Rod Davis <km6sn@...>
Investigating transmit gain in UBITX V3 using LTSPICE, Aug 28, 2018
all modeling done at 13.8vdc
q90 collector circuit modeled as parallel feed with 400 ohm load
q911-q912 collector circuit modeled as parallel feed with 200 ohm load
q92,q93 and q96,q97 collector circuit modeled as parallel feed
with 50 ohm load,each side
in the bidir amps the 2n3904 gain rolls off 1.2 db from 5MHz
to 45 MHz...not real bad; I believe 2n3904s are quite usable
in the bidir amps at 45 MHz.
In the TX:
q90 rolls off less than .5 db from 5 to 30 MHz
same for Q911-Q912
same for Q92,Q93,Q96,q97
so, adding q90 rolloff, and q911-912 rolloff, and q92,q93,q96,q97
rolloff I get (.5+.5+.5)= 1.5 db rolloff from 5 to 30 MHz.
My most recent ubitx v3 has 10W out on 40M, but less than 2W out on 15M.
That is more than 7dB drop.
I take it from the above that replacing the TX driver transistors with
higher Ft devices may not substantially improve the 15M performance.
WARNING: q911,q912,q92,q93,q96,q97 all dissipate at least 296 mW. That
means 250 mW transistors are not acceptable.
So, in searching for a better Ft device for the TX chain:
the BFR106 has the dissipation, but the 3.5 to 5GHz Ft invites instability
the MMBT10 lists dissipation at 225 mW, so it may not be a good candidate
the MMBT2369 lists dissipation at 225 mW, so it may not be a good candidate
the DTX2222A has the dissipation, but the Ft is not better than 2n3904, and
the footprint is not compatible with the board.
So at this point, I do not see a real winner for 2n3904 replacement.
I am sure open to suggestions on this front.
All of the above really leads me to question the power amp circuit..
and I do not have the wherewithall to model that successfully.
Any ideas there?