Skip to main content

Short note: 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.

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

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:

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.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Advanced Electronics chapter 2 questions →