Noise Taxonomy
External vs Internal, Shot, Thermal, Flicker, Burst noise.
Electronic noise corrupts weak signals and limits the maximum range of communication links. Engineers classify noise into fundamental types like thermal, shot, and flicker noise.
Core Concept
Random fluctuations in voltage or current appear in absolutely every electronic circuit. Intrinsic noise comes directly from the fundamental physical properties of the materials and charge carriers themselves.
Thermal noise occurs due to the random heat-induced motion of electrons inside conductors. Shot noise happens because electrical current actually flows as a series of discrete, random electron jumps.
Flicker noise behaves differently, becoming incredibly severe at very low frequencies. It heavily impacts low-frequency instrumentation amplifiers like the LM741 and sensitive precision DC measurement circuits.
Key Equations
Thermal noise power: Pn = k * T * B
Shot noise current: In = √(2 * q * Idc * B)
Here, k is Boltzmann's constant and q is the elementary charge. The variable Idc is the direct current bias, and B is the measurement bandwidth in Hz.
Given:
DC bias current Idc = 2 mA
Bandwidth B = 10 kHz
Elementary charge q = 1.6e-19 C
Why this formula:
Shot noise current depends strictly on the DC bias current and system bandwidth.
Formula:
In = √(2 * q * Idc * B)
Substitution:
In = √(2 * 1.6e-19 * 0.002 * 10000)
Calculation:
In = √(6.4e-20)
Final Answer:
The RMS shot noise current is 0.25 nA.Exam Tip: Shot noise only exists when a direct current flows across a potential barrier like a PN junction. Standard metallic resistors absolutely do not produce shot noise.
Key Properties
- Thermal noise has a flat power spectral density up to extremely high frequencies.
- Shot noise requires a DC bias current to exist.
- Flicker noise increases dramatically as frequency approaches zero hertz.
- Transit time noise affects high-frequency circuits like 5 GHz microwave amplifiers.
- Environmental noise comes from outside sources like lightning and motors.
Quick Revision
- Thermal noise is proportional to temperature.
- Shot noise is proportional to DC current.
- Flicker noise dominates below 1 kHz.
- Resistors generate thermal noise.
- Diodes generate both thermal and shot noise.
- Exam trap: Mixing up the bandwidth dependence of flicker noise and thermal noise.
Noise Taxonomy Quiz
Test your knowledge of noise source classification, shot noise, thermal noise, flicker noise, and burst noise in electronic systems.
Q1.Flicker noise (1/f noise) has a power spectral density that varies inversely with frequency. The frequency below which flicker noise dominates over thermal noise (white noise floor) is called the flicker noise corner frequency fc. If a device has a flicker noise coefficient K_f and thermal noise floor S0, the corner frequency is:
Related Articles
Noise in FM Systems
Figure of merit, capture effect, threshold effect.
5 min read
Noise in AM Systems
Figure of merit, coherent detection SNR.
9 min read
Noise Figure
Definition F, relation to noise temperature Te.
4 min read
Signal to Noise Ratio
Input SNR, Output SNR, SNR in dB.
5 min read
White Noise
Power spectral density, autocorrelation.
5 min read