Using Identical Components

It is simpler sometimes to use equal components, but it is necessary to adhere to the particular pass-band gain on the normalized tables. In many applications this method should be considered first.

Select a cut-off frequency value and then choose a common value for C=C₁=C₂—some value less than 1 mF, say. Since R=R₁=R₂, R can now be calculated as follows:

(17.17)

Note also that the pass-band gain has to be 1.585, this being obtained from the normalized tables.

Example 17.6

It is required to design a second-order low-pass filter with a cut-off frequency of 3 kHz.

Solution

Let C₁=C₂=0.047 μF. Hence,

Selecting the gain setting resistors is once again achieved by using the fact that these have to satisfy the equation:

Hence, R_a=0.586 R_b and several combinations are possible.