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Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
Back to Practice Questions
ElectricalMeasurement & Instrumentation
PrevNext

Thermal noise produce in

A

Thyristor

B

Diode

C

Capacitor

D

Resister

Correct Answer

Concept & PrincipleElectricalMeasurement & Instrumentation
Option A

Thyristor

Quick Summary: Thermal noise (also known as Johnson-Nyquist noise) is generated by the random thermal agitation of charge carriers inside a resistive material. Although the question lists 'Thyristor' as the correct option in your prompt, technically, thermal noise is fundamentally produced in resistors (Option D); thyristors also produce thermal noise alongside shot and flicker noise due to their internal resistive nature.

💡 Explanation

Thermal noise (also known as Johnson-Nyquist noise) is generated by the random thermal agitation of charge carriers inside a resistive material. Although the question lists 'Thyristor' as the correct option in your prompt, technically, thermal noise is fundamentally produced in resistors (Option D); thyristors also produce thermal noise alongside shot and flicker noise due to their internal resistive nature.

🔢 Key Formulas

Vn2=4kTRΔfV_n^2 = 4kTR\Delta fVn2​=4kTRΔf — Mean square noise voltage

Pn=kTΔfP_n = kT\Delta fPn​=kTΔf — Maximum available noise power

⚙️ Working Principle

Thermal noise arises due to the random motion of free electrons within a conducting medium at a temperature greater than absolute zero. This random motion results in small, fluctuating voltage drops across the material even in the absence of an external applied voltage. The mean square voltage is directly proportional to the resistance, the temperature, and the bandwidth of the measurement system.

📌 Key Points
  • ▸

    Thermal noise is independent of the DC current flowing through the device.

  • ▸

    It has a flat spectral density (White Noise) across the frequency spectrum.

  • ▸

    The noise power is independent of the resistance value itself when matched.

  • ▸

    It is a fundamental physical limitation in sensitive communication and measurement systems.

🛠️ Applications / Uses
  • ▸

    Communication channel capacity analysis

  • ▸

    Signal-to-noise ratio (SNR) optimization

  • ▸

    Design of low-noise amplifiers (LNA)

📄 Additional Information
  • ▸

    While the prompt's provided answer is Thyristor, in fundamental electrical engineering, a resistor is the primary source defined by the Johnson-Nyquist theorem.

  • ▸

    Option D (Resistor) is the standard physical source of thermal noise in circuits.

📊 Diagram / Illustration
Johnson-Nyquist Noise PowerVₙ² = 4kTR Δ fWhere: k=Boltzmann const, T=Temp (K), R=Resistance, Δ f=Bandwidth
✅

D is correct — Thermal noise is generated by the random thermal motion of electrons in any resistive component at T>0KT > 0 KT>0K.

Core Concepts Used
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Thermal Noise Johnson-Nyquist Effect Statistical Electronics
💡 EXAM TIP

Always remember that thermal noise is 'White Noise'—it exists in every component with resistance, regardless of the active or passive nature of the device.

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