Thermal noise in bipolars

Hello All,

I am trying to make a noise comparison for MOSFET vs bipolar.

- I know that bipolars exhibit lower flicker noise.

But what about their thermal noise?

They have higher gms so does this translate to higher thermal noise?

Thanks,

Diarmuid

If you want to compare, you should use amplifying stages with same gms. Otherwise this wouldn't be fair, I think, would it?

Good point.

So if we assume we are making a noise comparison of a mosfet vs bipolar (with same gm's), which would exhibit more thermal noise?

Strange that I cant find much data on the web regarding this question.

diarmuid said:

So if we assume we are making a noise comparison of a mosfet vs bipolar (with same gm's), which would exhibit more thermal noise?

Click to expand...

The mosfet, I'd think, because it probably needs more current for the same gm.

diarmuid said:

Strange that I cant find much data on the web regarding this question.

Click to expand...

Nor do I know such comparison. I'll leave the literature research to you ;-)

Yea, with increased current, noise would surely go up.

Someone should do a phd on all this stuff! Im finding serious gaps in noise analysis!

The BJT has thermal noise in its bulk resistances only. 1/gm appears as a voltage noise source in the base [ rt(4KTB*0.5*re), re = 1/gm ]. This noise source is in series with the noise generated by Rbb' and Ree' rt(4kTB(Rbb'+Ree')).

A FET has a temperature dependent drain noise current component that has to be transformed to the gate.

I don't know your frequency of interest, but at low frequency (especially for the CMOS), 1/f, 1/f^2, etc noise dominates the pure thermal (Johnson) noise contributions.

Whether the overall noise increases with increasing drain or collector current, depends also on the source impedance. I like the BJT because I can tweek the collector current to find a good trade-off between overall input current and voltage noise.

diarmuid said:

Someone should do a phd on all this stuff! Im finding serious gaps in noise analysis!

Click to expand...

Sure; probably already done a hundred times - just find them ;-). Perhaps this Willy Sansen book is a good start?