Filters Used In Electronics Circuits

A filter is a circuit capable of passing certain frequencies while attenuating (to reduce or block) other frequencies. Thus, a filter can extract important frequencies from signals that also contain undesirable or irrelevant frequencies. In the field of electronics, there are many practical applications for filters. Examples include:

• Radio communications: Filters enable radio receivers to only "see" the desired signal while rejecting all other signals (assuming that the other signals have different frequency content).
• Power supplies: Filters are used to eliminate undesired high frequencies (i.e., noise) that are present on AC input lines. This is done by the filters which stops the noise from entering and disrupting the operation of the electrical equipment. Also, it stops the electrical equipment on putting forward any noise to the power supply lines. Additionally, filters are used on a power supply's output to reduce ripple.
• Audio electronics: A crossover network is a filter network that channels low-frequency audio to woofers, mid-range frequencies to midrange speakers, and high-frequency sounds to tweeters.
• Analog-to-digital conversion: Filters are placed in front of an ADC input to minimize aliasing. Aliasing is a phenomenon or an effect  that causes different signals to become indistinguishable when sampled.

Passive and Active Filters

Broadly filters can be classified as passive and active filters.

Passive filters consist of passive elements such as resistors, capacitors and inductors. They are most responsive to a frequency range from 100 Hz to 300 MHz. The limitation at lower frequency is the requirement of higher range of the inductance and capacitance. The upper-frequency limit is due to the effect of parasitic capacitances and inductances. They have no amplifying elements so they have no signal gain, therefore the output level is always less than the input. One of the most common examples of passive filters is a R-C circuit which is responsible to eliminate noise from incoming signals. 

Active filters are capable of dealing with very low frequencies (approaching 0 Hz), and they can provide voltage gain (passive filters cannot). Active filters can be used to design high-order filters without the use of inductors; this is important because inductors are problematic in the context of integrated-circuit manufacturing techniques. However, amplifier bandwidth limitations makes active filters less suitable for very-high-frequency applications. Radio-frequency circuits must often utilize passive filters.

Four Major Types of Filters-

Filters can be placed into broad categories that correspond to the general characteristics of the filter’s frequency response. If a filter passes low frequencies and blocks high frequencies, it is called a low-pass filter. It is a high-pass filter if it blocks low frequencies and passes high frequencies. There are also band-pass filters, which pass only a relatively narrow range of frequencies, and band-stop filters, which block only a relatively narrow range of frequencies. The four types of filters can be listed as-

• Low pass filters
• High pass filters
• Band pass filter
• Band stop(band reject or notch) filter

The above diagram shows the variation of signals and lists the response curves for different types of filters. The signal vs frequency graph shows how much of the signal lies within each given frequency band. It shows which frequencies to be passed and which to be attenuated. 

In low pass filters, frequency response of the filter is nearly flat for low frequencies and all of the input signal is passed directly to the output. Any high frequency signals applied to the low pass filter circuit above the cut-off frequency (Cut off frequency lies at that point when the amount of attenuation due to the filter starts to increase swiftly) will become greatly attenuated, that is they rapidly decrease. In case of high pass filters the case is completely different. Similarly, is the case for bandpass and notch filters. The frequency response can be analysed using analytical and graphical methods. The graphical methods are bode plot, root locus, M and N circles etc. 

LOW PASS FILTER

Low pass filters allow the passage of signals with frequency lesser than the cut-off frequency and attenuates the signals whose frequency is higher than the cut-off frequency. When the low pass filter consists of a passive element in the circuit it is called a passive LPF and when it consists of an active element in the circuit it is an active LPF. 

Passive low pass filter

A simple passive RC Low Pass Filter or LPF, can be easily made by connecting together in series a single Resistor with a single Capacitor as shown below. 

In this type of filter arrangement, the input signal (Vin) is applied to the series combination (both the Resistor and Capacitor together) but the output signal (Vout) is taken across the capacitor only. This type of filter is known generally as a “first-order filter” or “one-pole filter” because it has only one reactive component, i.e., the capacitor, in the circuit.

When the frequency of the input signal is low, the impedance of the capacitor is high relative to the impedance of the resistor; thus, most of the input voltage is dropped across the capacitor (and across the load, which is in parallel with the capacitor). When the input frequency is high, the impedance of the capacitor is low relative to the impedance of the resistor, which means that more voltage is dropped across the resistor and less is transferred to the load. Thus, low frequencies are passed and high frequencies are blocked.

Active low pass filter

Active LPF’s principle of operation and frequency response is exactly the same as those for the passive filter, the only difference  is that it uses an op-amp for amplification and gain control. The simplest form of a low pass active filter is to connect an inverting or non-inverting amplifier to the basic RC low pass filter circuit as shown.

This first-order low pass active filter consists simply of a passive RC filter stage providing a low frequency path to the input of a non-inverting operational amplifier. The amplifier is configured as a voltage-follower (Buffer) giving it a DC gain of one, Av = +1 or unity gain as opposed to the previous passive RC filter which has a DC gain of less than unity. The advantage of this configuration is that the op-amps high input impedance prevents excessive loading on the filters output while its low output impedance prevents the filters cut-off frequency point from being affected by changes in the impedance of the load. 

