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Old 11th April 2013, 01:03 PM   #11
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Which part types exactly did you test?

I'd like to say, with regard to hFE matching, it's an easy measurement for manufacturers and users to make (but perhaps relatively hard for manufacturers to meet).

I'm always disappointed when I see an hFE matching spec but no Vbe-matching spec. That's because hFE matching isn't a very useful parameter. hFE is not predictable, and well-done designs work around that parameter and depend instead on Vbe and Vbe matching. Vbe is the parameter that counts for differential-amplifier stages, etc. One can hope that well-matched hFE will imply well-matched Vbe, but one cannot count on it.

I wonder if you tried Vbe matches for your 10 parts?
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Old 11th April 2013, 02:51 PM   #12
AndrewT is offline AndrewT  Scotland
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Yes, I agree.
Set up an LTP pair and apply a common Vbe to both devices in the pair.
Discover just how well matched the output currents are into the exactly equal Collector loads.
Once I have found potential pairs I always test them thermally coupled as LTP pair. That really does show up mismatching of supposed good pairs.

The 2sa1312 datasheet is impressive.
1dB noise figure for 50r @ 7mA & 1kHz
Jumps to 6dB ref same 50r @ 10Hz.
Maybe for lower frequencies, around 2 to 3mA may be a better bias.
Any thoughts?
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Last edited by AndrewT; 11th April 2013 at 03:00 PM.
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Old 4th October 2014, 05:04 PM   #13
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Quote:
Originally Posted by winhill2 View Post
(edited) This line of parts are in my estimation the lowest-noise BJTs now available (lowest r_bb') that have official low-noise specs**, now that Rohm's 2SD786 and 2SB737 have disappeared.

Sadly the 2SC2240 and 2SA970 (npn, pnp) super-low-noise trannies in TO-92 packages no longer appear on Toshiba's Bipolar Transistors Product List, and have been designated NRND by Toshiba (Not Recommended for New Designs). [link] This means the factory is presumably still taking (large) orders, but they are destined for the dust-bin of history. FYI, last chance, B&D still has some inventory.

But, not to fear, Toshiba offers what appears to be the same (or better) die in SOT-23 packages, the 2SC3324 and 2SA1312. These are stocked by Mouser, and cost only 15 cents qty 100. Their datasheets are fairly complete, including two NF contour plots. The 1kHz plot shows the 3dB contour dropping 35 ohms at 1mA and below 20 ohms at 10mA. That corresponds to voltage noise levels e_n of 1.4, 0.75 and 0.56nV at 0.1, 1 and 10mA. Using the 100Hz plots, we can calculate that the 1/f corner frequencies are about 14Hz, 35Hz and 170Hz, not too bad.

The hn3c51F and hn3a51F dual transistors, original subject of this thread, come in easy-to-solder SC-74 (6-pin SOT-23) packages, and apparently use the same dies as the 2sc3324 and 2sa1312. Their datasheets have the same two impressive noise-figure contour plots. But even though several years have gone past since this thread started, no distributors have picked up these parts.
Update:
Happily, Mouser has been carrying these Toshiba dual transistors, wow! They're priced at only 18 cents, qty 100. We've measured these parts. For the npn and pnp, they have r_bb' of 35 and 20 ohms, and e_n at 10mA of 0.75 and 0.58 nV/rt-Hz, resp. The Early voltage is 560 and 180 volts.

There are lower-noise transistors, lower e_n anyway, with r_bb' down to the under 2-ohm region, see Table 8.1 in AoE 3rd edition for full details, but they're not specified as low-noise transistors. They're low-saturation-voltage designs made by Zetex / Diodes, Inc. For example the ztx618 and ztx718 (fmmt618 and 718 for sot-23 packages) with r_bb' of 9.3 and 7.3 ohms, and the ztx851 and ztx951 with r_bb' of 1.67 and 1.24 ohms. We use the latter (multiple pnp transistors in parallel) to make a record-breaking low-noise amplifier, only 70 pV/rt-Hz. That's low enough to be a ribbon-mic pre-amp, without the usual step-up transformer!

Last edited by winhill2; 4th October 2014 at 05:07 PM.
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