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

摘要

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.