TWIN-T NOTCH FILTER Circuit Design Using TLC271 Op Amp

The theory of a bipolar twin- T notch filter was discussed in the section under Active Filter Applications. That twin-T filter required ± 15-V power supplies. This filter however, can be powered with a single 5-V supply. Active filters built with LinCMOS op amps will operate well at low frequencies from a single low-voltage supply. They also require minimum space because of their low component count. The 6O-Hz filter illustrated in Figure 3-42 has only one op amp, three resistors, and three small capacitors.

Figure 3-42. Twin-T Notch Filter for 60 Hz

If the resistors and capacitors are carefully matched, you theoretically obtain almost complete rejection of the 6O-Hz null frequency. The TLC271 has an input bias current of about 1 pA and will not generate adverse offset voltages even though the source impedance is 20 MΩ. Low-level 6O-Hz ripple will also be attenuated due to the TLC271’s capability of handling input signals at the ground rail while operating from a single supply. As illustrated in Figure 3-43, a 39-dB notch was achieved with this circuit.

Figure 3-43. TLC271 Twin- T Notch Filter Frequency Response