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LM318: Input impedance of inverting amplifier circuit

Part Number: LM318

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:




    Lastly, substitute (v+ - v-) back into the original equation:


    Let me know if there is any confusion in solving for the equation.




  • Sincerely thank you for your answer, I know the process