Only thermal noise is significant in passive components like a resistor. Resistor noise can be represented as a noise voltage source in series with the
resistor as shown below..
The noise voltage En primarily depends on three factors:
The bandwidth and
The noise Voltage is given by,
En= the Root-Mean-Square or RMS voltage level
k= Boltzmans constant (1.38*10-23)
T= Temperature in Kelvin (Room temp = 27 °C = 300 K)
R= Resistance in Ohms
Δf = Circuit bandwidth in Hz
As an example, that 100 kΩ resistor with 1MHz bandwidth will add noise to the circuit as below:
En= ( 4 *1.38*10-23 *300 * 100*103*1*106)1/2
=40.69 μV RMS
2. Noise Power:
The noise power, or Mean-Square noise, is the square of noise voltage a given here.
P = E2n = 4 k T R Δf
Note that the Noise power increases proportionately with temperature. By limiting the circuit bandwidth to the desired frequencies, we can limit the noise
power contributed to the circuit.
3. Junction Diode and Transistors: A junction diode will typically have two components of noise. One is thermal noise, and the other is shot noise. Note that if the active
device provides amplification, the noise also gets amplified along with the signal.