Question on Thermal Noise

[electronicsguy123]

Hi everyone, I have a few questions about thermal noise

1. In the representation of thermal noise, why it is represented in the form of resistor but not a voltage source? I thought noise is some kind of irregular voltage?

2. In the thevenin representation, why the Rn is noiseless?

Thanks for the help:-D

 

[FvM]

A represents the physical nature of noise (it's in the resistor itself, there are no noiseless resistors), B and C are equivalent circuits (theoretical models).

 

[chuckey]

1,2, The noise voltage exists behind the "noiseless" resistance, which is its source impedance, else putting a short circuit on it would generate an infinite current!!
Frank

 

[jiripolivka]

While the above is correct, the principle of the thermal noise is the noise power or noise temperature.
Nyquist has derived that the noise power is independent of resistor value and equal to its physical temperature, P=kTB, where k is the Boltzmann constant and B is the frequency bandwidth in Hz.

From this you can derive the noise voltage (which depends upon resistor value) and noise current (as well).

 

[hfj1010]

B and C are just equivalent circuits of A...

 

[electronicsguy123]

Ok, I think i get it, the resistor in second and third circuit representation is noiseless is because of Nyquist has derived that the noise power is independent of resistor value. Thanks everyone for the explanation