Abstract: An amplifier for an MR head comprises a means for switching cut-off frequencies of a plurality of lowpass filters, when a data signal superposed by a disturbance signal (a composite signal) is input from an MR head, a means for extracting the disturbance signal from the composite signal, a means for adding the extracted disturbance signal to the composite signal. Accordingly, it possible to obtain a data signal less influenced and less distorted even if a disturbance signal appears.

Abstract: A signal reproduction circuit used for an MR head has a simple circuit configuration to suppress a disturbance signal caused by the contact between the MR head and a disk, and to remove the ripples completely from the reproduced signal. The signal reproduction circuit used for the MR head includes a first circuit for generating a switching signal in accordance with a disturbance signal extracted from an input data signal, a second circuit including a filter circuit and a switching circuit, the filter having a first and a second cut-off frequency, the switching circuit for switching either the first of second cut-off frequency for outputting an un-suppressed data signal and a suppressed disturbance signal.

Abstract: A single end input type high frequency amplifier of the present invention decreases noise superposed on a power supply of a pre-amplifier in a read amplifier which amplifies a signal from MR head. The single end input type high frequency amplifier includes a transistor connected to an input side terminal of a differential input amplifier. A single end type resistor is connected to the transistor and a loop amplifier is connected to the single end type resistor to feedback a difference voltage between a reference side terminal and an input side terminal of the differential input amplifier.

Abstract: There is disclosed an amplifier circuit wherein a first differential amplifier consisting of an I/O circuit (11) and a constant-current circuit (31) and a second differential amplifier consisting of an I/O circuit (12) and a constant-current circuit (32) are connected to both first and second input terminals (1, 2), and current synthesizing circuits (21 to 24) synthesize the signals output from the first and second differential amplifiers to apply the synthesized signal to a current-to-voltage converter circuit (9), and wherein a clamp circuit (51) inhibits the operation of the first differential amplifier when the input signals at the input terminals (1, 2) are not less than (V.sub.cc -V.sub.sat), and a clamp circuit (52) inhibits the operation of the second differential amplifier when the input signals at the input terminals (1, 2) is not more than V.sub.sat.