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# Designing of High Pass Filter

### Written By:

A.M. Bhatt

1st order high pass filter

Step 1: select or choose required cut off frequency. Suppose we want to suppress all the frequencies below 100 Hz. These frequencies contributes to humming sound as well as power line frequency noise (50 Hz or 60 Hz). Thus our fc = 100 Hz

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

Fig. 1: Screenshot of calculations required to find resistance and pass band gain for 1st Order High Pass Filter

Final design with component values are as shown. Op-amp is active component and it requires +ve and -ve biasing voltages. One can test the circuit by applying input through signal generator and observing output on DSO or oscilloscope as well as bode plotter as shown in figure.

Fig. 2: Circuit Diagram of LM741 IC based 1st Order High Pass Filter

Note: - I have simulated above circuit in NI's multisim 11 software. The schematic design is also prepared in same software. Software is available as free for 1 month trial period from NI's website. Following all the circuits are also prepared in multisim 11 software and tested in it.

## 2nd order High Pass Filter

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

Step 2: select desired cut off frequency. Suppose we take fc = 500 Hz

Step 3: assume capacitor value C as 100nF

Fig. 3: Screenshot of calculations required to find resistance and pass band gain for 2nd Order High Pass Filter

Fig. 4: Circuit Diagram of LM741 IC based 2nd Order High Pass Filter

## Higher order High Pass Filters

Higher order filters like 3rd, 4th or 5th order filters can be designed by cascading 1st order and 2nd order LPF sections. Increasing order increases stop band attenuation by 20 db. The figure given below gives better idea. So using higher order filter we can get better response with stiff slop. We may get response like idle LPF.

Fig. 5: Overview of 3rd, 4th and 5th Order High Pass Filter

The cut off frequency for all the stages is same. So the RC value of all stages are also same.

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