High pass filter design

1st order high pass filter

Step 1: Select or choose the required cutoff frequency. Suppose we want to suppress all frequencies below 100 Hz. These frequencies contribute to hum and also to power line frequency noise (50 Hz or 60 Hz). Thus, our fc = 100 Hz

Step 2: Assume the required value of the capacitor. It must be less than 0.1 micro Farad. This is necessary for better frequency stability. Suppose we assume the same C value as 100 nF (nano farad)

Fig. 1: Screenshot of calculations required to find resistance and passband gain for 1st order high pass filter

The final project with component values ​​are shown. The operational amplifier is an active component and requires +ve and -ve bias voltages. You can test the circuit by applying the input through the signal generator and observing the output on the DSO or oscilloscope, as well as on the goat plotter, as shown in the figure.

Fig. 2: Circuit diagram of 1st order high pass filter based on IC LM741

Note: – I simulated the above circuit in NI multisim 11 software. The schematic design is also created in the same software. The software is available free of charge for a 1-month trial period on the NI website. Then all the circuits are also prepared in multisim 11 software and tested on it.

2nd order high pass filter

Step 1: For simplicity, assume R1 = R2 = R and C1 = C2 = C

Step 2: select the desired cutoff frequency. Suppose we take fc = 500 Hz

Step 3: Assume the value of capacitor C as 100nF

Fig. 3: Screenshot of calculations required to find resistance and passband gain for 2nd order high pass filter

Fig. 4: Circuit diagram of 2nd order high pass filter based on IC LM741

Higher-order high-pass filters

Higher order filters, such as 3rd, 4th or 5th order filters, can be designed by cascaded 1st and 2nd order LPF sections. Increasing the order increases the stopband attenuation by 20 db. The figure below gives a better idea. So using a higher order filter we can get a better response with hard slop. We can get responses like LPF idle.

Fig. 5: Overview of 3rd, 4th and 5th order high pass filter

The cutoff frequency for all stages is the same. Therefore, the RC value of all stages is also the same.

Circuit diagrams

Project video