[time-nuts] high rev isolation amps

Charles Steinmetz csteinmetz at yandex.com
Wed Mar 30 06:56:46 EDT 2016


Poul-Henning  wrote:

>I would have expected them to use capacitance optimized transistors, 
>also known as UHF transistors ?   Something like BFQ19 maybe ?

One of the main problems in isolation (and distribution) amplifiers 
is excessive additive (historically called "residual") phase noise 
due to AM to PM conversion in the amplifier.  The worst AM noise 
turns out to be flicker noise (1/f noise) in the transistors at 
baseband frequencies.  RF transistors typically have much higher 
flicker noise (by orders of magnitude), including 1/f corner 
frequencies that are decades higher, than general-purpose 
transistors.  The fact that most of the converted PM noise is at 1/f 
frequencies is doubly insidious, because it is close in to the 
carrier (and, therefore, essentially impossible to remove by filtering).

So, the best transistors for low phase noise design (of both 
amplifiers and oscillators) are transistors that have low flicker 
noise and low 1/f corners -- consistent, of course, with having 
sufficient gain at the RF frequencies of interest.  These days, there 
is a whole new class of BJTs that were designed for high current 
density and very low saturation voltage, some of which have 
astonishingly low flicker noise and also good gain-bandwidth products 
(f-sub-Ts).  Today's low phase noise designs often take advantage of 
these "accidental" characteristics of the new transistors.

Best regards,

Charles




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