Twin T High Q Bandpass Filter

Problem:
How does a designer make a Twin T High Q Bandpass filter?

Solution:
Use the circuit below:



This circuit is most suitable for use as a high sensitivity single tone detector. Absolute levels of amplitude will be hard to obtain. High Q circuits like this can also be unstable, and it may be best to de-tune it slightly.

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 sharpest peak.
  
  
• 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 width of the peak and temperature stability will be affected, however.