A certain op amp has the maximum offset voltage magnitude of Vos = 1 mV, the max

Question

A certain op amp has the maximum offset voltage magnitude of Vos = 1 mV, the maximum bias current of Ib = 290 nA and the maximum offset current magnitude of Ios = 34.8 nA. The op amp is used in the noninverting amplifier shown in the diagram below.

1. Find the worst-case DC output voltage range if vi = 0 and Rbias = 0.

2. What should be the value of Rbias to cancel the effect of bias currents?

3. Repeat p.1 for vi = 0 and Rbias calculated in p.2.

Explanation / Answer

1) Let Worst case output voltage when Vi=0 and Rbias = 0 be VooT and is given by

VooT = Voo + Voib    ( Voo = output voltage due to Vos and Voib = output voltage due to Ib )

VooT = ( 1 + R2/R1)Vos + R2Ib

VooT = ( 1 + 51K/5.1K )1mV + 51K*290nA

VooT = 11 mV + 4.59 mV

VooT = 15.59 mV                       .....................1

This is the required worst case DC output voltage

2)Rbias should have following value to cancel effect of bias current as below:

Rbias = (R1R2) / (R1 +R2 )

         =51*5.1/ (51 + 5.1 )

Rbias =4.64 K Ohm    ........................2

This is the required value of Rbias to cancel effect of bias current.

3)When we use Rbias calculated in 2 i.e. 4.64 K Ohm the output voltage VooT is given by

VooT = ( 1 + R2/R1)Vos + R2Ios

VooT = ( 1 + 51K/5.1K )1mV + 51K*34.8nA

VooT = 11 mV + 1.77 mV

VooT = 12.77 mV                       .....................3

This is the required worst case DC output voltage when Rbias = 4.64 K ohm is used

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