Integrated CRC filter circuit

A filter circuit comprising,

• first and second circuit elements each comprising a semiconductive body portion having a bulk of one conductivity type, a pair of spaced surface zones of the opposite conductivity type and of relatively low resistivity, and a localized surface channel of said opposite conductivity type and of relatively high resistivity extending between said spaced zones,
• a dielectric gate layer overlying said channel,
• a conductive gate layer overlying said dielectric layer,
• separate terminal connections to each of said spaced zones, the bulk portion and the conductive gate layer,
• the doping of said channel and the thickness of the dielectric layer being so related that in the range of between plus or minus ten volts between said channel and said conductive gate the resistivity of said channel is substantially independent of such voltage,
• an operational amplifier configured to operate in the noninverting unity gain mode and having an input and an output terminal,
• means interconnecting the two circuit elements to connect their two channels serially,
• means interconnecting the end terminal of the second of the elements to the input of the operational amplifier, and
• means interconnecting the output of the operational amplifier to the conductive gate terminal of the first of the two elements.

In combination

• a semiconductive integrated capacitor-resistor-capacitor filter adapted for use in a filter circuit comprising,
• a semiconductive body having a bulk portion of one conductivity type, a pair of spaced surface zones of the opposite conductivity type and of relatively low resistivity, and a localized surface channel of said opposite conductivity type and of relatively high resistivity extending between said spaced zones,
• a dielectric gate layer overlying said channel,
• a conductive gate layer overlying said dielectric gate layer,
• separate terminal connections to each of said spaced zones, the bulk portion and the conductive gate layer,
• the doping of said channel and the thickness of the dielectric layer being so related that in the operating voltage range of the input means the resistivity of said channel is substantially independent of such voltage,
• the pair of terminals to the spaced zones providing therebetween a useful filter circuit resistance, and each of the pairs of terminals consisting of one from the connections to the spaced zones and the other from a connection to either the conductive layer or the bulk providing therebetween a useful filter circuit capacitance, and
• input means for supplying to said filter a signal having substantial components with frequencies comparable to the inverse of the RC time constant of said filter, whereby such components are filtered.