[time-nuts] how to find low noise transistors

[Brooke Clarke]

Hi Charles:

Does hFE (DC) have much relevance to this?  Would hfe (AC) be the important one?
Mail_Attachment --
Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html
http://www.prc68.com/I/DietNutrition.html
Charles Steinmetz wrote:
> Rick wrote:
>
>> optimum noise figure is a function of the ratio between base spreading resistance and (beta)(r-sub-e).  If base 
>> spreading resistance is high, you make r-sub-e high by reducing collector current.
>
> I replied:
>
>> reducing transistor current to raise the noise resistance causes undesirable collateral effects (including reduced 
>> bandwidth, which increases phase noise due to baseband noise modulation of transistor capacitances and generally 
>> increases nonlinearity).
>
> I should also have mentioned:
>
> Reducing transistor current also frequently reduces beta (sometimes by a large factor, depending on the transistor's 
> beta vs. current curve and where you are on it).  This directly affects (beta)(r-sub-e) and, therefore, directly 
> reduces the noise figure.  I've pasted in the beta vs. collector current graphs for the ubiquitous 2N3904 and 2N4401 
> to illustrate this.  Some transistors are better than these over a useful range of collector currents, others are much 
> worse.  The beta of PNPs, which are generally quieter than NPNs, also generally falls off faster with reduced 
> collector current.
>
> Note that these are static (DC) curves, which are good approximations for the 1/f region.  The curves in the white 
> noise region, even at relatively low frequencies like 1 kHz, generally fall off faster than this as current is 
> reduced, so the effect of reduced beta on in-band noise figure is greater.
>
> Best regards,
>
> Charles
>
>
>
>
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