Abstract: A differential amplifier includes an input stage differential amplifier circuit and an output stage differential amplifier circuit. Differential common-mode output voltages of the output stage differential amplifier circuit are converted into common-mode currents by transistors having sources connected in common. The common-mode currents are taken as input mirror currents of a current mirror circuit. Output mirror currents of the current mirror circuit are supplied to the drain side as output electrode of a constant current transistor of the input stage differential amplifier circuit to form a common-mode feedback circuit. Since the common-mode feedback circuit is constructed only by the common-mode voltage/current converting transistors and the current mirror circuit, the feedback paths can be reduced to establish a transfer function with a single pole for stability of operation of the differential amplifier.

Abstract: A gain-controllable output buffer amplifier circuit includes an operational amplifier having a grounded non-inverting input and an inverting input connected through an input resistor to a signal input node. A tapped feedback resistor having a plurality of intermediate taps is connected between an output and the inverting input of the operational amplifier. Each of the intermediate taps is connected through a transistor switch in common to a non-inverting input of an output buffer amplifier, which has its output connected to a signal output node and an inverting input of the output buffer amplifier itself. The transistor switches are controlled by a switch control circuit in such a manner that only one of transistors switches is selectively turned on.