[time-nuts] how to find low noise transistors

[Richard (Rick) Karlquist]

OTOH, the other cure for high base spreading resistance is
to simply parallel multiple devices.  This avoids the bad
side effects you mention.  The other key noise parameter
in a BJT is RF current gain, and this cannot be "cured"
by any circuit design tricks.

Rick Karlquist N6RK

On 7/23/2015 8:29 PM, 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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