A filter is a frequency selective circuit that, passes a specified band of frequencies and blocks or attenuates signals of frequencies out side this band. Filter may be classified on a number of ways.
- Analog or digital
- Passive or active
- Audio or radio frequency
Analog filters are designed to process only signals while digital filters process analog signals using digital technique. Depending on the type of elements used in their consideration, filters may be classified as passive or active.
Elements used in passive filters are resistors, capacitors and inductors. Active filters, on the other hand, employ transistors or OP-AMPs, in addition to the resistor and capacitors. Depending upon the elements the frequency range is decided.
RC filters are used for audio or low frequency operation. LC filters are employed at RF or high frequencies.
The most commonly used filters are these:
- Low pass filters
- High pass filter
- Band pass filter
- Band reject filter
- All pass filter
Figure 1, shows the frequency response characteristics of the five types of filter. The ideal response is shown by dashed line. While the solid lines indicates the practical filter response.
Figure 1
A low pass filter has a constant gain from 0 Hz to a high cutoff frequency fH. Therefore, the bandwidth is fH. At fH the gain is down by 3 dB. After that the gain decreases as frequency increases. The frequency range 0 to fH Hz is called pass band and beyond fH is called stop band.
Similarly, a high pass filter has a constant gain from very high frequency to a low cutoff frequency fL. Below fL the gain decreases as frequency decreases. At fL the gain is down by 3 dB. The frequency range fL Hz to ∞ is called pass band and below fL is called stop band.
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