Designing of an active high-pass filter

The main function of filters is to suppress or filter out components from mixed and unwanted frequency signals to ensure clear communication. In the first article of this series, we learned about the different types of filters, including low-pass filters which we covered in the last tutorial.

In this article, we’ll learn how to design a high-pass filter or HPF. These filters allow all of the frequencies that are higher than their cut-off frequency to pass while stopping all of the others.

The first order of a high-pass filter

Step 1: Select or choose the required cut-off frequency. For this tutorial, let’s suppose that we want to suppress all of the frequencies below 100 Hz. These frequencies are similar to a humming sound or power-line frequency noise (50 or 60 Hz). This means that: FC = 100 Hz
Step 2: Now, assume the required value of the capacitor. It should be less than 0.1 micro Farad. This is required for better frequency stability. Let’s use the same value for C as 100 nF (nano farad)

Steps 3 and 4 are shown here with the calculations required to find the resistance and pass-band gain for the first order of the high-pass filter.

The final design with the component values is shown below. Since the op-amp is an active component, it requires +ve and -ve biasing voltages. It’s possible to test the circuit by applying input through the signal generator and observing the output on the DSO or the oscilloscope.

A circuit diagram of the LM741 IC-based for the first order of a high-pass filter.

Note: I have simulated the above circuit in NI’s multisim 11 software. The schematic design is also prepared using the same software. The software is available as a free one-month trial period from National Instrument’s (NI) website. The below circuits are also prepared using the multisim 11 software and tested in it.

The second order of a high-pass filter

Step 1: For simplicity, let’s assume that R1 = R2 = R and C1 = C2 = C
Step 2: Select the desired cut-off frequency. For our purposes, let’s use FC = 500 Hz
Step 3: Now assume the capacitor value of C as 100nF

Steps four and five are shown here with the calculations required to find the resistance and pass-band gain for the second order of a high-pass filter.

A circuit diagram of the LM741 IC-based for the second order of a high-pass filter.

Higher-order high-pass filters
Higher-order filters, such as the third, fourth, or fifth-order filters can be designed by cascading the first and second-order LPF sections. In this case, increasing the order increases the stop-band attenuation by 20 DB.

The figure below provides a clear image of this idea. By using the higher-order filter, we can get a better response with the stiff slop. For example, we may get a response such as an idle LPF.