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Abstract

An active filter circuit has a biquadratic transfer function and is constructed without coils with the smallest possible number of passive components, particularly capacitors, and comprises a single operational amplifier. A first resistor is connected from the filter input to the non-inverting input of an operational amplifier and a second resistor connects the same input of the operational amplifier to a reference potential. A third resistor is connected between the output of the operational amplifier and the non-inverting input. A first capacitor is connected from the output of the operational amplifier to the inverting input and the same input is connected, by way of a fourth resistor to the reference potential for representation of a low-pass transfer function and a fifth resistor is connected between the input of the filter and the inverting input of the operational amplifier for representing a high-pass transfer function. In addition, a sixth resistor connects the inverting input of the operational amplifier to a junction which is connected to the output of the filter by way of a seventh resistor and to the reference potential by way of an eighth resistor. A second capacitor also connects the filter input to the aforementioned junction. The reference potential is connected to a line extending through the filter and forming one terminal of a pair of input terminals and one terminal of a pair of output terminals. The circuit can be constructed, in particular, in layer technology and is suited as a selection device, particularly in the low-frequency range in communications systems.

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