[time-nuts] how to find low noise transistors

Charles Steinmetz csteinmetz at yandex.com
Thu Jul 23 23:29:28 EDT 2015


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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