Single-Supply Twin T High Q Notch Filter

Problem:
How does a designer make a Twin T High Q Notch filter that operates from a Single-Supply?

Solution:
Use the circuit below:



There are some general techniques to use with the Twin T topology that will greatly improve its performance:

• 1% components - R's AND C's - are a necessity for the deepest notch and best stability.
  
  
• The sharpest response will come when the components are matched (by using components from the same batch and by creating R3 and C3 by paralleling 2 of the values used for R1 and C1, respectively).
  
  
• It is possible to tune the center frequency over a narrow range if R3 is reduced by one or two standard resistor values and a potentiometer added in series. The depth of the notch and temperature stability will be affected, however.

This circuit has a fixed gain of 1 (0 dB).

The designer, if not familiar with the topics, may want to review Coupling Capacitors (Cin in the schematic) and Virtual Grounding (Vcc/2 in the schematic).