Filter: Band Pass Filter

Today, we are going to talk about bandpass filter. The roles of bandpass filter is to emphasize the interval of frequency. It meas that it will emphasize the signal which its frequency are locate between f_low and f_high. To build bandpass filter, we use the lowpass and highpass filter concatenate it together. The signal will pass through high pass filter which its cutoff frequency is f_low and then lastly, the signal is passed low pass filter with f_high cutoff frequency.

To describe this bandpass filter, the transfer function is applied. The bandpass transfer which is in s domain, so, to concatenate transafer function we can triavially multiply it together. For example, we want to build bandpass filter which have f_low equal to 1 KHz, f_high = 10 KHz and gain is one. Then, we design the high pass filter with 1 KHz cutoff frequency and low pass filter with 10 KHz. The circuit and transfer function is shown below.

From this transfer function, it can plot the frequency response and phase response like figure below with the MATLAB code.

Now, we are going to do the real experiment with this designed circuit. We measure the voltage input and output to calculate the gain and phase. We use the gain and phase to plot the frequency and phase response in the figure below.

The data point is written in the code below.

To discuss the result, from the frequency response, the pass band is a little bit short because the f_low and f_high is not much different. When it going up and then it will going down in a short time because the cutoff frequency is locate at the -3dB of the frequency response. The construction of circuit is show with this figure below. We use the LM741 as the op amp.

Thanks for reading.