In this post I will discuss about the Operational Amplifier as the Comparator. Among the vast applications of the Operational Amplifier; Operational Amplifier as the comparator is very important and also commonly used. In the previous post I have discussed the Inverting and Non-Inverting Operational Amplifier configurations in which the feedback network was designed to act as the voltage divider circuit thus feeding the portion of the output signal to the input of the Operational Amplifier. It is also discussed there that the task to be performed by the Operational Amplifier depends upon the feedback network of the Operational Amplifier. It must be noted here that the Operational Amplifier as a Comparator should have a unique type of feedback network so that operation of comparison can be performed. The point I am trying to make is, just like the feedback network of the Inverting and Non-Inverting Operational Amplifier every configuration of the Operational Amplifier for a specific purpose will have a unique feedback network. Here I am discussing the Operational Amplifier as the comparator which has the unique feedback network for implementing the comparison operation.
After reading this post you will learn about the Comparator, working of the comparator, operational Amplifier as the comparator, transfer equation for the Operational Amplifier based Comparator and how to design the Operational Amplifier as a comparator to customize our application. So sit back, keep reading and enjoy learning.
What is a Comparator?
Before diving into the discussion of the Operational Amplifier acting as Comparator let us discuss what basically the comparator is. As the name implies the Comparator is the circuit that is used to make the comparison between the two voltage levels and the digital output of the comparator which of the voltage variable is larger than the other. So the comparator has two inputs and one digital output. The transfer equation of the comparator is given below:
The transfer equations of the comparator circuit shows that one of the two inputs of the Operational Amplifier acts as the reference voltage and the other input is the signal whose voltage level needs to be compared with the reference voltage.
op amp as a comparator:
Operational Amplifier circuit can be designed in number of ways to perform the particular task, among the various Operational Amplifier configurations one is the Comparator. As described earlier the operation of the Comparator is designed by designing the feedback network of the Operational Amplifier. The basic circuit for the Operational Amplifier as the Comparator is shown in the figure below:
Let us now understand the working of the Comparator based on the Operational Amplifier. The Comparator as can be seen in the image above has two inputs. One of the two inputs is called Vref and the other is called the Vin. The input Vref is connected to the Operational Amplifier via resistive feedback network and the input Vin is directly connected to the OP-amp. The input signal Vin is the signal whose level needs to be compared with some reference voltage which is here the input signal Vref. When the input signal Vin reaches the level that is higher than the reference voltage the output of the comparator gets saturated and become high that is gives the maximum voltage at the Output and similarly when the voltage level of the input signal Vin becomes lower than the reference voltage the output of the comparator circuit gets low that is gives the minimum value and the level of the Output voltage determines whether the input signal is higher than the reference voltage or lower than it. Thus in this way the Operational Amplifier acts as the comparator circuit.
The input and output waveforms of the Operational Amplifier comparator are shown as follows:
In the above diagram it is clear that when the input signal becomes greater than the reference voltage the output of the comparator circuit gets low and when the input signal becomes lower than the reference voltage the output of the comparator gets high.
Comparator with Variable Reference Voltage:
In this section we will see how the comparator circuit can be used to have variable reference voltage by modifying the circuit. Consider the following circuit:
Here we can see that the input reference voltage is connected to the Operational Amplifier inverting input terminal via a potentiometer so by rotating the knob of the Potentiometer the voltage at the inverting input terminal of the Operational Amplifier can be altered so basically the reference voltage is being altered at the input and thus gives the variable reference voltage for the Input signal Vin.
hysteresis comparator :
An important consideration while designing the Operational Amplifier comparator is the Hysteresis. The effect of Hysteresis is introduced in the comparator circuit in order to make it more reliable and give immunity for the noise that is inherent in the circuit. The idea of the Hysteresis can be understand by that if the voltage level of the input signal becomes equal to the reference voltage then the output of the comparator gets saturated bu due to presence of the noise in the input signal the comparator makes oscillation due to these noise in the signal which creates false comparison so in order to avoid this situation the circuit is designed in such a way that input signal needs to be a bit higher than the reference voltage to be saturated in one direction and similarly it needs to be a little bit lower than the reference voltage to get saturated in other direction. This is called the Hysteresis. The idea of Hysteresis can be seen in the following picture:
That is all for now I hope this article would be helpful for you. In the coming posts I will come up with more interesting applications of the Operational Amplifier. Till then stay connected. Keep reading and enjoy learning.
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