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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
