Electronic – A questions about negative resistance

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

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

With most opamps the problem you describe would exist and could cause damage or unusual operation of the device.

However, the data sheet for the LMC6482 does cover this specific mode of operation. The device will operate within specification provided the current into the input is limited to 5mA or less. It is guaranteed that inversion will not occur and that the output will remain at zero volts. The 1K input resistor does limit the current as required.

As Spehro comments the current is right at the limit and it would be better if R1 and R5 were both increased to 10k or more to give greater margin. Providing the two resistors are the same value the operation of the circuit wouldn't be changed. (avoid very large values or capacitance effects will affect operation).

See section 7.3.2 in the datasheet.

LMC6482 Datsheet