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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