Question E7G04:
What is meant by the term “op-amp input offset voltage”?  

Choices:  
A: The output voltage of the op-amp minus its input voltage  
B: The difference between the output voltage of the op-amp and the input voltage required in the immediately following stage  
C: The differential input voltage needed to bring the open loop output voltage to zero  
D: The potential between the amplifier input terminals of the op-amp in an open loop condition    

Correct Answer is  C: The differential input voltage needed to bring the open loop output voltage to zero  

To understand “op-amp input offset voltage”:

1) Why the correct answer (C) is correct
- Real op-amps aren’t perfectly balanced internally. Even with both inputs tied together at the same voltage, a small differential voltage must be applied between the inputs to force the op-amp’s output to exactly zero (in open-loop terms).
- That small required input difference is the input offset voltage, typically denoted $V_{OS}$.
- Conceptually, model it as a tiny DC source in series with one input.

2) Why the other choices are incorrect
- A: “The output voltage minus its input voltage”  
  That describes something like gain-related behavior, not the inherent input imbalance. Offset is referenced to the inputs, not output minus input directly.
- B: “The difference between the op-amp output and the next stage’s required input”  
  That’s about interstage signal levels and has nothing to do with the op-amp’s internal input imbalance.
- D: “The potential between the amplifier input terminals in an open loop condition”  
  Too vague and misleading. In open loop, the output saturates with virtually any nonzero input difference. Offset voltage is the specific differential input required to make the open-loop output zero, not just any input potential that happens to exist.

3) Useful formulas and rules of thumb
- Input-referred model:
  $$V_{out} = A_{OL}\,\big[(V_{+}-V_{-}) + V_{OS}\big]$$
  where $A_{OL}$ is the open-loop gain and $V_{OS}$ is the input offset voltage.
- In a closed-loop amplifier with gain $A_{CL}$, the output offset due to $V_{OS}$ is approximately:
  $$V_{out,\,offset} \approx A_{CL}\,V_{OS}$$
- Typical magnitudes: older general-purpose op-amps ≈ 1–5 mV; precision/modern devices ≪ 1 mV (down to µV).
- Input bias currents flowing through source resistances create additional offset at the inputs. Balancing input resistances helps minimize that contribution.
- Many op-amps provide offset null/trimming pins to adjust $V_{OS}$ toward zero.