# How to design Twin T Active Notch Filter?

Notch Filter are basically band stop filter which blocks signals with single frequency and allows signals with other frequencies to pass through it. Band stop filters also called band reject filters can be classified as wideband or narrowband which are just terminology to say whether the filter blocks wide range of frequencies or just some narrow band of frequencies. Narrow band stop filters designed to stop a specific narrow band of frequencies. Notch filter falls into narrow band band stop filter but the difference is that Notch filter stops a single frequency. One of the popular usage of notch filter is to filter the electrical 60/50Hz noise from the power supplies and electrical appliances. Thus, notch filter are also often called noise suppression filter or EMI suppression filters.

### Active Twin T Notch Filter or Active RC Notch Filter

An active Twin-T notch filter can be designed by adding amplifying stage using operational amplifier after the Passive twin-T notch filter. Adding op-amp to the passive Twin-T provides gain, high input impedance and provides low output impedance.

The circuit diagram of Active Twin-T Notch filter or active RC notch filter is shown below.

The center frequency of this active Twin-T notch filter is given by,

$f_{c}= \frac{1}{2 \pi R C }$

which is the same as that of passive Twin-T notch filter.

The gain of the active Twin-T notch filter is given by,

$A =1+\frac{R_{A}}{R_{B}}$

The quality factor or Q-factor of the circuit is given by,

$Q=\frac{R_{B}}{2(R_{B}-R_{A})}$

#### Active Notch filter online calculator

Inputs:

Theoretical Results:

Design Example

Here an active notch filter with Twin-T filter is designed with center frequency of 50Hz. LM358 operational amplifier is used. The Q-factor for the filter is 2.5.

Let the Capacitor C be 0.47uF and RB be 1KOhm.

Using the formula provided above or using the online active Twin-T notch filter calculator we get value of resistor R as 6.78KOhm, resistor RA as 800Ohm and gain A is 1.8.

The following shows the frequency response of the above active notch filter.

In this way we can design passive and active Twin-T notch filter which can be used for noise supression and also as EMI suppression filters. Other filter tutorials and online filter calculator are as follows: