During signal processing, a device called a Band Pass Filter or BPF is needed to allow a particular range of required frequencies and to avoid unwanted frequency ranges or components. By this filtering noise signal can be reduced by eliminating frequencies that are not required. Different types of filters like linear and nonlinear, analog, digital, active and passive, etc. are available to use based on the requirement. This article discusses an overview of Band Pass Filter.
What is Band Pass Filter?
The bandpass filter is defined as a device that allows the frequencies which are within the required frequency range and reject the remaining frequencies which are not in that specific range. Coming to high pass filter, this filter will attenuate signals which are below the cut-off frequency and low pass filter will attenuate the signals which are having frequencies higher than the cut-off frequency. But if we only require a particular range of frequencies and want to eliminate the frequencies which are out of that range, another filter can be formed by cascading high pass filter and low pass filter which can be called a bandpass filter.
There are two cuts off frequencies in this bandpass filter, one is from the high pass filter and its high cut off frequency. This will be the highest frequency limit of that band. There is another cut-off frequency from the low pass filter and its second cut off frequency known as the lower cut off frequency which decides the low-frequency range of that band.
This bandpass filter only allows the particular band and attenuates all the remaining signals above or below that cut off frequencies.
Band Pass Filter Circuit
This is the circuit diagram of the RC passive bandpass filter.
Band Pass Filter
The cascading or the combination of high pass filter and low pass filter forms a bandpass filter as it allows a particular band of frequencies and will attenuate the signals which are higher and lower than cut off frequencies. This bandpass filter will allow the input signals without distortion and Noise. The width of this band of frequencies is called bandwidth. To clearly define bandwidth, it is the particular frequency range that lies in between two cut-off points i.e. high cut-off frequency and low cut-off frequency.
Bw= fH – fL
The difference between the lowest cut-off frequency and the highest cut-off frequency is known as bandwidth.
To be an ideal bandpass filter, the filter has to filter or attenuate certain frequencies that even lies within the band because to eliminate noise. Bandpass filters are also known as second-order filters due to the factor that there exist two capacitors, reactive components within the single circuit. One capacitor in the high pass circuit and another capacitor in the low pass circuit.
Please refer to this link for Band Pass Filter MCQs
Different Types of Bandpass Filters
These bandpass filters are mainly categorized into two types of filters. They are
- Wide Band Pass Filter
- Narrow Band Pass Filter
Wide Band Pass Filter
The circuit diagram of the wide bandpass filter is shown below. Several practical circuits exist in this wide bandpass filter circuit. As we discussed this wide bandpass filter can be formed by cascading the two sections i.e. high pass and low pass.
Wide Band Pass Filter
For the formation of + or – 20db/decade bandpass filter, two sections first order high pass section and first-order low pass section are cascaded, in the same way for the formation of + or – 4odb/decade bandpass filter second order high pass section and second-order low pass section is connected. It implies that the order of low pass and high pass filters will govern the order of the bandpass filter.
BPF Characteristics
The above-shown image is the characteristics of the bandpass filter. Here wide bandpass filter composed of first-order high pass filter and first-order low pass filter is demonstrated in the figure.
Narrow Band Pass Filter
Here in this diagram, a narrow bandpass filter employing multiple feedbacks is shown. Only one OP-Amp is employed in this filter. Compare to other filters, this filter has some unique features which are very useful.
Narrow BPF
- It is called a multiple feedback filter due to two feedback paths.
- OP-Amp here existed is used in inverting mode
- We can see the frequency response of the narrow bandpass filter in the above figure. The designing of a narrow bandpass filter is done for specific values of fc or Q & BW.
For the calculation purpose
C1 = C2 = C
R1 =Q/2π fc CAf
R2= Q/2πfc C(2Q2-Af) &
R3= Q/πfc C
Here the gain at center frequency is defined as Af
Af=R3/2R1
Fc can be changed to fc’ without changing bandwidth or gain .this can be formed by simply changing R2 to R2’
R’2= R2[fc/f’c]2
Bandpass Filter Calculator Calculation
This is the passive bandpass filter circuit and the bandpass calculations can be done through this circuit.
BPF Calculation
By using the formula we can find out the required frequency in this passive bandpass filter.
For high cut off frequency = 1/2ΠR1C1
For low cut off frequency = 1/2ΠR2C2
Even we can also calculate required frequencies for the different bandpass filters like active inverting op-amp BPF and active non inverting op-amp BPF
Applications of Bandpass Filter
The bandpass filter has a wide range of applications
- It is used in optics like LIDARS, LASER, etc.
- These filters are widely used in wireless transmitters and receivers.
- In audio signal processing, these bandpass filters play a vital role by allowing the particular frequencies and removing the unwanted.
- These bandpass filters are also used in instruments and applications of medical and seismology.
One of the best applications of bandpass filters is audio amplifier circuits. We can observe simply as the speakers will play the desired range of frequencies and will attenuate the remaining. - The bandpass filter is effectively used in optimizing Signal to Noise ratio, S/N ratio, and even the sensitivity of the receiver.
- As these bandpass filters are specially designed to allow a particular band of frequencies, so it is widely used in communication systems.
Please refer to this link to know more about FIR Filter MCQs and High Pass Filter MCQs.
Thus, this is the overview of bandpass filters, its circuit, types & its applications. The bandpass filter has a wide range of applications in the present world. Here is a question for you, what is the RLC bandpass filter and RLC band-reject filter?