# Opamp output offset help

• Engineering
• Callum Plunkett
In summary, the conversation is about finding the value of Rf in a circuit with known values for R1 and R2. The equation ##\frac{dV_{20}}{dI_{IO}} = R_f## is discussed and it is suggested to redraw the schematic and consider the effects of ##I_{IO}## and the grounded input. The goal is to find where ##I_{IO}## must flow and what voltages it will produce at the output.

#### 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.

#### Attachments

• Untitled.png
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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 RF 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?

DaveE said:
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 RF 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

• DaveE