[sdiy] Resistors in feedback loop of noninverting op amp buffer

[Seb Francis]

Thanks for this answer Don.

Donald Tillman wrote:
>    
> In a real world bipolar opamp, the inputs are the bases of a pair of
> diff amp transistors.  These transistors are running at some quiescent
> CE current, and therefore need some dc current in their bases, 1/beta
> of the CE current.  That's called the input bias current, and it gets
> sucked in off of whatever the voltage source is on the inputs.
>
> That feedback resistor is to insure that any voltage drop due to the
> input bias current times the input resistance at the positive input is
> roughly equal to the voltage drop due to the input bias current at the
> negative input.  So it corrects for any offsets due to input bias
> current.
>
>   

So it's the same reason as for putting an input resistor on the -ve 
terminal in a normal non-inverting amp configuration.
Like it says here when talking about Rz:
http://www.play-hookey.com/analog/non-inverting_amplifier.html


> The resistor is not necessary for FET input opamps.
>   

And I guess the same is true for using an input resistor on the -ve 
terminal - this is also unnecessary for FET input opamps.

Seb