Science, Maths & Technology

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Electronic applications

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# 2.4 First-order filters

In addition to the filter categories already introduced (low-pass, band-pass, etc.), filters are categorised by their order. The order of a filter is determined by the form of the differential equation governing the filter’s behaviour. The simplest type of filter, with the simplest equation, is called a first-order filter. Higher-order filters are more complex than first-order filters, both in their circuitry and in the differential equation that governs them. The higher the order, the more effective the filter.

An example of a first-order filter is the simple circuit in Figure 8.

Figure 8 First-order filter

## Activity 3

Which of the four categories of filter shown in Figure 5 does the filter in Figure 8 belong to? You should be able to work it out from the behaviour of the capacitor at low frequencies and high frequencies. Explain your answer.

Figure 5 (repeated) Four types of ideal filter: (a) low-pass filter; (b) high-pass filter; (c) band-pass filter; (d) band-stop (notch) filter