---
title: "Noise in op-amp circuits and its reduction"
subject: "Advanced Electronics"
chapter: 2
key: "semester_4/advanced_electronics:opamp-noise-minimize"
source: https://www.bctengineer.com/q/semester_4/advanced_electronics/opamp-noise-minimize
site: BCT Engineering
---

# Noise in op-amp circuits and its reduction

Advanced Electronics · Chapter 2: Operational Amplifier Characterization

- Asked in: 2071 Chaitra, 7 marks
- Video: [Types of Noise in Amplifiers (Amplifiers #11)](https://www.youtube.com/watch?v=_1WsVgEaJ3U) — Aaron Danner
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/advanced_electronics/opamp-noise-minimize
- Whole chapter: https://www.bctengineer.com/md/semester_4/advanced_electronics/chapter-2

## Question

Discuss noise in operational amplifier circuits with necessary diagram and suggest the measures to be taken to minimize the internal noise.

## Short answer

Internal noise includes thermal, shot, flicker, and popcorn noise. Minimize by using low-value metal-film resistors, limiting bandwidth, selecting low-noise op-amps, low noise gain, supply decoupling, shielded cables, and balancing input resistances with bypass capacitor.

## Answer

**Noise** is any unwanted random signal that appears at the output of an op-amp circuit and is not part of the input signal. It sets the smallest signal the amplifier can handle and limits the signal-to-noise ratio. It can be **external** (pickup from power lines, motors, radio sources, ground loops) or **internal** (generated inside resistors and transistors).

**Types of noise in op-amp circuits:**
- **Thermal (Johnson) noise:** random motion of electrons in every resistor. RMS voltage vn = √(4kTRB), where k = 1.38 × 10⁻²³ J/K, T in kelvin, R in ohms and B the bandwidth. A 1 kΩ resistor at 300 K gives about 4 nV/√Hz. It is white (flat with frequency).
- **Shot noise:** due to random crossing of charge carriers across a junction; noise current in = √(2qIB·B), q = 1.6 × 10⁻¹⁹ C. It is present in the bias currents of the input transistors.
- **Flicker (1/f) noise:** power increases as frequency falls; dominant below the corner frequency (about 10 Hz to 1 kHz), caused by surface defects.
- **Popcorn (burst) noise:** sudden step changes in bias current due to contamination; heard as popping in audio.

**Noise model:** an op-amp's internal noise is represented by a noise voltage source en (nV/√Hz) in series with the input and noise current sources in (pA/√Hz) at each input, driving a noiseless op-amp.

```text
              Rf (noise 4kTRf)
        +-----/\/\/-----+
        |  in           |
 R1 ----+-----(-)       |
 (4kTR1)          >-----+---- eno
 Rs --[en]--+--(+)
 (4kTRs)    in
```

All the input-referred noise sources are amplified by the **noise gain** (1 + Rf/R1), not by the signal gain. The independent sources add as root-sum-of-squares:

```text
eni² = en² + (in·Rs)² + (in·R1‖Rf)²
       + 4kT(Rs + R1‖Rf)          per Hz
Eno  = (1 + Rf/R1) · eni · √B     (rms)
```

**Measures to minimize noise:**
1. Use low-value resistors (thermal noise and in·R terms grow with R); use metal-film resistors, not carbon.
2. Limit the bandwidth to what the signal needs (noise ∝ √B): add a small capacitor across Rf or a filter.
3. Choose a low-noise op-amp: low en (bipolar input, e.g. OP27) for low source resistance; low in (FET input) for high source resistance.
4. Keep the noise gain low; take gain in the first stage so later-stage noise is less significant.
5. Balance the resistances at the two inputs but bypass Rcomp with a capacitor so its noise is not amplified.
6. Decouple power supply pins with 0.1 µF capacitors near the IC; use proper grounding (star ground) and shielded cables against pickup.
7. Keep the circuit cool and away from noisy (switching) sources; use ac coupling or chopper op-amps to avoid 1/f noise at low frequencies.

## Check yourself

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

1. Thermal (Johnson) noise voltage is given by:
   A. vn = sqrt(4kTRB)
   B. vn = sqrt(2qIB)
   C. vn = 4kTRB
   D. vn proportional to 1/f
   Answer: A. It depends on temperature, resistance and bandwidth and is white. (Key point: Thermal noise)

2. Shot noise arises from:
   A. Electron motion in resistors
   B. Random crossing of charge carriers across a junction
   C. Surface defects causing 1/f
   D. Contamination causing bursts
   Answer: B. In = sqrt(2qIB.B); thermal is resistors, flicker is surface defects, popcorn is contamination. (Key point: Shot noise)

3. Input-referred noise sources are amplified at the output by:
   A. Only the signal gain Rf/R1
   B. A gain of 1
   C. The noise gain (1 + Rf/R1)
   D. The open-loop gain A
   Answer: C. Eno = (1 + Rf/R1) eni sqrt(B), same for inverting and non-inverting. (Key point: Noise gain)

4. Which measure reduces noise?
   A. Use large carbon resistors and wide bandwidth
   B. Raise the noise gain
   C. Remove supply decoupling
   D. Use low-value metal-film resistors and limit bandwidth to what is needed
   Answer: D. Noise grows with R and sqrt(B); decoupling and shielding also help. (Key point: Minimising noise)

## Related in Advanced Electronics

- [Short note: Noise in op-amp circuit](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Ashort-note-opamp-noise) — 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
- [Single op-amp IA with mismatched resistances](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Asingle-opamp-ia-mismatched) — Advanced Electronics, chapter 4
- [3 op-amp IA advantages; electromagnetic isolation](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Athree-opamp-ia-electromagnetic-isolation) — 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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- [Summing amplifier using op-amp](https://www.bctengineer.com/question.md?key=semester_1%2Ffundamental_of_electrical_and_electronics_engineering%3Asumming-amplifier-op-amp) — Fundamental of Electrical and Electronics Engineering, chapter 6
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