Bandstop Filter
R-C Bandstop Filter
It is a series combination of low-pass and high-pass RC filters as shown in Figure (A). In fact, it can be obtained by reversing the cascaded sequence of the RC bandpass filter. As stated earlier, this filter attenuates a single band of frequencies and allows those on either side to pass through. The stopband is represented by the group of frequencies that lie between f1 and f2 where response is below – 60 dB.
For frequencies from fc1 to f1, the following relationships hold good :
For frequencies from f2 to fc2, the relationships are as under :
In practices, several low-pass RC filter circuits cascaded with several high-pass RC filter circuits which provide almost vertical roll-offs and rises.
Moreover, unlike RL filters, RC filters can be produced in the form of large-scale integrated circuits. Hence, cascading is rarely done with RL circuits.
Series-and Parallel-Resonant Bandstop Filters
The series resonant bandstop filter is shown in Figure (C) where the output is taken across the series resonant circuit. Hence, at resonant frequency f0, the output circuit ‘sees’ a very low resistance R over which negligible output voltage V0 is developed. That is why there is a shape resonant dip in the response curve of Figure (D). Such filters are commonly used to reject a particular frequency such as 50-cycle hum produced by transformers or inductors or turn table rumble in recording equipment.
For the series-resonant bandstop filter shown in Figure (C), the following relationships hold good :
At any other frequency f,
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Parallel-Resonant Bandstop Filter
In this filter, the parallel-resonant circuit is in series with the output resistor R as shown in Figure (F). At resonance, the parallel circuit offers extremely high impedance to f0 (and nearby frequencies) as compared to R. Hence the output voltage V0 at f0 developed across R is negligibly small as compared to that developed across the parallel-resonant circuit. Following relationships hold good for this filter :
Read article – high-pass RC filters
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