A filter is a reactive network that allows the desired frequency band to pass freely while suppressing all other frequency bands. Filters are constructed of reactive components and if this is not the case, the attenuation in the passband of the filter network will never become zero. A filter should not provide any attenuation to the passband, which is called the transmission band or passband. It should provide attenuation to all other frequencies, which is called the attenuation band or stopband. Filter networks are widely used in communication systems to separate various voice channels in carrier frequency telephone circuits. Filters can also be applied in instrumentation, telemetry equipment, etc. Filters can have multiple passbands separated by attenuation bands and they are classified into four types: low pass, high pass, band pass, and band stop filters.

Low Pass Filter

This type of filter resistors and capacitors are used in its construction and this circuit removes the unwanted high frequencies from the electrical signal and will only pass the desired signal. A passive low pass filter can be easily created by connecting a single resistor in series with a single capacitor. In this circuit, the input signal is applied to the series combination but the output signal passes only through the capacitor. It is also called a first order filter because it has only one reactive component, the capacitor. Active low pass filter, its operation and principle is same as passive filter, but op amp is used for amplification and gain control.

High pass filter

High pass filter will only let high frequency signals pass, while it will block or hinder low frequency signals. High pass RC filter will only let high frequency signals pass, its circuit is made up of resistor and capacitor. To create this circuit, capacitor is connected in series with the power signal entering the circuit. Capacitor provides high resistance to low frequency or DC signal and low resistance to high frequency signal. The operation of active high pass filter is same as passive high pass filter, except that active high pass filter uses op amp in it.

Band pass filter

This type of filter will only let predefined frequencies pass, it will remove all frequencies above and below the set value. It is a combination of high pass and low pass filter. Band pass filter can be obtained by cascading high pass and low pass filter. This filter will only let a certain range of frequencies, band pass filter has many applications, they are used in audio amplifier circuit and wireless transceiver etc.

Band stop filter

This filter is also called band stop filter, it will let most frequencies pass unchanged, but will attenuate the frequencies in a certain range to very low level. It is the opposite of a bandpass filter. A bandstop filter blocks and rejects frequencies between its two cutoff frequency points and passes all frequencies on both sides of the range. It can be made up of a combination of a lowpass filter and a highpass filter.