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

[Donald Tillman]

   > From: Aaron Lanterman <lanterma at ece.gatech.edu>
   > Date: Sat, 22 Nov 2008 02:18:50 -0500
   > 
   > Every once in a while, I see something that's clearly a unity gain  
   > noninverting op  buffer, but the designer has put a resistor in the  
   > feedback loop. There's no resistor from the negative input terminal to  
   > ground, so it's not like they're trying to provide gain. All the  
   > textbooks show a wire from the output to the negative input terminal.
   > 
   > Why do people sometimes put a resistor there? When inputting  
   > schematics into Eagle I keep wanting to just replace them with wires,  
   > but I don't want to do that if there's a good reason they are there.

Hey Aaron,

A fine question!

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.

The resistor is not necessary for FET input opamps.

  -- Don

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