The key to solve the seven major noises inside the circuit(3)

3. High frequency thermal noise

High frequency thermal noise is caused by the random movement of electrons inside the conductor. The higher the temperature is, the more intense the electronic motion will be. The random motion of the electrons inside the conductor creates many small current fluctuations inside it. Because it is disordered, its average total current is zero, but when it is used as a component (or as part of the circuit) it is amplified. After the circuit, its internal current is amplified into a noise source, especially for high-frequency thermal noise of circuits operating in the high-frequency band.


Usually in the power frequency, the thermal noise of the circuit is proportional to the passband, and the wider the passband, the greater the influence of the thermal noise of the circuit. Taking a 1kΩ resistor as an example, if the passband of the circuit is 1MHz, the effective value of the open circuit voltage noise appearing across the resistor is 4μV (the temperature is room temperature T=290K). It seems that the electromotive force of the noise is not large, but if it is connected to an amplifying circuit with a gain of 106 times, the output noise can reach 4V, and the interference to the circuit is great.


Post time: Aug-18-2021

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