# inverting op amp

Discussion in 'Electronic Basics' started by smokie, Apr 2, 2005.

1. ### smokieGuest

Ri = 1K Rf = 10K and a 910 ohm from the non-inverting input to
ground.

how would the output be afffected if i swapped the 910 ohm with a 10K
resistor?

2. ### Andrew HolmeGuest

Depends on the input bias current - a (very) small current which flows out
of op-amp inputs. For a JFET-input op-amp, with a bias current of 200pA, it
would shift the ground reference level by (10e3-910) * 200e-12 = 1.8
micro-volts ! I leave it for the student to calculate the effect of this on
output voltage (of a x10 amplifier) ...

3. ### BanGuest

There would be a change in offset voltage by offset current x 9090.
There would be also an increase in noise voltage by around 3-5dB, depending
on the current- and voltage noise figures of the opamp.
The lowest noise would be achieved with the +input grounded.

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