In computer data storage and retrieval systems, magneto-resistive heads, otherwise known as MR heads, are employed for reading data from a storage disk. The magnetically stored data is read by means of the change in resistance of the MR head as it passes through the magnetic field representing the data. For monitoring the varying resistance of the head, a dc bias current is passed through the head. However, this presents a dc offset to the amplifier of the head circuit, and to reduce this effect and provide suitable amplification of data signals, a capacitor is employed to reduce the dc voltage offset.
The large size of the capacitor, however, results in its relatively slow charging such that a large dc bias is present at the input of the amplifier when the head is switched to its read state. In turn, this large signal saturates the amplifier and causes it to clip the data signals. Hence, in the above described prior art circuit, data is substantially unavailable from the disk during the relatively extended time of charging of the capacitor.
Additionally, thermal asperities such as those which occur when the head strikes the disk, also result in a reading failure. The head striking the disc produces a rapid rise in head temperature, followed by a delayed return to normal, due to the slow dissipation of heat from the head. Since the resistance of the head is related to its temperature, the voltage at the input to the amplifier rises rapidly and then decays slowly when a thermal asperity occurs. As can be expected, this large signal tends to both distort the data signal and also clip the signal from the amplifier such that retrieval of data from the disk is essentially lost during this interval of increased head temperature.