5 FILTER DESIGNS

Filters Designs
All designs of filters are either one of the following
A low pass filter
A High pass filter
A All pass filter
A Band pass filter
A Band – stop filter

Low pass filters are used for attenuating the undesired higher frequencies while letting the lower frequencies to passes through.
A High – pass filter is used to attenuate the lower undesired frequencies while allowing the high frequencies to pass.
All­-pass filter passes all frequencies at the same gain. Its main use is to change the phase of the signal and as a phase – correction circuit.
A Band pass filter is a combination of low and high pass-filters where the low pass will alternate the undesired high frequencies and the high pass alternate the undesired low frequencies living a band of frequencies in between as a Bandwidth.
A Band stop filter passes all frequencies but attenuates a certain band in between (middle). Allows upper and lower frequencies to pass. This is double pole circuit or a second order filters.
The first three are first order circuit or single pole circuits.

Sallen –Key
It is the mostly widely known second-order topology filter. It requires a single op-amp, and four passive components.
With easy tuning, the circuit is a unity again and can change style to a unity gain Butterworth.

Multiple feedback (MFB)
It is a very versatile, low cost and easy to implement but calculations are somewhat complex.

Fliege filter
Has good control over the tuning and the Q style. The gain is fixed at two for low-pass and band-pass filters and unity for Notch.

Akerberg – Mossberg filter
Easiest to use from the rest. Easy to change the gain style of low –pass and high pass and the Q of band-pass and Notch filters.
Bi Quad
Only available in a low-pass and band-pass varieties. The low-pass filter is useful whenever simultaneous normal and inverted outputs are needed.