operational amplifier resistor selection

[csdave]

hi all,
Can anyone point me to an explanation of how to best choose resistor sizes for an op-amp? I mean what range of values Kohms Mohms once the ratios have been chosen?

Back in school it was always 10k-100k but I guess I can go higher depending on the accuracy I need ...

thanks!

 

[steve.hunch]

Actually it will depend on your application. For example bias currents: if Ib=10uA so you cannot put 1M resistors. Also input capacitance, it will form LPF with resistor and slow down signal. Noise. Current feedback Opamps have specific resistance in order of hundreds of ohms that you can put in the feedback... There are lots of arguments, you have lots of freedom, but at the end it will depend on OpAmp and application.

 

[FvM]

I guess, that you are mainly dealing with low frequency (e.g. audio, slow measurement etc.) applications. Then 10 - 100k resistance level isn't a bad starting point. For low noise audio or low voltage measurement applications (e.g. thermocouple), lower values may be suitable, for low power battery operated circuits higher values. You'll also find reference circuits for typical applications in OP datasheets.

 

[chuckey]

In the classic op-amp configuration, the input resistor determines your input impedance, so if its too low, then you have to drag more current out of your source. Likewise if the feedback resistor is too low then you use up more of your op-amps output current draining down this resistor. If the input resistor too high the effect of any thermal drift currents will be worse. As you say 10 ->100k will be workable, but if you want a gain of say 100, then it will be 10K(about) and 1M (about). In general resistors in the range 100 - 1k have the best tolerance +- .1%, high value resistors generate more thermal noise voltages.
Frank