First-order analog active bandpass filter with variable center frequency
simulate this circuit &ndash; Schematic created using CircuitLab

The shown bandpass circuit (GIC bandpass) uses an active simulated inductor. The corresponding equations are:

R1=R, R5=R6, k2=R2/R1, k4=R4/R1, Co=C3=C

Midband gain: Ao=2

Midband frequency: wo=1/[RC*SQRT(k2k4)] 

Quality factor: Q=1/[SQRT(k2k4)]=woRC

Because bandwidt BW=wo/Q=1/RC we can tune the frequency with k2 or k4 (single-element tuning) without changing the bandwidth BW.

As an alternative, we can tune the midfrequency wo by varying R1=R without touching the quality factor Q.