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Recently, I was looking at the calculation of the input impedance of the op-amp
This article describes in detail how to calculate the input impedance of the inverting amplifier circuit, not just the RIN，
But how to understand v+ and v-, in the article , the v- is vtest, v+=itest*Ra, I do not know how to get this formula, look forward to the expert's detailed answer.
Hello,
The equation is simplified via substituting v+ and v- into equation 26.
Figure 10 notes that the V+ node of the inverting amplifier is equal to 0V. To solve for V-, start from ground in figure 10 and sum the voltage rise from Vtest with the voltage drop due to the current flowing through Ra:
Therefore:
Lastly, substitute (v+ - v-) back into the original equation:
Let me know if there is any confusion in solving for the equation.
Regards,
Jacob
