Band Stop filter or Notch filters can be designed in many ways. The basic idea is to connect low pass filter(LFP) in parallel with a high pass filter(HPF) and sum it and use summing amplifier to sum the output from the LPF and HPF. The LPF and HPF can be either passive or active filters. Thus we can have an entire active filter in which the LPF, HPF and amplifier are all active component or we can use passive LPF and HPF and use an active amplifier summing and amplifying the passive filter output. Here we will show and explain design of passive twin T bandstop filter using passive LPF and HPF.
Passive Twin T Notch Filter or Passive RC Notch Filter
A simple notch filter can be designed with only resistors and capacitors called twin-T notch filter or RC notch filter. The following shows the circuit diagram of a Twin-T or RC notch filter.
The frequency response of twin-T notch filter or RC notch filter is given by,
where fc is the center frequency(the frequency to be stopped/rejected).
The formula to calculate the center frequency of the notch filter is given by,
\[f_{c}= \frac{1}{2 \pi R C }\]
Passive Notch filter online calculator
Design example
Consider the design of a Twin-T notch filter with center frequency of 50Hz to suppress or reject 50Hz noise from electrical power supplies. Let choose capacitor(C) of 0.47uF. Then using the above formula for center frequency of the notch filter and using the online notch calculator we can calculate the resistor value R as 6.78KOhm.
The following shows circuit diagram of notch filter with center frequency of 50Hz with calculated resistor and capacitor values.
The following shows the frequency response of the above RC notch filter circuit.
As can be seen in the frequency response graph at 50Hz the attenuation is maximum.
