Electronic – Why is non-inverting op-amp input resistance on a tee to ground

operational-amplifier

In many of the explanations I've read of non-inverting op-amps, the non-inverting input is given a resistance to meet the specs of the op-amp. I'm confused by how the resistance is applied. Since the resistor is just acting to control the input current, I expected it to be inline with the input voltage. However, the resistor is usually attached as a tee from the input going to ground. Why is the input resistor on a tee and not just inline with the non-inverting input?
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Best Answer

Let the resistance looking into the non-inverting input be \$R_{in+}\$

If a resistor \$R_i\$ is placed in series with the non-inverting input, the resistance seen by the source \$v_I\$ is

$$R_I = R_i + R_{in+} \ge R_{in+}$$

If a resistor \$R_i\$ is placed in parallel with the non-inverting input, the resistance seen by the source \$v_I\$ is

$$R_I = R_i||R_{in+} \le R_{in+}$$

Since, for an ideal op-amp, \$R_{in+} = \infty\$, the series case gives

$$R_I = R_i + \infty = \infty$$

while for the parallel case

$$R_I = R_i||\infty = R_i$$

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