Electronic hardware filters can be classified into two types: active and passive.
Active Filters: Filter circuits consisting of active components like transistors and operational amplifiers with passive components like resistors, capacitors, etc. are called active filters.
Figure 1: Active filter
Passive Filter: Filter circuits that consist of only a resistor, capacitor and inductor and are tuned to resonate at a certain frequency are called passive filters.
Figure 2. Passive filter
There are mainly four types of filters in electronics based on their frequency response and they can be designed using active and passive components. These are low-pass, high-pass, band-pass and band-reject filters (notch filters).
Low pass filter
A filter that provides a constant DC output up to a cutoff frequency and then passes no signal above that frequency is called an ideal low-pass filter.
The relationship between the output voltage and the input voltage is called voltage gain, which is constant over a frequency range from zero to the cutoff frequency. Consequently, the output will be faithfully available from zero to the cutoff frequency with a constant gain. Frequencies between zero and the cutoff frequency are called passband frequencies, while frequencies above the cutoff frequency are called stopband frequencies.
In an ideal low-pass filter, low frequencies are in the pass band and higher frequencies are in the stop band. A low-pass filter attenuates high frequencies and passes low-frequency components of a signal passing through a low-pass filter circuit.
First-order analog filters can be designed using a series resistor and a shunt capacitor or a series inductor and a shunt resistor.
Figure. 3: Passive Low Pass Filter System Block Diagram
…where Input signal —> Low pass filter —> Output signal (attenuated low frequencies)
For example, a signal has a frequency band of 100 MHz and we need a signal with a frequency band of less than 50 MHz. In this case, we need a low pass filter that will attenuate the other frequency band that we don't need. In power lines, a low-pass filter is used as an EMI filter to reduce the proportion of frequency components.
Figure 4: Circuit design and frequency response of the low-pass filter
High pass filter
A filter that passes frequency band signals above the cutoff frequency is called a high-pass filter. The high-pass filter has zero gain at a frequency called the cutoff frequency, and above this frequency the gain is constant. Consequently, a signal of any frequency beyond the cutoff is reproduced faithfully with a constant gain, and frequencies from zero to the cutoff will be blocked.
The high-pass filter is the functional complement of the low-pass filter. In a high-pass filter, the low-frequency components are in the stop band and the high-frequency components are in the pass band. This means that a high-pass filter passes the high-frequency components and blocks the low-frequency components of a signal while passing it through a high-pass filter. High-pass filters can be implemented using a capacitor in series and a shunt resistor or a resistor in series and a shunt inductor.
Figure 5: Passive High Pass Filter System Block Diagram
…where, Input signal —> High pass filter —> Output signal (high frequencies attenuated)
Figure.6: Circuit design and frequency response of the high-pass filter
If a signal has a frequency band of 50 MHz and we need a signal beyond the frequency band of 30 MHz, then we need this type of filter which will stop the frequency band below 30 MHz and allow frequency above 30 MHz in cases ideals. These filters are used in speakers to remove low-level noise.
Bandpass filter
The cascaded version of a high-pass filter and a low-pass filter is called a band-pass filter. The bandpass filter passes a set of frequencies between a lower cutoff frequency and a higher cutoff frequency. Frequencies below a lower cutoff frequency and above a higher cutoff frequency are in the stop band. The ideal bandpass filter is shown in the figure below. A bandpass filter passes one band of frequencies while attenuating other parts of frequencies (left and right) that are outside the band of a signal.
Figure 7: Passive bandpass filter system block diagram
…where, Input signal —> Low pass filter —> High pass filter —> Output signal
Figure 8: Circuit design and frequency response of the bandpass filter
If, for example, a signal consists of a frequency band of 200 MHz and we need a signal that consists of a frequency band above 50 MHz and below 100 MHz, then we need a bandpass filter. One application of these filters is modern telephones or cell phones, as the audio side is approximately 250 Hz to 5 Khz.
Band rejection filter (Notch filter)
The complement to the bandpass filter is called a band-reject, also called a notch filter. In this filter, frequencies below the lower cutoff frequency and above the higher cutoff frequency are in the passband. The frequencies between the lowest cutoff frequency and the highest cutoff frequency come in the stop band. The figure below shows a notch response.
Figure.9: Band-rejection filter system block diagram
Figure 10: Circuit design and frequency response of the band-rejection filter
The idealized filters defined above cannot be constructed easily. There must be an instantaneous transition from the passband to the endband. But there is a transition region that is reduced by the designer when carrying out multiple order filter designs.
Suppose we have a signal that consists of a frequency band of 200 MHz and we need a signal that consists of a frequency band below 50 MHz and above 100 MHz. In this case, we need a band-rejection filter. One example is notch filters that are used in medical equipment to remove 50 Hz hum.
This article reviewed basic hardware filters used in signal filtering, noise reduction, and removing unwanted frequency components from an information signal.