In this post I will discuss the uses of the Operational Amplifier as the active filters. In the previous post I have discussed about the Operational Amplifier as the Instrumentation Amplifier and it was also mentioned that Operational Amplifier can be used to implement a lot of circuits and specialty amplifiers due to its diverse attributes so one of the applications of the Operational Amplifier is its use as the Active Filter which I will discuss in this post.
Operational Amplifier Active Low Pass Filter
After reading this post you will learn about the Operational Amplifier implementation as the Active Filter, various types of filter topologies, filter topologies implementation using the Operational Amplifier, low pass filter, high pass filter, band-pass filter and band-stop filter. So sit back, keep reading and enjoy learning.
Before diving deep into the discussion of the Operational Amplifier as the filters let us discuss what basically the Filter circuits are. As the name implies the filter is type of electronic that allows certain frequencies in the signals to be passed while attenuating other unwanted frequencies. The filtering can be thought as the signal processing in which the signal is processed so that the signal contains the desired frequencies and depleted of all the unwanted frequencies. Filters can be classified into two classes according to the design technology point of view:
- Passive Filters.
- Active Filters:
For the sake of relevance I will stick to the Active Filters only here. The Active Filters are the type of filters which are implemented using at least active component which in our case is the Operational Amplifier. The active filter can be designed to allow the higher frequencies to be passed through it while attenuating the lower frequencies; this type of filter is called the high pass filter. In the similar way the filter can also be designed to pass the low frequencies through it while attenuating the higher frequencies; these types of filters are called the low pass filters. Following the same context band-pass filters allow the certain frequencies while attenuating the upper and lower frequencies of the band that ought to be passed and opposite in the case for band-stop filter which attenuates the particular set frequencies while allowing all the frequencies to be passed.
An important consideration while designing the filter of any the above mentioned types is the cut-off frequency. The cut-off frequency of the filter can be designed as the threshold for the filter that is in case of low pass filter cut-off frequency is that above which the filter will not allow the signal to pass. In the similar way cut-off frequency for the high pass filter is that frequency below which no signal can pass through the filter. As you must have figured out that the band-stop and band-pass filters have two frequencies one is called the upper cut-off frequency and the other is called the lower cut-off frequency.
Operational Amplifier and active filters:
As mentioned in the above section that the Operational Amplifier is one of the most popular devices among the analog engineers because of its diverse applications among these applications one of the most is its use as active filters. Operational Amplifier can be configured to function as low pass filter, high pass filter, band pass filter or band stop filter. The Operational Amplifier acting as one of these filters depends upon the feedback network. The point I am trying to make here is that the Operational Amplifier feedback network can be altered to function any of these filters. Let us discuss the Operational Amplifier active filters in some detail.
Low pass Filter:
As mentioned in the above section that the feedback network of the Operational Amplifier can be designed to exploit it as the Low pass filter. The Low pass filters are of types with respect to the design point of view. One is the passive low pass filter which can be implemented using resistors, capacitors or inductors and the second is the active low pass which is implemented using the Operational Amplifier or any other active component by the way the Operational Amplifier is the most popular.
The cut-off frequency of the low-pass filter is the frequency above which the filter will attenuate all the frequencies considerably. In the Operational Amplifier based low pass filter the cut-off frequency can be customized by choosing the appropriate values of the resistor and capacitors in the feedback network of the Operational Amplifier. The circuit implementation of the Operational Amplifier based Low pass filter is shown below:
The frequency response of the Filter is shown below:
As you must have noticed that the above circuit for the low pass filter is designed to have the cut-off frequency of 40 kilo Hertz. All the frequencies above the 40 KHz have been attenuated considerably.
The step response of the above designed circuit is shown below:
The input impedance of the above circuit corresponding to the frequency is given below:
The simulation of the low pass filter in Proteus is shown in the figure below:
While studying the filters you will continuously read the term “Frequency Response” so it is recommended that you must know the meaning of the frequency response of the filters or any other system. As the name implies the frequency response of the system gives the knowledge about the relationship between the input and output from the frequency point of view. That is the frequency response diagram of the filter shows the relation between the input frequencies to the filter and the magnitude of the output signal in decibel. That is the frequency response of the filter shows the magnitude in decibel of the output signal corresponding to the frequency applied at the Input of the Filter.
That is all for now I hope this article will be helpful for you. In the next post I will come up with more interesting applications of the Operational Amplifier in the coming posts. Till then stay connected, keep reading and enjoy learning.