generally larger transistors running at much lower current than they
inteded for have lover noise figure, but much larger input and [Cbe] and
feedback Ccb] capacitance, the same effect could be achieved putting
--for AC!-- low noise transistors paralell [ signal voltage adds
arithmetically, noise voltage adds vectorial -- quadratic like power ]
most of the case power transistors are also "not so fast" lower fT.
On 7/18/2015 3:16 AM, Dr. David Kirkby (Kirkby Microwave Ltd) wrote:
On 18 July 2015 at 04:25, Charles Steinmetz <csteinm...@yandex.com> wrote:
Dave wrote:
1) Do you have any suggestions for a replacement for the LM394CN, which is
obsolete and unobtainable from any reputable source? There are plenty on
eBay for a few $'s from China, but the probability of them being fakes is
greater than 0.99. The MAT12 seems to be one possible candidate for a
replacement and while not cheap, is available from reputable sources like
Farnell.
The MAT12 should certainly work, but there is *plenty* of DC degeneration
in the circuit (1v at each emitter), so there is no advantage to matched
transistors. A pair of Zetex (Diodes, Inc.) ZTX849 or FZT849 actually have
significantly lower voltage noise than either the LM394 or MAT12.
Best regards,
Charles
Do you really mean the ZTX849? It is a power transistor (5A continuous
current collector current, 20 A peak collector current), with no
specification for noise on the data sheet. It would on the face of it
hardly seem a wise choice for a low noise amplifier, but perhaps you know
something I don't.
The LM394CN or MAT12 are at least a low-noise devices, even if the matching
between the two transistors in the package is not needed.
Dave
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