  For the need of measurements focused in condensed matter physics and
especially Bernoulli effect in superconductors we have developed an active
resonator with dual operational amplifiers. A tunable high-Q resonator is
performed in the schematics of the the General Impedance Converter (GIC). In
the framework of frequency dependent open-loop gain of operational amplifiers,
a general formula of the frequency dependence of the impedance of GIC is
derived. The explicit formulas for the resonance frequency and Q-factor include
as immanent parameter the crossover frequency of the operational amplifier.
Voltage measurements of GIC with a lock-in amplifier perfectly agree with the
derived formulas. A table reveals that electrometer operational amplifiers are
the best choice to build the described resonator.
