THE LOW NOISE INPUT STAGE
Even a low noise amplifier contains different noise sources and all resistive sources create thermal noise depending on the
temperature.
As Mr. James Bryant from Analog Devices mentioned:
“So anywhere at all that there is resistance in a circuit, whether it is carrying current or not, there is a noise generator with
an output voltage noise spectral density of
4
kTBR
We can reduce the noise by reducing the resistance
R
(this may increase current and/or power consumption), but reducing
the temperature
k
is not usually practicable (if we cool a resistor from room temperature (298K) to liquid nitrogen
temperature (77K), its noise voltage is still more than half its room temperature value).
And, of course, we can't change
Boltzmann's Constant because Professor Boltzmann is dead.”
