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Filters
A filter is basically a frequency selective circuit designed to pass specific band of frequencies and block or attenuate input signals outside this band
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2. Classification of filters
Depending on signal nature:
1. Analog filter Digital filter.
Depending on components:
1. active filter Passive filter.
Depending on frequencies:
1. low pass filter high pass filter
3.band pass filter band stop.
5. All pass filter

3. Ideal Filter Response Curves

4. Actual freq.response

5. RC Low Pass Filter Circuit

6. Frequency Response of a 1st-order Low Pass Filter

7. Applications
Applications of passive Low Pass Filters are in audio amplifiers and speaker systems to direct the lower frequency bass signals to the larger bass speakers.
To reduce any high frequency noise or “hiss” type distortion.

8. Terms related to filters
1.Cut-off frequency- it is the frequency at which signal strength drops 3dB(i.e. signal power becomes half.)
2. Pass band-The range of frequency which is permitted to pass.
3.Stop band- The range of frequency which is (not permitted)attenuated to pass.

9. Cont….
4.Roll off rate-The gain of filter falls rapidly in the stop band. The rate at which gain falls off is called the roll off rate.

10. Contd…
5.Center frequency- the center frequency of a filter or channel is a measure of a central frequency between the upper and lower cutoff frequencies.OR geometric mean of lower and higher frequencies.

11. 6.Band width-It is the difference between the two cut-off frequencies.
Contd…
6.Band width-It is the difference between the two cut-off frequencies.
7. Q(quality) factor-it is the ratio of center frequency to bandwidth of filter.
Q=fc/B.W.

12. Contd…
8. Order of filters-The filter characteristics is given by order of filter i.e. no.of poles.
Filter order
Roll of rate 1
20 dB/decade 2 3
40 dB/decade
60 dB/decade

13. Passive Filter vs Active Filter
R,C or L,C components are used
Gain less than 1.
Isolation between input and output absent.
Source loading can take place.
Frequency response is not sharp.
RC,LC and OP-amp are used.
Gain greater than 1.
Isolation between input and output present.
Source loading can not take place.
Frequency response is sharp

14. Second-order Low Pass Filter

15. First Order Active Low Pass Filter

16. Frequency Response Curve

17. Normalised Low Pass Frequency Response

18. First Order Active Low Pass Filter

19. Second Order Low Pass Filter

20. Second Order High Pass Filter
Graphs from Prentice Hall

21. Second Order High Pass Filter
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