Band‑Stop Filters: Design, Twin‑T Implementation, and Notch Frequency Analysis

How to Build a Band‑Stop (Notch) Filter

Band‑stop, or notch, filters suppress signals within a specified frequency band while allowing all lower and higher frequencies to pass. The most common topology pairs a low‑pass and a high‑pass section in parallel, mirroring the series connection used for band‑pass circuits.

Below is a system‑level block diagram that illustrates the parallel arrangement:

System‑level block diagram of a band‑stop filter.

Twin‑T Band‑Stop Filter

One of the most widely used passive designs is the Twin‑T filter, which employs two "T" networks:

• The low‑pass leg consists of resistors R₁ and R₂ with capacitor C₁.
• The high‑pass leg consists of capacitors C₂ and C₃ with resistor R₃.

When the component ratios satisfy the following relationships, the filter delivers a sharp, symmetrical rejection band:

Notch Frequency

With the ratios above, the frequency at which maximum attenuation occurs – the notch frequency – is given by:

Below is a SPICE simulation that demonstrates the filter’s response. The AC sweep shows a deep attenuation dip at the calculated notch frequency.

twin-t bandstop filter
 v1 1 0 ac 1 sin
 r1 1 2 200
 c1 2 0 2u
 r2 2 3 200
 c2 1 4 1u
 r3 4 0 100
 c3 4 3 1u
 rload 3 0 1k
 .ac lin 20 200 1.5k
 .plot ac v(3)
 .end

Response of the Twin‑T band‑stop filter.

Key Takeaways

• A band‑stop filter rejects frequencies within a defined band, passing all others.
• Parallel low‑pass and high‑pass "T" networks create the classic Twin‑T topology.
• The deepest attenuation point is called the notch frequency and can be calculated from the component ratios.

Related Resources

• Passive Filter Circuits Worksheet