RC Notch Filter (Twin T)

The twin T notch filter can be used block an unwanted frequency or if placed around an op-amp as a bandpass filter. The notch frequency occurs where the capacitive reactance equals the resistance (Xc=R) and if the values are close, the attenuation can be very high and the notch frequency virtually eliminated. The insertion loss of the filter will depend on the load that is connected to the output, so the resistors should be of much lower value than the load   for minimal loss.
 

  At audio frequencies, the filter could function as a bass and treble boost circuit by attenuating the mid range frequencies. Using 1.5K 
resistors and 0.1uFcapacitors, the band stop at -10dB is about 500 Hz to 2Khz. The depth and width of the response can be adjusted
somewhat with the 0.5R
value  and by adding some resistance across the C values.
 

 If the circuit is used
around  an op-amp as a bandpass filter, the response
may need to be dampened to
avoid  oscillation.

• What is the function C in this circuit?
• Why adding some resistance across the C values?
     

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