Phil Hobbs wrote: : Looks good. Another approach would be something like a BFP650 follower,
Phil, I was about to suggest that if you've ended up using the BFP650 by following my example, you might be better off with the BFP640. Or more modern ones with a higher hFE (I'm fond of the NESG3031 and NESG4030).
Namely, my choice of BFP650 (and not the then more widely available BFP640) was driven by the search of the lowest possible R_BB, because I was deemed to encounter lower source resistances than the 50ohms for which SiGe devices are generally optimized. I reasoned that the base geometry of the BFP650, as a medium power device, is more likely yield a low R_BB. In your applications BFP640 may work better with the "more ordinary" impedance levels and ambient temperatures.
The "I was about.." part means that I'm somewhat puzzled now that I looked up the recent Infineon data sheets. The BFP650 is now listed as a SiGe:C part (the datasheet 2010-10-22) and its Gummel-Poon model reads RB=6.376 . The preliminary datasheet I have, dated Aug-16-2004, lists it as an ordinary SiGe part with RB=1.036 . So, I'm wondering how much the device has changed from the versions I've been using. I seem to recall that also in some other more recent data sheet versions it was listed as a SiGe, not SiGe:C .
Given the fact that the most recent BFP640 datasheet (dated 2007-05-29) reads RB=3.129, the BFP650 does not seem to have any noise advantage any more, either.
In fact, there was a recent paper in the RSI where the Jena group claim to have achieved 15 pV/rtHz with BFP640 in LHe, as compared with my 75 pV/rtHz with BFP650. This is a puzzling result, actually, because it is better than one would expect in the picture where u_N originates from the collector shot noise acting on the r_E - provided that the thermal voltage saturates at ~5..7 meV even when the ambient temperature keeps going down. According to my data it does saturate in BFP640 just like it does in all other SiGe's I've tried, when measured by the transconductance. I'm wondering whether my transistors have always been oscillating so high that I cannot see it (ie. >27 GHz) and the V_T saturation is an artefact due to it...
Regards, Mikko