In this post I will discuss about the DC imperfections of the Operational Amplifier. I have discussed in my previous posts that the practical Operational Amplifier does not exactly match the attributes of the Ideal Operational Amplifier due to which various problems arises in the operational of the practical circuit based on the Operational Amplifier. I will discuss here one of them that is the DC offset voltage of the Operational Amplifier. In my previous posts I have discussed some of the commonly known and widely used Operational Amplifier configurations. This post will be oriented around the DC offset voltage of the practical Operational Amplifier and the effects on the operation of the circuit due to this offset voltage.
After reading this post you will learn about the DC offset voltage of the Operational Amplifier, causes that leads to the DC offset voltage, effects of the DC offset voltage on the performance Operational Amplifier based circuit and also the methods for catering the effects of the offset voltage and op amp offset voltage. So sit back, keep reading and enjoy learning.
Let us now understand what is the offset voltage and after this we will discuss how it manifests itself in the Operational Amplifier circuits. The Operational Amplifier is the most commonly known analog ICs and it is capable of performing a large variety of operations that depends upon the feedback network of the Operational Amplifier for example the operational amplifier finds applications in Signal conditioning circuit, signal processing circuits, data acquisition systems and many more however all the operational amplifiers are subjected to some DC imperfections and DC offset voltage is one of them. As we know that the Operational Amplifier is a differential amplifier with two input terminals and one output terminal. By the term differential amplifier it means that the Operational Amplifier amplifies the difference of the signal voltages applied at Inverting and Non-inverting input terminal and the amplified difference of these signals appear on the output of the Operational Amplifier. However due to the limitations in the design of the Operational Amplifier internal circuitry it is subjected to some imperfections. For example the input stage of the ideal Operational Amplifier should be exactly matched so that the Operational Amplifier can effectively sense the difference of the signal voltages applied at the two input terminals but in the practical operational amplifier there is always some mismatch due to which the operational amplifier is unable to assess accurate difference of the signals. The point I am trying to make can be understand by a simple experiment. Consider an operational amplifier with both terminals that is inverting and non-inverting terminal shorted to each other and then grounded so that the terminals are forced to have the same potential. By definition the operational amplifier should give zero voltage at the output but due to the mismatch between the inverting and non-inverting input terminals as described in the above discussion some voltage drop happens to exist between the inverting and non-inverting terminals and the output voltage at the Operational Amplifier output terminal is non-zero. This difference of the voltage between the inverting and non-inverting terminal is called the offset voltage.
The dc offset voltage can be nullified by various strategies for example by capacitor coupling of the input terminals however some models of the Operational Amplifier has terminal called input offset terminal through which we can nullify the effect of the DC offset voltage at the output of the Operational Amplifier. The Operational Amplifier equivalent model representing the DC offset voltage is shown in the figure below:
Effects of DC offset Voltage:
Let us now understand what are the effects of the DC offset voltage on the performance of the circuits based on the Operational Amplifier?
The Operational Amplifier is basically the direct-coupled amplifier. By the term direct coupled means that the Operational Amplifier can amplify the DC signal in the same way as the AC signal. Therefore, the Operational Amplifier becomes more vulnerable to the DC imperfections such as the DC offset voltage. Let us try to understand the effect of the DC offset voltage with an example. Consider that you are using the Operational Amplifier in inverting configuration with the closed loop gain of 1000. That is the signal applied at the inverting terminal will amplified by 100 times. Now tie the Inverting and Non-Inverting terminals together and connect it to ground so that both the input terminals are at same potential. However as we noticed in the earlier discussion that due to the DC offset voltage the output voltage will be non-zero so let us assume that the magnitude of the DC offset voltage is 5 millivolts and you must have figured out that at the output the voltage will be 1000 times the 5 millivolts that is 5 volts. If you apply the signal at this configuration the signal will have a DC offset of 5 volts from the ground and if you are using the Operational Amplifier with low power supply this DC offset voltage will act to saturate the Operational Amplifier and thus the complete recursion of the amplified signal cannot be obtained. The overall effect can be understood from the figure below:
As can be seen in the picture above that the offset voltage can be either negative or positive. Notice in the image above that in case of the negative offset voltage the output waveform is below the zero reference line and in case of the positive voltage the output waveform is above the zero reference line.
That is all for now I hope this post would be helpful for you. In next post I will come with more important concepts and applications of the Operational Amplifier. till then stay connected, keep reading and stay connected.