Input Resistance

The input resistance of a circuit is given by  

$$R_{in}=\frac{V_{in}}{I_{in}}$$ (12)

As we said above, the input resistance of the op-amp is extremely high. As a result, the input resistance of the noninverting amplifier is also extremely high since the input is directly into the op-amp terminal. It turns out that, due to feedback, the resistance of the noninverting amplifier is even larger than that of the op-amp by itself. The large input resistance can be verified qualitatively by placing a resistor in series between V_in and V⁺, and determining the current that flows through it. This can be done by measuring the voltage difference on either side of the resistor, and dividing by the resistor value.