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Noise simulation shows wrong result when ideal current sources are present

mhasan14

I tried to measure noise contribution of a MOSFET in the following circuit arrangement. The current source is ideal. I would measure noise current from ADE noise simulation. However the noise current  does not seem to be what is expected. The values are way too low. The noise current does show 1/f pattern over 10mHz to 1kHz, however the noise is much lower than the thermal noise value. The  thermal noise should be around 2*q*I and at lower frequencies 1/f would dominate. But I see the noise current six orders of magnitude lower than the thermal noise. Shorting out the current source displays expected result however. I measure noise current by attaching a zero volt voltage source at the source of the MOSFET and probe current from it.

The question is what is going wrong here. If I replace the MOSFET with an analog resistor noise current output is zero.

In had calculations we would open up the ideal current source and all the noise current of MOSFET flows through the MOSFET. So I expect to see the same result from simulation.

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  • It's not entirely clear how you're measuring the noise current - but my guess is that because you have an ideal current source, the current through the device is fixed - the noise is not influencing (it may have a small influence about how much current flows into the bulk hence the small numbers). 

    Showing your real test bench and the analysis setup might clarify that for certain though.

    Regards,

    Andrew.

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  • It's not entirely clear how you're measuring the noise current - but my guess is that because you have an ideal current source, the current through the device is fixed - the noise is not influencing (it may have a small influence about how much current flows into the bulk hence the small numbers). 

    Showing your real test bench and the analysis setup might clarify that for certain though.

    Regards,

    Andrew.

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