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

[Donald Tillman]

   > Date: Sat, 22 Nov 2008 10:24:46 +0000
   > From: Seb Francis <seb at burnit.co.uk>
   > 
   > Donald Tillman wrote:
   > >    
   > > 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.

Yes, indeed.  (Uh, you mean the "+" input terminal.  For both the
classic inverting or non-inverting cases.)

   > > 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.

Yes, that's right.

  -- Don

-- 
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com