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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