Abstract: An operational amplifier and a digital signal transfer circuit in which an output voltage does not decrease instantly even if an abrupt change of input signal due to noise causes a high-to-low transition to be superposed on an input signal. The operational amplifier includes a differential pair of transistors with a first transistor having a collector that is grounded and a second transistor having an emitter connected to the emitter of the first transistor; a current mirror circuit with a third transistor having a collector connected to an output node of the differential pair of transistors and a fourth transistor having a base connected to a base of the third transistor; a fifth transistor having a collector connected to an output terminal and a base connected to an output node of the current mirror circuit; and a capacitive element connected between the output node of the current mirror circuit and the output terminal.

Abstract: A sensor drive circuit which accurately detects the amount of variation in inductance of a coil serving as a variable sensor. A peak detector circuit detects a peak value of a difference signal between a variable-amplitude signal and an intermediate signal to output a peak detection signal as a verification signal to a first input of an error amplifier. The error amplifier has a second input receiving a reference voltage and outputs a control signal to an AGC circuit on the basis of the result of comparison between the peak detection signal and the reference voltage. A peak detector circuit detects a peak value of a difference signal between the variable-amplitude signal and an inverted variable-amplitude signal to output the peak value as an inductance detection signal to the exterior. The AGC circuit performs automatic gain adjustment to output a signal of determined amplitude to a buffer in response to the control signal from the error amplifier.