Home / Topics / Current Electricity & Circuits
Noise in circuits
Advanced
Thermal Johnson noise and shot noise limit measurements.
Thermal (Johnson) noise V² = 4kTRΔf and shot noise limit the smallest detectable signals. Cooling and narrow bandwidth reduce them.
1 MΩ at 300 K, 10 kHz: 13 μV rms.
Amplifiers add their own noise.
In more detail
Electrical noise is random fluctuation in current or voltage, arising from thermal motion and quantum effects. Thermal noise in a resistor is proportional to its temperature and resistance.
Real-world uses
- Designers of sensitive amplifiers minimise noise in input stages.
- Signal processing removes noise from measurements.
- Engineers designing sensitive amplifiers consider thermal and shot noise, which set the smallest signal they can detect.
Open the interactive lesson, with simulation and quiz →
Frequently asked questions
What causes thermal noise?
Random thermal motion of electrons in a resistor, also called Johnson noise.
Can noise be eliminated?
It can be reduced by cooling and careful design, but not fully removed.
What causes thermal noise?
The random thermal motion of electrons in any resistor, which creates a small fluctuating voltage even with no current.
Related topics in Current Electricity & Circuits
Series vs parallel in homes · Drude model · Hall effect · Network analysis with nodes and meshes