Single-Supply Twin T High Q Bandpass Filter

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

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.

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).