# Questions about Johnson Noise

Thread Starter

#### Hassan mahmoud

Joined Jan 23, 2016
19
1. Is this noise consider DC Noise? Could I remove it with a cap?
2. Based on formula an infinite resistor will cause an infinite voltage noise, and zero current noise. Well, it's obvious that there is a contradiction here since the open circuit(which is infinite resistor) isn't produce an infinite noise. Could someone explain?
3. The equation tells that the short circuit with infinite bandwidth will cause noise approaches to infinity. How much is that correct?

#### dl324

Joined Mar 30, 2015
12,931
You know the term but, clearly, you haven't done much research.

In answer to your first question, Johnson Noise (also referred to as Thermal Noise) is related to charged carrier movement in a resistance. It is also in the categories of noise considered White Noise which are uniformly distributed across frequency. So no, it can't be filtered with a cap.

#2 That's the theory, but it makes no sense in the real world.

#3 Infinite bandwidth has little application in the real world.

Thread Starter

#### Hassan mahmoud

Joined Jan 23, 2016
19
You know the term but, clearly, you haven't done much research.

In answer to your first question, Johnson Noise (also referred to as Thermal Noise) is related to charged carrier movement in a resistance. It is also in the categories of noise considered White Noise which are uniformly distributed across frequency. So no, it can't be filtered with a cap.

#2 That's the theory, but it makes no sense in the real world.

#3 Infinite bandwidth has little application in the real world.
Thanks for the reply.
According to #3, Actually, this is the first time that there is bandwidth in real world. Anyway, for what ranges of Resistor and bBandwidth, the equations is valid?

#### dl324

Joined Mar 30, 2015
12,931
Anyway, for what ranges of Resistor and bBandwidth, the equations is valid?
I'd think that it would hold for anything within reason; i.e. no zero/infinity resistance and/or bandwidth. Things might also get interesting at, or near, absolute zero.

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