---
title: "Output offset voltage of 741 inverting amplifier"
subject: "Advanced Electronics"
chapter: 2
key: "semester_4/advanced_electronics:output-offset-741-inverting"
source: https://www.bctengineer.com/q/semester_4/advanced_electronics/output-offset-741-inverting
site: BCT Engineering
---

# Output offset voltage of 741 inverting amplifier

Advanced Electronics · Chapter 2: Operational Amplifier Characterization

- Asked in: 2070 Asar, 2+2+1+2 marks
- Video: [Op-Amp: Input Offset Voltage Explained](https://www.youtube.com/watch?v=hUu3SqRSYyA) — ALL ABOUT ELECTRONICS
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/advanced_electronics/output-offset-741-inverting
- Whole chapter: https://www.bctengineer.com/md/semester_4/advanced_electronics/chapter-2

## Question

For the inverting amplifier shown in figure below, R1 = 100 KΩ and Rf = 10 MΩ. Calculate: i) Maximum output offset voltage caused by the input bias current IB. ii) Maximum output offset voltage caused by the input offset voltage, Vios. iii) The value of compensation resistor Rcomp needed to eliminate the effect of input bias current. iv) The maximum output offset voltage even if the Rcomp is connected in the circuit. The op-amp used is 741 with Vios = 6 mV, IB = 500 nA and Ios = 200 nA. [Figure: inverting amplifier: source Vi through R1 to the inverting input, feedback resistor Rf from output V0 to the inverting input, non-inverting input grounded.]

## Short answer

i) Output offset from I_B: 5 V. ii) From V_ios: 0.606 V. iii) Compensation resistor R_comp = 99.01 kΩ (R1∥Rf). iv) With R_comp, maximum total output offset is 2.606 V (2 V from I_os plus 0.606 V from V_ios).

## Answer

Given: R1 = 100 kΩ, Rf = 10 MΩ, 741 with Vios = 6 mV, IB = 500 nA, Ios = 200 nA (maximum values).

```text
Noise gain: 1 + Rf/R1 = 1 + 10 MΩ/100 kΩ = 101
```

#### (i) Output offset due to input bias current
With the + input grounded, the − input is at virtual ground, so IB flows through Rf:

```text
Vo(IB) = Rf · IB = 10 × 10⁶ × 500 × 10⁻⁹ = 5 V
```

#### (ii) Output offset due to input offset voltage
Vios is amplified by the noise gain:

```text
Vo(Vios) = (1 + Rf/R1) · Vios = 101 × 6 mV = 0.606 V
```

#### (iii) Compensating resistor
To cancel the effect of IB, both inputs must see the same dc resistance:

```text
Rcomp = R1 ‖ Rf = (100 kΩ × 10 MΩ)/(100 kΩ + 10 MΩ)
      = 10¹² / 10.1 × 10⁶ = 99.01 kΩ
```

(A standard 100 kΩ resistor would be used.)

#### (iv) Maximum output offset with Rcomp connected
With Rcomp = R1‖Rf, the bias-current error becomes Ios·Rf:

```text
Vo(Ios) = Rf · Ios = 10 × 10⁶ × 200 × 10⁻⁹ = 2 V
```

The offset voltage error is still present, so in the worst case both add:

```text
Vo(total, max) = Vo(Ios) + Vo(Vios) = 2 + 0.606 = 2.606 V
```

**Answers: (i) 5 V, (ii) 0.606 V, (iii) Rcomp = 99.01 kΩ, (iv) 2 V due to Ios, or 2.606 V total including Vios.** Without Rcomp the worst-case total would be 5 + 0.606 = 5.606 V. The remaining offset can be removed with the 741 offset-null pot.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. For R1 = 100 kohm and Rf = 10 Mohm, 1 + Rf/R1 equals:
   A. 101
   B. 100
   C. 99
   D. 10
   Answer: A. 1 + 10M/100k = 101. (Key point: Noise gain)

2. The output offset due to IB = 500 nA with Rf = 10 Mohm is:
   A. 0.5 V
   B. 5 V
   C. 50 mV
   D. 0.606 V
   Answer: B. Vo = Rf x IB = 10^7 x 500x10^-9 = 5 V. (Key point: Bias-current offset)

3. The compensation resistor value is:
   A. R1 + Rf = 10.1 Mohm
   B. Rf = 10 Mohm
   C. R1||Rf = 99.01 kohm
   D. R1/Rf = 0.01
   Answer: C. Rcomp = 100k x 10M/(10.1M) = 99.01 kohm, about 100 kohm standard. (Key point: Rcomp)

4. With Rcomp connected, the remaining worst-case offset is:
   A. 0 V
   B. 5.606 V
   C. 0.606 V only
   D. 2 V from Ios plus 0.606 V from Vios = 2.606 V
   Answer: D. Ios x Rf = 200 nA x 10 Mohm = 2 V; Vios term (101 x 6 mV) remains. (Key point: Remaining offset)

## Related in Advanced Electronics

- [Offset voltage/current and bias current compensation](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Aoffset-voltage-bias-compensation) — Advanced Electronics, chapter 2, asked 2×
- [Offset/bias definitions; closed-loop input resistance](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Aoffset-bias-closed-loop-input-resistance) — Advanced Electronics, chapter 2, asked 2×
- [Output of log-ratio amplifier with RT compensation](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Alog-ratio-temperature-compensated) — Advanced Electronics, chapter 5
- [Bias current effect; closed-loop output impedance](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Abias-current-output-impedance) — Advanced Electronics, chapter 2
- [Effect of input offset voltage; PSRR](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Aoffset-voltage-effect-psrr) — Advanced Electronics, chapter 2
- [Input offset voltage; proof of constant GBP](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Ainput-offset-voltage-gbp) — Advanced Electronics, chapter 2

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