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why is the input referred noise not proportional to the output noise?

MarcoL

Hi all,

     Recently I found the input referred noise is not equal to the output noise divided by the ac gain in specter noise analysis, a little bit confusing.

    Such a testbench is built. Generally it 's a common-source amplifier with an ideal resistor as the load of a transistor. A DC voltage source, of which the AC voltage is set to 1V is connected to the gate, and the output noise is obtained at the drain using the expression:

    getData("out" ?result"noise").

The input referred noise is obtained at the gate, using the expression:

   getData("in" ?result"noise").

Besides, the gain is obtained using :

  db(v("/Vout") ? result "ac") ), where Vout denotes the drain voltage.

  Thus I expected to get two curves illustrating the foregoing noises, of which the input referred noise should be equal to the output noise divided by the gain. It turned out, however, while the ratio of two noises seems fixed, the value deviates from the gain. For instance,

    vn,o= 255.19 nV/sqrt(Hz), vn,i = 49.32 nV/sqrt(Hz), the ratio = 5.18 or 14.29dB at f= 1M Hz, and this ratio is almost fixed before 3dB bandwidth.

   gain = 15.82 dB until f reaches beyond 500M Hz.

   So why is that? where does the 1.53dB difference come from? In my understanding, the noise analysis is based on the ac model, so the gain for ac analysis and noise analysis should not be different. Can anyone explain that?

Thanks a lot,

Marco

2016/11/18

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  • Andrew Beckett

    Marco,

    noise analysis is actually based on xf analysis (not ac analysis), although the results should be similar. The gain should also be output by the noise analysis - worth looking at that - this is the gain from the specified input source to the output (rather than ac analysis which only gives the gain if it's an ideal input source and the source value is 1).

    If you can't get consistency on this, we'll need to see the circuit - it's not obvious to me from your description what might be wrong. 

    Regards,

    Andrew.

Reply
  • Andrew Beckett

    Marco,

    noise analysis is actually based on xf analysis (not ac analysis), although the results should be similar. The gain should also be output by the noise analysis - worth looking at that - this is the gain from the specified input source to the output (rather than ac analysis which only gives the gain if it's an ideal input source and the source value is 1).

    If you can't get consistency on this, we'll need to see the circuit - it's not obvious to me from your description what might be wrong. 

    Regards,

    Andrew.

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