Figure 17.36 : Active circuit (twin-T)
This is the frequency at which the signals passing along the two branches appear to be in antiphase and hence cancel. This cancellation effect causes a sharp dip in the response at and close to the resonant frequency.
For signals above the critical frequency the low-pass response disappears and prevents the input signal from passing through the integrators. This results in the bandpass output peaking sharply at the critical frequency.
The Q factor or selectivity of the fi lter is determined by R5 and R6 in Figure 17.37 and may be calculated from the expression: