Opmap feedback resistor values

[Techman_7]

Hello,
In an inverting opamp amplifier if,

feedback R = 2kohm & input series R = 1kohm, then gain is = 2.
Also if,
feedback R = 2Mohm & input series R = 1Mohm, still the gain is = 2.

So, which resistor combination should be chosen? And why?

 

[FvM]

It depends, on:
- noise requirements
- source impedance
- OP bandwidth, high resistance circuit might become unstable with high MHz OPs

What's you intended application, used OP?

 

[danadakk]

Since Zin of ~= R1, higher R values aid this. But keep in mind as the OpAmp
signal goes up in freq, and OpAmp open loop G drops, the virtual ground
starts rising, eg. OpAmp no lionger has enough G to hold it at ~ 0V, so
Zin is impacted.

High values of Rf and stray C to ground at Vin iunvewrting input, cause phase shift and
'can impact stability, so additional cap is used across Rf to add a zero and lift
the phase before G drops to 1, to add phase margin. Huge topic on web, many
ap notes on this.

High valued Rs also produce more thermal noise, another huge topic.

High valued Rs produce lower power, but bigger offsets due to bias current
of OpAmp. Also create bigger external noise pickup due to stray C coupling.

So tradeoffs, always tradeoffs.

https://www.analog.com/en/education/education-library/op-amp-applications-handbook.html

https://www.ti.com/lit/an/sboa092b/sboa092b.pdf

Using guards - https://www.ti.com/lit/an/snoa664/snoa664.pdf

Google "high value resistors versus low value resistors op amps"



Regards, Dana.

 

[KlausST]

Hi,

and for low power applications (like coin battery operated) you may choose higher value reistors.

Klaus