On 18 July 2015 at 09:45, Ole Stender Nielsen <ols...@mail.tele.dk> wrote:
> I had a few LM394CN left and used one for the pre-amp. > I would try to use another low-noise transistor. Furthermore, you do not > necessarily need a matched pair. The transistor on the right in the > schematic is merely used for biasing the amplifying transistor on the left. > The prices for matched pairs can get ridiculously high, so I too would try > to look for alternatives. If you still want a matched pair, there's the > SSM2212 from Analog Devices, which is priced lower than the MAT12. You may > also be able to find the earlier SSM2210, which now is obsolete. Another > alternative is the THAT300 from THAT Corporation, which has 4 low-noise > NPNs in a 14-pin DIP (great for experiments...) > Useful single NPN transistors I can think of would likely be the BC337 or > perhaps the 2SC3324 which both have low Rbb, and they are quite cheap. > Hi, I ordered the parts for this, including a MAT12. I do consider some arguments put by others - the first on the mailing list, the second privately. 1) There are cheaper low noise devices - that in itself has generated into another thread. 2) The atmospheric noise at 100 kHz is high But given * A MAT12 is not a fortune from reputable semiconductor suppliers like Farnell (I would not consider eBay, due to fakes) * I am only building one, so cost is not a huge issue. * The MAT12 is specified for low noise, without me wasting time selecting devices. * MAYBE the argument about atmospheric noise is valid - I tend to agree with it, but again due to the one-off nature of this, I am going to ignore that argument. So I bought a MAT12. I'm *not* going to change that - it is bought, paid for and here. > > The "Ca. 3 Ohm" was indeed the estimated input resistance, including the > series resistance of the input capacitor as I recall. > If the input resistance of that common-base amplifier is 3 Ohms, does it make sense to have the loop resistance 3 Ohms too? For maximum power transfer one wants the source and load to be the complex conjugate, but I know for microwave devices, you do not design the input match for maximum power transfer, but generally minimum noise. I just checked my HP 4284A precision LCR meter, which covers from 20 Hz to 1 MHz in 8000 and something discrete steps including 100 kHz. I can set the voltage as low as 5 mV, so I could measure the input impedance (R + j X) of the amplifier at 5 mV RMS. So while Ole's estimate is no doubt good, I can actually measure it with the MAT12 in place. I have 95 m of 2.5 mm^2 mains cable, with a PVC (?) jacket. The copper conductor which should have a DC resistance of about 0.7 Ohms. I'm wondering if I would be better * Having a longer piece (more resistance, higher cost, a larger thermal noise voltage, but better impedance matching) * Use thinner wire, which would be less costly, but again aim for a DC resistance of 3 Ohms - or whatever my LCR meter says in the input Z at 100 kHz. In the short term I am just going to put on the 95 m, as I have that here, but longer term I wonder if I would be better increasing the number of turns, to put the DC resistance closer to the input resistance of the amplifier. Any comments? Dave _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.