OpAmp as a follower, with a resistor from out to -in instead of a wire

[Bricolo]

Hi,

I'm working on a project, and I need an opamp used as a follower, but for practical reasons, instead of having a wire from output to -in, there will be a resistor (but no resistor from -in to ground)

Is this worse than a direct connection? Can this cause problems? Is there an optimal value (impedance matching between -in and +in?) ?

 

[richie00boy]

In a critical application to minimise DC offset due to input bias currents it's actually good practice to put a resistor there that is equal to the impedance seen by the non-inverting input.

 

[jcx]

feedback R in unity gain can be destablizing if the R value is too high and adds phase shift to the feedback path due to op amp input Cin * Rfdbk

depends on the op amp, a few hundred Ohms is usually ok, much higher R and you may need to add a small C in parallel with the R

 

[Bricolo]

isn't adding a small cap there always a good idea?

 

[00940]

if it's just for "practical" reasons, why not using a 0 ohms resistor ?

 

[Bricolo]

the "practical reason" isn't routing the connection 😉

 

[Nicke]

Hi,

I'm working on a project, and I need an opamp used as a follower, but for practical reasons, instead of having a wire from output to -in, there will be a resistor (but no resistor from -in to ground)

Is there an optimal value (impedance matching between -in and +in?) ?

Check the data sheet for AD797,that opamp needs a 100ohms resistor in the feedback when used as a follower

isn't adding a small cap there always a good idea?

Not with an current feedback opamp 😉