[time-nuts] Re: +1/f of transistors

[ed breya]

It depends on context and your definition of low noise. I was picturing 
ones commensurate with the 1/f discussion, trying to get down toward DC. 
Many are spec'd for so many nV or uV p-p over 0.1 to 10 Hz. The OP-07 is 
a good example of this class.

A lot of opamps are spec'd ok for their intended use, but you have to 
look fully at the charts and numbers for your actual use. For instance, 
the ADA4817 looks great as a HF/RF amp, with 4 nV/rt-Hz, but that's at 
100 kHz. Similar for the OPA656. At 10 Hz, it looks like it's well over 
100 nV/rt-Hz.

The JFET types are fine for high-Z amplifiers and such, but can't reach 
the near-DC region in the few nV/rt-Hz (at 10 Hz, say).

The CFAs may be OK too - they're built for low-Z, so may tend to be 
operating the + inputs at fairly high bias currents, like the low noise 
opamps.

The good part overall, is that to get near the nV/rt-Hz range, the 
source resistance has to be very low anyway, so at least the bias 
currents are manageable, although zero would be nicer.

Ed