---
title: "Short note: Noise in op-amp circuit"
subject: "Advanced Electronics"
chapter: 2
key: "semester_4/advanced_electronics:short-note-opamp-noise"
source: https://www.bctengineer.com/q/semester_4/advanced_electronics/short-note-opamp-noise
site: BCT Engineering
---

# Short note: Noise in op-amp circuit

Advanced Electronics · Chapter 2: Operational Amplifier Characterization

- Asked in: 2070 Asar, 3 marks
- Video: [Noise in Op Amp Circuits Part 1](https://www.youtube.com/watch?v=TBEsukcdPOk) — Darryl Morrell
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/advanced_electronics/short-note-opamp-noise
- Whole chapter: https://www.bctengineer.com/md/semester_4/advanced_electronics/chapter-2

## Question

Write a short note on noise in op-amp circuit.

## Short answer

Noise in op-amp circuits comes from thermal, shot, flicker, popcorn, and external pickup. The op-amp is modeled as noiseless with input voltage and current noise sources; total output noise equals noise gain times the root-sum-square of sources over bandwidth. Reduce it with low-value metal-film resistors, bandwidth limiting, low-noise op-amps, shielding, and grounding.

## Answer

**Noise** is any unwanted random signal at the output of an op-amp circuit. It limits the smallest signal that can be amplified (signal-to-noise ratio).

**Sources:**
- **Thermal (Johnson) noise** in resistors: vn = √(4kTRB); white noise.
- **Shot noise** in junction currents: in = √(2qI·B).
- **Flicker (1/f) noise:** large at low frequencies, below the corner frequency.
- **Popcorn (burst) noise:** random step changes due to contamination.
- **External noise:** pickup from mains (50 Hz), switching circuits, ground loops.

**Model:** the op-amp is treated as noiseless with an input noise voltage source en (nV/√Hz) and noise current sources in (pA/√Hz). Resistors add 4kTR. All input noise is amplified by the noise gain (1 + Rf/R1), and independent sources add as root-sum-of-squares:

```text
Eno = (1 + Rf/R1)·√[en² + (in·R)² + 4kTR]·√B
```

**Reduction:** use low-value metal-film resistors, limit bandwidth (capacitor across Rf), choose a low-noise op-amp suited to the source resistance (bipolar for low R, FET for high R), keep noise gain low, decouple supplies and use shielding and proper grounding.

## Check yourself

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

1. Which noise is white noise in resistors given by vn = sqrt(4kTRB)?
   A. Thermal (Johnson) noise
   B. Shot noise
   C. Flicker (1/f) noise
   D. Popcorn noise
   Answer: A. Thermal noise comes from random motion of charge carriers in a resistor and has a flat spectrum. (Key point: Thermal noise)

2. How do independent noise sources add at the output?
   A. By simple arithmetic addition, unamplified
   B. As root-sum-of-squares, then multiplied by the noise gain (1 + Rf/R1)
   C. They cancel each other
   D. Only the largest source counts and it is not amplified
   Answer: B. Uncorrelated sources add in power, so their rms values add as root-sum-of-squares, and the noise gain amplifies them. (Key point: Combining noise)

3. Which step reduces noise in an op-amp circuit?
   A. Using large-value resistors
   B. Increasing bandwidth
   C. Limiting bandwidth with a capacitor across Rf and using low-value resistors
   D. Raising the noise gain
   Answer: C. Noise is proportional to sqrt(B) and sqrt(R), so smaller R and smaller B reduce it. (Key point: Noise reduction)

4. Flicker (1/f) noise is largest:
   A. At high frequencies only
   B. At exactly 50 Hz mains
   C. It is independent of frequency
   D. At low frequencies, below the corner frequency
   Answer: D. 1/f noise rises as frequency falls, so it dominates below the corner frequency. (Key point: Flicker noise)

## Related in Advanced Electronics

- [Find coefficients of op-amp summing circuit](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Asumming-amplifier-coefficients) — Advanced Electronics, chapter 1
- [Noise in op-amp circuits and its reduction](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Aopamp-noise-minimize) — Advanced Electronics, chapter 2
- [Input impedance of inverting op-amp](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Ainverting-input-impedance) — Advanced Electronics, chapter 2
- [Output voltage of two op-amp IA](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Atwo-opamp-ia-output) — Advanced Electronics, chapter 4
- [Design three op-amp IA with gain 20](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Adesign-3opamp-ia-gain-20) — Advanced Electronics, chapter 4, asked 3×
- [Design three op-amp IA with gain 50](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Adesign-3opamp-ia-gain-50) — Advanced Electronics, chapter 4, asked 3×

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