[time-nuts] FTS 1200 transistor replacement

Charles Steinmetz csteinmetz at yandex.com
Sat May 17 18:39:21 EDT 2014


Bert wrote:

>what  transistor will have even lower noise than the than low noise BC 109.

Generally speaking, the baseband noise of a bipolar transistor is 
most strongly influenced by the transistor's base spreading 
resistance.  Of course, the transistor also needs to have whatever 
other characteristics are required in the application circuit -- in 
particular, adequate current gain and transition frequency at the 
operating point and sufficiently low input capacitance -- and these 
may be mutually exclusive with lowest base spreading resistance.

The 2SD786 and MPSA18 may be good candidates.  That said, I seriously 
doubt you will get usefully better phase noise by using a "better" 
transistor in this location.

Best regards,

Charles






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