Describe thermal noise and shot noise and their effect on SNR.

• A. Thermal noise ∝ sqrt(kTΔfR) and shot noise ∝ sqrt(2qId); both set fundamental SNR limits
• B. Thermal noise and shot noise are the same phenomenon
• C. Noise only affects DC levels
• D. Noise sources do not limit SNR in analog systems

Answer: (A)

Thermal noise and shot noise are inherent, fundamental sources of randomness in electronics that set the floor for how well any signal can be measured. Thermal noise comes from the random motion of charge carriers in a resistor due to temperature, producing a noise power that scales with temperature and bandwidth. This shows up as an RMS voltage across a resistor described by v_n,rms ≈ sqrt(4 k T R Δf), meaning higher temperature, higher resistance, or wider bandwidth all raise the noise level.

Shot noise arises from the discrete, particle-like nature of charge carriers. In a circuit with a steady current, the arrival of individual electrons fluctuates, giving a current noise with i_n,rms ≈ sqrt(2 q I Δf). This noise grows with the DC current and with the measurement bandwidth.

Both types of noise are white over the frequency band of interest, so they form a constant noise floor that cannot be eliminated. Since SNR is the ratio of signal power to noise power, increasing bandwidth or current increases the noise, lowering SNR, while higher temperature or resistance also raises the noise floor and reduces SNR. The statement reflects that both thermal and shot noise contribute to the unavoidable limits on SNR in analog systems.