Passive Band Pass Filter : Circuit, Working, Gain & Its Applications

Filter circuits filter out frequencies within electronic circuits. These circuits utilize a combination of resistors & capacitors as their fundamental construction blocks. This filter circuit is necessary in the power supply block diagram after the rectifier circuit because it changes a pulsating AC to DC and it supplies in a single direction only. A filter circuit detaches the available AC component within the rectified output & permits the DC component to arrive at the load. There are different types of filters available, among them band pass filter (BPF) is one of the types. This filter allows frequencies in a specific range of frequency & attenuates frequencies when it is outside range. These filters are available in different types but passive BPF is one of the types. So, this article provides brief information on a passive bandpass filter, its working, and its applications.

What is a Passive Band Pass Filter?

The combination of both the low pass filter & high pass filter is known as the passive band pass filter. This type of filter allows a certain band of frequencies & blocks all the remaining frequencies. This is an electrical circuit that uses passive elements only like R, C & L. So this filter is made by cascading two filters like LPF & HPF. The main use of a passive bandpass filter is in an audio amplifier. Sometimes in audio amplifiers, we require a certain frequency range that does not begin from 0 Hz & not a high frequency, although we require a certain range of frequency band, either it is wider or narrow range.

Passive Band Pass Filter Circuit Diagram

The passive filter uses only passive components such as; resistors, inductors & capacitors. Thus, the passive bandpass filter can also use passive components & it does not utilize the operational amplifier for amplification. The amplification part similar to an active band pass filter is not present within a passive band pass filter. The passive band pass filter circuit diagram also includes high-pass & low-pass filter circuits. So the first part of the circuit is for the passive HPF whereas the second half of the circuit is for the passive LPF.

Passive Band Pass Filter Design

The passive bandpass filter design can be done simply using resistors & capacitors. The passive band pass filter circuit doesn’t need any power and is not utilized for any active amplification. These types of bandpass filters are used in addition to an active circuit for providing amplification but by itself, they don’t provide any amplification. These filters are designed with a combination of an HPF & an LPF.

The required components to make this circuit mainly include; capacitors – 1nF & 1μF, resistors – 150Ω & 16KΩ. To build this circuit, this circuit needs only resistors and capacitors. For this filter circuit, the pass band ranges from 1KHz to 10KHz for the resistors and capacitor values chosen. If we modify these frequencies, the resistors & capacitors values need to be changed.

This circuit has two parts like high pass filter and a low pass filter. The first part of this circuit is composed of R1 & C1 to form the HPS. So this filter allows simply all frequencies over the point it is designed mainly to pass. This filter design simply forms the lower cut-off frequency point but the required lower cutoff frequency point in this circuit is 1KHz. So, the HPF allows above 1KHz frequencies.
The lower cut-off frequency can be calculated with the following formula.

The lower cut-off frequency = 1/2πR1C1.

We know the values of resistor and capacitor like; R1 = 150Ω and C1 = 1μF, so substitute these values in the above equation, and we can get;

The lower cut-off frequency = 1/2π(150Ω)*(1μF) => 1061 Hz => 1KHz.

This filter allows above 1KHz all frequencies & blocks simply all frequencies or attenuates greatly all frequencies under 1KHz.

Similarly, the second part of this circuit is composed of resistor R2 & capacitor C2 to form the LPF. This filter blocks all frequencies under the cut-off point.

Here we need the higher cut-off frequency to be 10 KHz within this filter circuit, thus this circuit allows simply below 10 KHz all frequencies to be passed & blocks all frequencies above 10 KHz point.
The formula to calculate the higher cutoff frequency is the same to lower cut-off frequency, frequency => 1/2π R2C2

We know the values of resistor R2 and capacitor C2 like; R2 = 16KΩ & C2 = 1nF, so substitute these two values in the above equation then we can get;

Higher cutoff frequency = 1/2π(16KΩ)*(1nF)= 9952Hz => 10KHz.

Thus, the HPF allows all frequencies above the lower cut-off point whereas the LPF allows all frequencies under the higher cutoff frequency. So this will create a band-pass filter where the filter has a passband in between the lower & higher cutoff frequencies.

To avoid the loading effect on LPF from the HPF, it is recommended that the R2 resistor value must at below 10 (or) above the R1 resistor. In this circuit, we make the R2 resistor value 100 times higher.

Working

This circuit works by allowing full-strength signals in between the low pass filter & the high pass filter frequencies. If the low-pass filter (LPF) is designed for 2KHz frequency whereas the high-pass filter (HPF) is designed for 200Hz frequency, then this circuit generates output signals between 200Hz & 2KHz with near full strength or complete strength.

When the generated signals are outside of this range frequencies will be attenuated greatly, thus their amplitudes are very low as compared to the amplitude of the signal within the pass band. The pass band refers to the signals in between the high pass and low passes filters which are passed throughout full strength.

Here, the pass band is 200Hz to 2 KHz then the low cutoff frequency is 200Hz & the high cutoff frequency is 2 KHz. In the pass band, these two frequencies are the two points within the passband where there is a 3dB drop within amplitude. So this drop is equivalent to 0.707VPEAK.

In the following bandpass graph, there is peak amplitude (VPEAK). Here the amplitude will drop whenever you get these two frequencies. Once it achieves 0.707VPEAK, then this is the 3dB cutoff point that signifies half the maximum power. After the 3dB cutoff points, there is a steep drop in amplitude, thus frequencies outside of the cutoff frequencies are highly attenuated.

Here we have two main frequencies; the lower cutoff frequency at 1 KHz & the higher cutoff frequency at 10 KHz. So the center frequency is known as the frequency in between higher and lower cutoff frequency which is measured by using the formula √(f1)(f2) => √ (1061)(9952) => 3249 Hz.

The output signal around this frequency has full strength & is at its highest peak value. When we get close to this frequency, then the value will attenuate or reduce within amplitude. The amplitude is 0.707VPEAK at the cutoff frequencies. For instance, if VPEAK measures 10Volts from peak to peak at the cutoff frequencies, then the amplitude is 7V approximately because 10V * 0.707V => 7V.

Gain of Passive Band Pass Filter

The gain of the passive band pass filter is always below the input signal, so the output gain is less than unity. The output signal at the center frequency is within phase, although the output signal below the center frequency leads the phase with a +90° shift & the output signal above the center frequency will lag within phase by -90° phase shift. Whenever we provide electrical isolation in between the two filters then we can obtain better filter performance.

Applications

The applications of passive bandpass filters include the following.

• The Passive Band Pass Filter is used for isolating or filtering out certain frequencies that lie in a specific band (or) range of frequencies.
• These filters are used within audio amplifier circuits or applications like; pre-amplifier tone controls (or) loudspeaker crossover filters.
• These apply to transmitter & receiver circuits within wireless communication medium.

Thus, this is an overview of a passive bandpass filter, circuits, working, and their applications. This filter is the combination of HPF and LPF and it allows a selective frequency range. This filter circuit allows a wide & narrow range of frequencies. The cut-off frequency of higher & lower mainly depends on the filter design. Here is a question for you, what is BPF?