Resistor Noise Calculator

About the Resistor Noise Calculator

Every resistor generates a small random voltage across its terminals — even with no current flowing. This phenomenon, known as Johnson-Nyquist noise or thermal noise, was first measured by John B. Johnson in 1926 and theoretically explained by Harry Nyquist in 1928. It is a fundamental consequence of the thermal agitation of charge carriers (electrons) inside the resistive material.

Thermal noise sets the ultimate sensitivity limit for electronic circuits. In a radio receiver, the noise floor of the front-end amplifier is largely determined by the thermal noise of its input resistance. In precision measurement systems, low-noise design starts with understanding and minimizing resistor noise. The RMS noise voltage is proportional to the square root of resistance, temperature, and bandwidth — meaning that reducing any of these three factors reduces noise.

This calculator computes Johnson-Nyquist noise voltage, noise current, available noise power, and voltage/current spectral density. It provides tables showing how noise changes with bandwidth and resistance, plus a signal-to-noise ratio chart for various signal levels. You can also analyze the combined noise of multiple identical resistors in series or parallel configurations.

When This Page Helps

Understanding resistor noise is essential for low-noise circuit design. Whether you\'re selecting components for an audio preamplifier, an RF front end, or a precision instrumentation amplifier, thermal noise sets the fundamental limit. This calculator lets you quickly quantify noise voltage, noise current, and spectral density, compare values across resistances and bandwidths, and estimate the signal-to-noise ratio for your application.

How to Use the Inputs

1. Enter the resistance value and select the unit (Ω, kΩ, MΩ).
2. Set the temperature (°C) — room temperature is 25°C.
3. Enter the measurement or system bandwidth and select the unit.
4. Optionally specify multiple identical resistors in series or parallel.
5. Read the RMS noise voltage, noise current, and spectral density from the outputs.
6. Review the noise vs. bandwidth and noise vs. resistance tables for design insight.
7. Check the SNR table to see how your noise compares to typical signal levels.

Example Calculation

Tips & Best Practices

• Use the lowest resistance that meets your circuit requirements to minimize noise voltage.
• Reduce bandwidth with appropriate filtering to lower noise — halving bandwidth reduces noise by ~30%.
• Metal film and thin-film resistors have less excess noise than carbon composition types.
• At very high frequencies (>100 MHz), parasitic inductance and capacitance affect the noise spectrum.
• Paralleling identical resistors reduces noise voltage by √n — two parallel 10 kΩ = 5 kΩ effective with 0.71× noise.
• The thermal noise floor at 25°C is −174 dBm/Hz — memorize this as a design reference.

The Physics of Thermal Noise

Thermal noise arises from the random Brownian motion of charge carriers inside any conductor. At thermal equilibrium, the fluctuation-dissipation theorem guarantees that any dissipative element (resistance) will produce voltage fluctuations with a white power spectral density of S_v = 4kTR V²/Hz. This is a remarkably universal result — it depends only on resistance and temperature, not on material, geometry, or construction.

Noise in Circuit Design

In a transimpedance amplifier (e.g., for photodiode readout), the feedback resistor\'s thermal noise directly limits the minimum detectable current. A 1 MΩ feedback resistor at 25°C produces about 4 nV/√Hz, which translates to ~4 fA/√Hz of equivalent input noise current. Designers often face a tradeoff: higher resistance means more gain but also more noise (both scale as √R). Bandwidth limiting through capacitive feedback can reduce total integrated noise.

Beyond Johnson-Nyquist: Excess Noise

Real resistors exhibit additional noise beyond the thermal minimum. Carbon composition resistors generate significant 1/f (flicker) noise when current flows, quantified by a noise index in dB. Metal film and wirewound resistors have much lower excess noise. For the most demanding applications, bulk metal foil resistors (e.g., Vishay VHP) offer the lowest combined thermal and excess noise.

Frequently Asked Questions

• What is Johnson-Nyquist noise?

  It is the random voltage generated by any resistor due to thermal motion of electrons. It was first measured by Johnson and theoretically explained by Nyquist in the 1920s. Unlike shot noise or flicker noise, thermal noise has a flat (white) spectrum.

• Does noise depend on the resistor type (carbon, metal film, wirewound)?

  Johnson-Nyquist (thermal) noise depends only on resistance, temperature, and bandwidth. However, real resistors also generate excess noise (1/f noise and current noise) that varies by type. Carbon composition resistors are noisier than metal film.

• Why does noise power not depend on resistance?

  Available noise power P = kTΔf is independent of R because higher R produces more voltage but also limits the current proportionally. The maximum power transfer occurs when load impedance matches the source, and the result is always kTΔf.

• How does temperature affect noise?

  Noise voltage scales as √T (absolute temperature in Kelvin). At room temperature (298 K), cooling to liquid nitrogen (77 K) reduces noise voltage by about half, which is why cryogenic preamplifiers are used in radio telescopes.

• How do series and parallel resistors affect noise?

  Series: total R increases, so V_noise increases (but I_noise decreases). Parallel: total R decreases, so V_noise decreases but I_noise increases. Paralleling n identical resistors reduces voltage noise by √n.

• What is the noise floor?

  The noise floor is the minimum detectable signal level in a system, set by thermal noise (and other noise sources). At 25°C over a 1 Hz bandwidth, the thermal noise floor is −174 dBm regardless of resistance.