That depends somewhat on the application and whether you are concerned about noise. You would not likely have to do it for a MOS or JFET input amplifier since they have very low bias currents. And the resistor does add some noise thermal noise to the circuit. It can also add a high frequency rolloff pole due to stray input capacitance, which may be important in high frequency amplification applications.Even in non-inverting applications with a bipolar opamp, it is a good practice to include compensation resistors on the inputs in order to cancel out the effects of the input bias current. For example, if the impedance seen by the inverting input is 100 kΩ, then a 100 kΩ resistor would be added to the non-inverting terminal. You would not simply connect the input signal to the opamps non-inverting input.
In general true. But there are op amps that have a noise level comparable to a 100Ω resistor and for very low noise applications you don't want to add any more resistance at the input than you absolutely need. For a first stage with reasonable gain, the input resistor values are the most critical and contribute the most noise relative to the signal.The noise generated by a single resistor is not an issue in the vast majority of applications because op amp noise, power supply noise, stray noise picked up through the wiring, other resistor noise, etc. usually swamp out the noise generated by a single resistor. If the resistor is a low value its generated noise is minimal.
Other considerations such as impedance, signal polarity, etc. normally overshadow the resistor noise issue.
by Duane Benson
by Jake Hertz
by Jake Hertz
by Jeff Child