Integrated CRC filter circuit

A capacitor-resistor-capacitor (CRC) element for active filter realization, which is fully integrable and compatible with MOS technology, is described. The incorporation of the CRC element in a semiconductor integrated circuit active filter also is described. The structure of the CRC filter element is closely analogous to a depletion mode MOS field effect device, except that the channel zone 26 is doped to a level which substantially precludes conductivity modulation at the usual operating voltages. However, the doping level is such as to enable the use of the channel zone as a semiconductor resistance element. Thus, the N-channel CRC element realized in the NMOS technology comprises a first capacitance composed of the gate 27, gate dielectric 38, and resistive channel 26, paralleled by the resistive channel 26 itself constituting a resistor, and then the underlying PN junction capacitance between the N-type resistive channel 26 and the underlying P-type semiconductor body portion 21. An active low-pass filter consists of two CRC elements and an operational amplifier and utilizes the positive feedback principle.