I often see Rb added to the input of the op amp as shown in the figure to match the input impedance, but I haven't known why for a long time. The following is my understanding of this:
According to our usual understanding, the input impedance of the op amp is very high. Ideally, there is no current on Rb, but in reality it is not the case. Figure (b) is the input pre-stage differential amplifier circuit of the general op amp. Ra and Rb are actually Upstream is the base current of the two transistors. If Ra is not equal to Rb, the voltage drops on Ra and Rb are not equal, which will cause Vin to not be amplified by the differential amplifier circuit in equal proportions, thus affecting the accuracy of the amplifier circuit This is why I understand why the amplifier circuit needs to make the input impedance of the positive and negative input equal.
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