What is a Band Pass Filter? Circuit Diagram, Types, and Applications

In signal processing, filters are one kind of devices used for allowing required frequency components as well as removes unwanted frequency components. Filtering can be defined as; the interfacing signal background noise can be diminished by removing some frequencies. The circuit of the filter can be used for uniting the LPF and HPF properties into an only filter which is termed as a band pass filter. There are different kinds of filters available such as analog/digital, active/passive, linear/nonlinear, time-variant/time invariant. This article discusses an overview of the band pass filter with applications

What is Band Pass Filter?

The definition of the band pass filter is a circuit which permits the signals to flow among two particular frequencies, although divides these signals at other frequencies. These filters are available in different types; some of the BPF-band pass filter design can be done with an external power as well as active components such as integrated circuits, transistors, which are named as an active band pass filter. Similarly, some of the filters use any kind of power source as well as passive components like capacitors and inductors, which are named as a passive band pass filter.

These filters are applicable in wireless transmitters as well as receivers. In a transmitter, a BPF can be used to limit the output signal’s bandwidth toward the minimum necessary level & transmitting data at the preferred speed & form. Similarly, in a receiver, this filter lets the signals in a favored frequency range to be decoded, whereas keeping away from signals at unnecessary frequencies. The signal to noise (S/N) ratio of a receiver can be optimized by a BPF.

Band Pass Filter Circuit

The best example of a band pass filter circuit is the RLC circuit that is shown below. This filter can also be designed by uniting an LPF and HPF. In BPF, Bandpass illustrates a kind of filter otherwise procedure of filtering. It is to be differentiated from passband that refers to the real section of the influenced spectrum. An idyllic bandpass filter doesn’t have gain and attenuation, so it is totally level passband. That will totally attenuate every one of frequencies exterior the passband.

Band Pass Filter Circuit
Band Pass Filter Circuit

Practically, the bandpass filter is not ideal and doesn’t attenuate every one of frequencies outside the preferred frequency choice totally. Particularly, there is a section just outside the proposed pass band wherever frequencies are attenuated, however not discarded which is called like the filter roll-off, & usually, it is specified in dB of attenuation for every octave otherwise decade of frequency. In general, the filter design looks to build the roll-off as thin as feasible, therefore letting the filter to do the proposed design. Frequently, this can be attained at the expenditure of passband ripple otherwise stopband ripple.

The filter bandwidth can be defined as the dissimilarity among the upper frequency as well as lower frequency. The form factor is the fraction of bandwidths calculated with two dissimilar attenuation values for determining the cut-off frequency, For example., a form factor of 2:1 at 20/2 dB means the bandwidth calculated among frequencies at 20 dB attenuation is double that calculated among frequencies at 2 dB attenuation. Optical BPFs are commonly used in photography as well as lighting work in theatre. These kinds of filters take the outline of a clear colored film otherwise sheet.

Different Types of Band Pass Filters

The categorization of the bandpass filter can be done in two types such as wide bandpass filter as well as narrow band pass filter.

Wide Band Pass Filter

A WBF or wide bandpass filter (WBF) can be formed by dropping low pass as well as high pass segments which is normally a different circuit intended for simple design & act.

Wide Band Pass Filter
Wide Band Pass Filter

It is recognized with a number of practical circuits. A bandpass filter with ± 20 dB/ decade can be formed by using the two sections like a 1st order low pass as well as high pass sections can be dropped. Similarly, a bandpass filter with ± 40 dB/ decade can be formed by connecting two second-order filters in series namely low pass and high- pass filter (HPF). This means the order of the bandpass filter (BPF) is ruled with the order of the low pass & high pass filters. The bandpass filter graph is shown below.

Frequency Response of BPF
Frequency Response of BPF

A bandpass filter with ± 20 dB/decade can be is composed of a 1st order HPF (high pass filter). A 1st order LPF (low-pass filter) is shown in the following figure by its frequency response.

Narrow Band Pass Filter

Generally, a narrow bandpass filter uses several feedbacks. This bandpass filter using an op-amp as shown in the following circuit diagram. The main features of this filter mainly include the following.

Narrow Band Pass Filter
Narrow Band Pass Filter

Another name of this filter is a multiple feedback filter because it includes two feedback lanes

An op-amp is utilized in the inverting mode

The frequency response of this filter is shown in the following figure.

Frequency Response of NBPF
Frequency Response of NBPF

Usually, the designing of this filter can be done for exact values of center frequency (fc) & bandwidth or center frequency & BW. The components of this circuit can be determined by the following relationships. Each of the C1 and C2 capacitors can be taken to C for the simplifications of design calculation.

R1 = Q/2∏ fc CAf
R2 =Q/2∏ fc C(2Q2-Af)
R3 = Q /∏ fc C

From the above equations, at middle frequency Af denotes the gain, so Af = R3 / 2R1

But, the Af should satisfy this statement Af<2Q2

The multiple feedback filters’ fc (center frequency) can be altered toward a novel frequency fc with no changing the bandwidth or gain. This can be attained just by altering R2 to R2’ so that

R2’ = R2 * (fc/fc)2

Band Pass Filter Calculator

The following circuit is the passive bandpass filter circuit. By using this circuit we can calculate the passive bandpass filter. The formula for passive bandpass filter calculator is shown below.

Passive Band Pass Filter Calculator
Passive Band Pass Filter Calculator

For low cut off frequency = 1/2∏R2C2

For high cut off frequency = 1/2∏R1C1

Similarly, we can calculate for active inverting op-amp BPF, and active non-inverting op-amp BPF.

Band Pass Filter Applications

The applications of bandpass filters include the following.

  • These filters are extensively applicable to wireless transmitters & receivers.
  • This filter can be used to optimize the S/N ratio (signal-to-noise) as well as the compassion of a receiver.
  • The main purpose of the filter in the transmitter is to limit the BW of the output signal to the selected band for the communication.
  • BPFs are also widely used in optics such as LIDARS, lasers, etc.
  • The best application of this filter is audio signal processing, wherever a specific range of sound frequencies is necessary though removing the rest.
  • These filters are applicable in sonar, instruments, medical, and Seismology applications
  • These filters involve communication systems for choosing a particular signal from a variety of signals.

Therefore, this is all about band-pass filter theory which includes, circuit diagram with working, types of bandpass filters and its applications. From the above information, finally we can conclude that the other fields of applications of these filters include in astronomy, these filters permit only an only section of the range of light into a device. These filters can assist in finding wherever stars recline on the major series, recognizing redshifts, etc. Here is a question for you, what is an active bandpass filter?

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