[time-nuts] FTS 1200 transistor replacement

EWKehren at aol.com EWKehren at aol.com
Sat May 17 19:05:39 EDT 2014


Thanks for all the help. I do have a couple of BC 109 but I thought with  
all the work to carefully disassemble the unit that after 30 years there 
would  be a better transistor.
Thanks again
Bert Kehren
 
 
In a message dated 5/17/2014 6:47:55 P.M. Eastern Daylight Time,  
csteinmetz at yandex.com writes:

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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