In this post I will discuss about the input bias current and input offset current of the practical Operational Amplifier. In the last post I have discussed about the DC offset voltage of the Operational Amplifier and there I have mentioned that the practical Operational Amplifier due to the limitation of the manufacturing technologies are subjected to some DC imperfections and these DC imperfections effects the Operation of the practical Operational Amplifier circuits. The input bias current and the input offset current are also these DC imperfections and thus also affect the ideal operation of the Operational Amplifier circuits.
After reading this post you will learn about the input bias and input offset current of the Operational Amplifier, the effect of these DC imperfections on the operation of the Operational Amplifier circuits and the output signals. So sit back, keep reading and enjoy learning.
Input Bias Current:
Let us first discuss about the input bias current of the Operational Amplifier. In order for the Operational Amplifier to operate properly it should be supplied with some input bias current. It is important to note here that this input bias current is independent and different from that that we discuss while considering the high input impedance of the Operational Amplifier. That is one of the important characteristics of the Operational Amplifier as I mentioned in my posts is its high input impedance which makes it important in designing the signal conditioning circuit as it minimizes the loading effect on the input signal source by draining as low current as possible so the point I am trying to make through this discussion is that the input bias current is different from the current that should be minimized in order to avoid loading effect. This input bias current is necessary for the proper operation of the Operational Amplifier. This input bias current is applied at both the inverting and non-inverting terminal of the Operational Amplifier. These current are represented by the current sources shown in the following image which can be considered as the equivalent circuit for the Operational Amplifier showing the input bias currents:
The current at the Inverting terminal is represented by Ib1 and at Non-Inverting is represented by Ib2. In an ideal Operational Amplifier these two current are exactly identical but as mentioned before due to the limitations of the manufacturing technologies these current are not exactly equal in the practical Operational Amplifier which leads to problems in the actual operation of the Operational Amplifier based circuits.
Input Offset Current:
Practically the input offset current is of significance which is simply the difference of the input bias current at the inverting terminal and non-inverting terminal which is given by the following equation.
It is important here to mention that in the Operational Amplifiers based on the Bipolar Junction Transistor the input offset current is relatively high that is 10nA and in the Operational Amplifiers based on the Field effect transistor technologies the input offset current is in the range of pico-amperes. The input offset current should be kept as low as possible because it effect the performance of the Operational Amplifier circuits as we will see in the next section.
Effect of non-zero input offset current:
The effect of the non-zero input offset current can be understood by the help of an example. Consider the Inverting configuration of the Operational Amplifier as shown in the following image:
Let us now assume that the Inverting and Non-Inverting input terminals of the Operational Amplifier are tied together and then connected to ground so that both input terminals are forced to have same potential as shown in the figure above. By definition and as we know that the Operational Amplifier should give zero output voltage as both the input terminals are maintained at same potential and no current should flow but in the practical circuit the Operational Amplifier will give some non-zero value of the output voltage and this is due to the non-zero offset current. Due to the presence of the offset current which is basically due to the mismatch of the input bias currents at the two input terminals the output signal will have some offset. This idea can be made clear by the following image:
op amp input bias current and offset current:
The picture above shows the effect of the offset current on the output waveforms. It is clear that there are two types of offset current one that causes the negative offset in the output waveform and one that causes the positive offset in the output waveform. The ideal case for zero offset current is also shown in the figure above.
That is all for now I hope this post would be helpful for you/ in the next posts I will come up with more interesting applications of the Operational Amplifier. Till then stay connected, keep reading and enjoy learning.