P
Phil Hobbs
- Jan 1, 1970
- 0
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?
Cheers,
Phil Hobbs
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?
Cheers,
Phil Hobbs