Bandstop filter

In electronics, a bandstop filter is a circuit with a frequency response that has one stopband in the centre, surrounded by two passbands. We can’t realise a fully vertical drop or rise, but we can realise a steeper slope. As this gap increases, the scope gets insignificant. Why a bandstop filter? When there’s a region of frequencies that we want to exclude, like ambient frequency (around 60 Hz).

The bandstop transfer function is the sum of the high-pass functions and the low-pass functions, i.e., the two sub-circuits are connected in parallel and then summed at the end.

$$T(s)=T_1(s)+T_2(s)=\frac{K_{1}s}{s+{\alpha }_1}+\frac{K_2}{s+{\alpha }_2}$$

Circuits

The inverting adder is usually used to sum the circuits at the end. We use two op amps to add the signals from the parallel connection of a high-pass and low-pass filter:

See also

• Low-pass filter
• High-pass filter
• Bandpass filter