A band reject filter is one which does not allow a specific band of frequencies to pass through it. It is also known as Notch filter, owing to the shape of its frequency response curve. This filter thus reject a narrow frequency range and allows all other frequencies. The name twin T is due to the twin T RC filters attached to each other via Ground.

In this tutorial, we will build a band reject filter for rejecting 50-60 Hz AC supply hum. We will see the circuit, do a simulation and finally make the circuit practically and will finally compare the simulated and practical results.

Step 1: Parts Required

2pc 470k resistor 0.25W
1pc 100k resistor 0.25W
3 10nF non polar ceramic capacitor
Male Berg strip
PCB and soldering kits

Step 2: Circuit and Simulation

Twin T notch is a very popular topology to build a band reject filter. This circuit is very popular to reject a particular narrow band of frequencies. Here I will build the circuit to remove supply voltage hum, consisting of the narrow frequency range 50-60 Hz. This circuit requires 3 resistors and 3 capacitors, The circuit has a very big transfer function and formula, so we use an online calculator tool to get the required frequencies. You can also see the transfer function and cutoff formulae at the same site

Link for the online calculator: http://sim.okawa-denshi.jp/en/TwinTCRkeisan.htm

R1=R2=470k. R3=100k, C1=C2=C3=10nF. This is a very good value to reject the 50-60 Hz supply hum.

Then I did a simulation on Multisim online

Circuit link: https://www.multisim.com/content/eZVUQzD8mxj68ijC9nvoB5/multiple-feedback-bandpass-filter/

So we see the circuit is working as I've expected and so moved on to build the veroboard circuit.

Step 3: Hardware and Testing

I build the veroboard circuit, then I took my DSO and used my mobile as Function Generator.

Link: https://www.keuwl.com/FunctionGenerator/

Then I apply various input frequencies to the circuit and observed the corresponding output on DSO. Noted down values and then calculated Gain factors.
I also did the same for the simulation circuit previously.

Then I plotted Bode plot curves for both simulation and hardware, you can see in the image that both curves are very much similar looking and can reject the aforesaid frequencies and hence proves that our circuit is successful. I can now attach an opamp or bjt (at input or output of this circuit, as per application) to act as buffer and thus can successfully remove AC supply hums.