Resistor values and Noise

[JTEX]

Resistors make noise. Thermal (Johnson) noise. The higher the value, the more noise. If you simply go with convenient, common values without paying attention to their noise contribution, you can easily swamp an op amp's self noise with the surounding resistors' noise. The following 3 circuits are functionally equivalent and cost about the same. The only significant difference is the output noise (and input impedance, if it matters to the preceding stage).

A 7dB difference is more than twice the noise! Don't leave performance on the table for no good reason.

 

[LukeFRC]

where's the variable resistor set in each of them? the middle?

 

[JTEX]

yes