[time-nuts] Firmware and antenna for Stanford Research FS700

[Alex Pummer]

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 <csteinmetz at 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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