Low noise transistors

I'm building a test jig to measure the noise floor of my antenna-coupled tunnel junctions. This is slightly awkward to do by cobbling test equipment, because they're only a few hundred ohms, and I need to be able to measure their noise under bias. I'm rooting through my Lista drawers full of ten-year-old parts for something to slap together.

So I have this nice way-way-sub-poissonian bipolar current source, with

1-Hz voltage feedback so that I don't blow anything up. Works groovy. Now I'm building a low-noise amplifier to do the noise measurement. It's a discrete diff amp made from Rohm 2SD786 BJTs, running at a voltage gain of about 250, with an inverted cascode on each input transistor to deal with the huge C_cb while bringing the signals out near ground. There's an LM6361 as the output stage, with overall AC-coupled feedback to get a gain of 1000, and an auxiliary bias correction loop dumping a bit of current into one of the collector loads to keep delta V_BE of the diff stage at zero. Should work fine sometime tomorrow.

Which brings me to my actual question: Are the 2SD786 and 2SD737 still the BJT noise champions at low source resistance?


Phil Hobbs

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Phil Hobbs
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