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Thermal Noise of Resistor

Themis M

Hello,

I am designing an instrumentation amplifier using the current balancing amplifier architecture. The differential voltage at the input is copied to the sources of the input pair and generates a current through the resistor R1.

I am running a noise simulation to obtain the output noise at the terminals of R1 which, for the specific circuit, is approximately equal to the input-referred noise. However, the resulting noise generated by the resistor R1 is much smaller than the predicted 4kTR by several orders of magnitude. On the contrary, it appears to be proportional to 1/R1.

I am not sure I can understand how increasing the value of a resistor reduces the noise generated at its terminals... 

  • Andrew Beckett

    Without seeing the architecture of your circuit, it's hard to be sure, but the mistake most people make is that the noise contributions you are seeing are output-referred noise rather than source-referred (i.e. the noise at the device itself). It is possible since SPECTRE161 (and later IC617 versions) to turn on "noise separation" on the noise form which allows you to see the noise at the device separately from the gain from the noise source to the output and the total noise.

    In my experience the noise analysis doesn't do the wrong thing - it's almost certainly correct circuit behaviour you're seeing or you're misinterpreting what the results mean. I'm happy to be proved wrong though!

    Regards,

    Andrew.

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