operation and to reduce noises by providing an FET amplification stage with the 2nd amplification stage and the 3rd and 4th LPF type amplification stages, and providing negative feedback from the 3rd amplification stage to the 2nd amplification stage and from the 4th amplification stage. CONSTITUTION:The 1st amplification stage consists of FETs 21 and 22; parallel- connected gates are connected to an input terminal 41, and parallel-connected drains are connected to an output terminal 42 through a differential amplifier 23 as the 2nd amplification stage and the sources of the FETs 21 and 22 are connected to a differential amplifier 25 as the 4th amplification stage of an LPF type. A DC component amplified by the amplifier 24 is fed back to the amplifier 23 negatively through a transistor (TR) 26. The output of the amplifier 25 is fed back to the sources of the FETs 21 and 22 negatively through a TR27 as a current source. Consequently, the DC output level of the amplifier itself is held at the zero potential and the FETs are operated with saturated drain currents.