# First order Active HPF Design and Test

Here an active first order high pass filter using LM358 is designed and tested. The high pass filter is build on a breadboard and tested with soundcard based software which features oscilloscope, spectrum analyzer and frequency response plotter. The signal on the oscilloscope, frequency spectrum and frequency response graph is determined in real time with actual filter on a breadboard. This can be useful to anyone who wishes to test circuit performance. The high pass filter component values were calculated using the online first order active high pass filter calculator.

### Active first order HPF on breadboard

The following shows the active 1st order high pass filter build with LM358 operational amplifier on a breadboard.

#### Circuit Schematic

The following is the circuit wiring diagram of the first order HPF using LM358 op-amp.

In the circuit above, the capacitor C and resistor R forms a high pass filter. The signal is applied to this high pass filter via Vi. This first order high pass filter is constructed in non-inverting configuration so the applied signal after passing through the high pass filter enters into the non-inverting terminal of the op-amp. The cutoff frequency of the high pass filter is given by,

$$f_c= \frac{1}{2 \pi R C}$$

where in this example we have used cutoff frequency of 1kHz. The resistor value is 16kOhm and the capacitor value is 0.01uF.

The op-amp provides a passband gain which is set by the resistor Rf and R1. The passband gain of the HPF is,

$$A_F = 1 + \frac{R_F}{R1}$$

By choosing how much gain in the passband we want and choosing either resistor Rf or R1 we can calculate the other resistor value. In this tutorial we have Rf=10Kohm and R=2.7Kohm which gives gain of 4.7. Without the proper gain in the passband the output signal may not be high enough in magnitude to process the signal further. So proper gain should be set. The above calculated values were directly obtained from the online high pass filter calculator whose link is provided above.

The power supply is dual split power supply suppling +2.5V and -2.5V to the LM358 op-amp. This was explained in LM741 non-inverting amplifier with dual supply from single supply and also in biasing LM741 for Single Supply with Split Resistor Biasing.

### Tests Results

The filter was tested and the following shows frequency response of the filter.

As you can see there is not much rolloff in the response. This is because it is a first order filter.

The following shows the signal waveform of the input to the filter and output from the filter.

In the above picture the blue waveform is the input signal and the yellow waveform is the output from the high pass filter. It shows that the magnitude of the output signal is decreased at the particular frequency relative to the magnitude of the input signal. This illustrates that the filter is decreasing the output signal magnitude as the frequency is increased.