US 4,383,230 AGranted

Voltage tuned active filter and circuitry simulating a capacitance and an inductance

OwnerDavid B. Manzoliniinventor

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Abstract

An active filter provides precision filtering of an output signal over a wide range of signal input frequencies. Overall the active filter comprises a simulated L/C resonating loop which includes: a first operational amplifier in a loop with an operational transconductance amplifier simulating a capacitance; and a second operational amplifier in a second loop with a second operational transconductance amplifier simulating an inductance. The apparent capacitance of the first loop and the apparent inductance of the second loop can be regulated by applying a control current to each of the operational transconductance amplifiers. In a preferred embodiment the control current is derived as an antilogarithm of a linear voltage where the linear voltage corresponds to the logarithm of the filter resonant frequency. The filter output provides a constant percentage band width signal independent of the tuned frequency. The amplitude gain at resonance similarly is independent of the tuned frequency.

Claims10

Dependent claims

A voltage-tunable active filter comprising a resonating closed loop circuit including

a simulated capacitance comprising a first operational amplifier connected in a first closed loop with a first operational transconductance amplifier and means for applying a control current to the said first operational transconductance amplifier for establishing the simulated capacitance of the said first closed loop;
a simulated inductance comprising a second operational amplifier connected in a second closed loop with a second operational transconductance amplifier and means for applying a control current to the said second operational transconductance amplifier for the purpose of establishing the simulated inductance of the said second closed loop;
conductor means joining the output terminal of the said second operational transconductance amplifier to the inverting input terminal of the said first operational amplifier;
the inverting input terminals of both operational transconductance amplifiers being connected through a resistor to ground;
the said first closed loop, said second closed loop, said conductor means and ground constituting the said resonating closed loop;
means for introducing a multi-frequency input signal into the said resonating closed loop;
means for withdrawing from the said resonating closed loop an output signal having amplified signal components within a constant percentage band width about the resonating frequency of the said resonating closed loop.

4 more elements

Dependencies are inferred from the claim text.