Special band-stop filter: Double T filters are used in many 11 circuits. This type of filter can be made into a perfect band-stop filter, at least in theory. In the present circuit (Fig. 1), a double T filter is used in a different manner. It can have the characteristic of the combination of either a band-stop filter and a low-pass filter (switch S1 in position 1) or a high-pass filter and a band-stop filter (S1 in position 2). The characteristic curves are given in Fig. 2 and 3 respectively.
The variable Kin Fig. 2 and 3 is determined by the setting of P1. Variable M depends on the setting of P2 and determines the Q factor. A drawback of the present design is that the maximum suppression is diminished slightly. At M= 0.75, maximum attenuation is (calculated) 50 dB. With components values as in Fig. 1, the frequency of that attenuation is I kHz. Other frequencies can be computed easily. The band-stop frequency, ƒbs, with S1 in position 1 is
ƒbs = 1/2Π RCK
With S1 in position 2,
ƒbs = K/2Π RC.
In all cases, 0< k <1; C is in farad and R is in ohms. Various types of IC may be used for the operational amplifier.
The current drawn by the circuit is around 2 mA.