Noise

Noise analysis quantifies the random fluctuations that fundamentally limit the resolution and dynamic range of analog circuits. The two dominant noise sources in MOSFETs are thermal noise, generated by the channel resistance (modeled as a current source with PSD = 4kT·gamma·gm), and flicker (1/f) noise, caused by traps at the Si-SiO2 interface that capture and release carriers, with a power spectral density proportional to 1/f and inversely proportional to device area (WL). Flicker noise dominates at low frequencies and is a key reason analog designers use large gate areas for input devices in precision amplifiers and why techniques like chopping and correlated double sampling exist.

Input-referred noise analysis involves referring every noise source in a circuit back to the input, allowing direct comparison against signal amplitude to compute SNR. For a differential pair, the input-referred noise voltage depends on the gm of the input devices relative to the gm of the current source and load devices — a higher gm ratio between input and noise-contributing devices improves noise performance, which is why bias current and device sizing trade directly against noise. Total integrated noise over a bandwidth, often expressed as kT/C noise in switched-capacitor circuits, sets fundamental limits on ADC resolution and sample-and-hold accuracy.

Designers must also account for noise folding in switched-capacitor and sampled systems, where wideband noise aliases into the signal band, and for correlation between noise sources when computing total output noise via superposition of variances (not amplitudes). Noise simulations in SPICE (.noise analysis) report both device-level contributions and total output/input-referred noise, guiding trade-offs between power consumption, bandwidth, and noise floor — a central trade-off in nearly every analog block from LNAs to data converters.

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