Opamp Output Offset: Solving for Rf in Circuit with Unknown Feedback Resistor

[Callum Plunkett]

Homework Statement

The amplifier of FIGURE 1 has a closed loop gain of 20 dB and R1 =
10 kΩ. The resistors have a tolerance of 5%.

Estimate the output offset voltage of the circuit of FIGURE 1 when
the op-amp used is
i) op-amp A of TABLE A
ii) op-amp B of TABLE A.

Relevant Equations

See attached

I’ve been given the attached question and equation in regard to output offset. I know all the required values apart from Rf. Normally id assume Rf was the feedback resistor however that (as far as I can tell from the circuit) is marked as R2. Would I be right if I assumed this value was Rf = r1ǁr2? Any help would be appreciated.

 

[DaveE]

OK, so you know R1 & R2 values. Look at the equation and focus on the $I_{IO}$ effect on the output, since it is the simplest. So $\frac{dV_{20}}{dI_{IO}} = R_f$ . You can solve for the output voltage offset due just to the input offset current in terms of R1 & R2, that will tell you what R_F must be. You will want to assume that the input is 0V (grounded) and that $I_{IB}=0A$ for this special case. It will help to redraw the schematic to include the $I_{IO}$ current source and the grounded input. Where must $I_{IO}$ flow? What voltages will that make, referred to the output?

 

[Callum Plunkett]

OK, so you know R1 & R2 values. Look at the equation and focus on the $I_{IO}$ effect on the output, since it is the simplest. So $\frac{dV_{20}}{dI_{IO}} = R_f$ . You can solve for the output voltage offset due just to the input offset current in terms of R1 & R2, that will tell you what R_F must be. You will want to assume that the input is 0V (grounded) and that $I_{IB}=0A$ for this special case. It will help to redraw the schematic to include the $I_{IO}$ current source and the grounded input. Where must $I_{IO}$ flow? What voltages will that make, referred to the output?

Wonderful Thank you that's helped a lot