While this configuration provides good stability to the filter, its main disadvantage is that it has no voltage gain above one. However, although the voltage gain is unity the power gain is very high as its output impedance is much lower than its input impedance.  If a voltage gain greater than one is required we can use the following filter circuit.

Applications of Active Low Pass Filters are in audio amplifiers, equalizers or speaker systems to direct the lower frequency bass signals to the larger bass speakers or to reduce any high frequency noise or “hiss” type distortion. 

HIGH PASS FILTER

Whereas, the low pass filter only allows signals to pass below its cut-off frequency point, the passive high pass filter circuit as its name implies, only passes signals above the selected cut-off point, eliminating any low frequency signals from the waveform. 

Passive high pass filter 

In a passive high pass filter the capacitor is connected in series with the resistor. The input voltage is applied across the series network but the output is drawn only across the resistor. High Pass filter allows the frequencies which are higher than the cutoff frequency  and blocks the lower frequency signals. Below is the image of the passive high pass filter. 

In this circuit arrangement, the reactance (In electric and electronic systems, reactance is the opposition of a circuit element to the flow of current due to that element's inductance or capacitance. Greater reactance leads to smaller currents for the same voltage applied) of the capacitor is very high at low frequencies so the capacitor acts like an open circuit and blocks any input signals at input  until the cut-off frequency point is reached. Above this cut-off frequency point, the reactance of the capacitor has reduced sufficiently to now act more like a short circuit allowing all of the input signal to pass directly to the output.

Active high pass filter

The basic operation of an Active High Pass Filter is the same as for its equivalent RC passive high pass filter circuit, except that the circuit has an operational amplifier providing amplification and gain control.

By connecting a passive RC high pass filter circuit to the inverting or non-inverting terminal of the op-amp gives us firstThe operation is same as that of the passive high pass filter, but the input signal is amplified by the amplifier at the output. The amount of amplification depends on the gain of the amplifier. order active high pass filter.

BAND PASS FILTER

Unlike the low pass filter which only pass signals of a low frequency range or the high pass filter which pass signals of a higher frequency range, a Band Pass Filters passes signals within a certain “band” or “spread” of frequencies without distorting the input signal or introducing extra noise. This band of frequencies can be any width and is commonly known as the filters Bandwidth.

Bandwidth is commonly defined as the frequency range that exists between two specified frequency cut-off points ( ƒc ), that are 3dB below the maximum centre while attenuating or weakening the others outside of these two points. For widely spread frequencies, we can simply define the term “bandwidth”, BW as being the difference between the lower cut-off frequency and the higher cut-off frequency  points. In other words, BW = ƒH – ƒL. For a pass band filter to function correctly, the cut-off frequency of the low pass filter must be higher than the cut-off frequency for the high pass filter.

Band Pass Filters can also be used to isolate or filter out certain frequencies that lie within a particular band of frequencies, for example, noise cancellation. Band pass filters are known generally as second-order filters, (two-pole) because they have “two” reactive components, the capacitors, within their circuit design, one capacitor in the low pass circuit and another capacitor in the high pass circuit.

BAND STOP / NOTCH / BAND REJECT FILTER

The Band Stop Filter, (BSF) is another type of frequency selective circuit that functions in exactly the opposite way to the Band Pass Filter. The band stop filter, also known as a band reject filter, passes all frequencies with the exception of those within a specified stop band which are attenuated.  The ideal characteristics can be shown as below- 

When the input signal is given, the low frequencies are passed through the low pass filter in the band stop circuit and the high frequencies are passed through the high pass filter in the circuit. This is shown in the below block diagram.

The summing of the high-pass and low-pass filters means that their frequency responses do not overlap, unlike the band-pass filter. This is due to the fact that their start and end frequencies are at different frequency points. For example, suppose we have a first-order low-pass filter with a cut-off frequency, ƒL of 200Hz connected in parallel with a first-order high-pass filter with a cut-off frequency, ƒH of 800Hz. 

The input signal is applied to both filters as the two filters are effectively connected in parallel. All of the input frequencies below 200Hz would be passed unattenuated to the output by the low-pass filter. Likewise, all input frequencies above 800Hz would be passed unattenuated to the output by the high-pass filter. However, input signal frequencies in-between these two frequency cut-off points of 200Hz and 800Hz, that is ƒL to ƒH would be rejected by either filter, forming a notch in the filter's output response.

In other words, a signal with a frequency of 200Hz or less and 800Hz and above would pass unaffected but a signal frequency of, say 500Hz would be rejected as it is too high to be passed by the low-pass filter and too low to be passed by the high-pass filter.

Applications Of Filters:

Frequency filters have so many applications, some of these applications are given below-

• Tuner in radio:  The bandpass filter in the radio's tuner allows a fixed frequency to the output speaker.
• Treble & bass of the speaker: The bass has lower frequencies & treble has higher frequencies. They are separated using high pass & low pass filters and are separately routed to corresponding bass speakers & treble speakers for clear music.
• Anti-Aliasing: A low-pass filter filters out the high-frequency components from a signal before sampling. It prevents the aliasing component from being sampled.
• Notch Filter: They are band reject filters with a narrow bandwidth that filters out any interfering signal.
• Power Supply Smoothing: The output of the power supply, which is a rectifier has an AC ripple in it. These frequencies are filtered out using a low pass filter, smoothing the output signal.
• Noise suppression: They are used in communication systems for noise removal from the received signals.