[time-nuts] high rev isolation amps
Charles Steinmetz
csteinmetz at yandex.com
Wed Mar 30 10:56:46 UTC 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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