[time-nuts] Re: Collector current that minimizes BJT noise

[Matt Huszagh]

Hi Gerhard, thanks for the thoughts.

Gerhard Hoffmann via time-nuts <time-nuts at lists.febo.com> writes:

> Not so sure about BJTs, but for FETs it is best to run them at
> high drain current. Gain is proportional to the root of Id,
> and voltage noise is proportional to 1/ sqrt(gain). So, things get
> better with the 4th root of current. One hits the limits quite soon.
> It is generally better to reduce Id and put more FETs in parallel.
> Unless used at high temeratures, there is not much noise current,
> but Cin hurts.
>
> On BJTs , Ib * Rbb is the worst problem. A large Beta helps to
> minimize that. But if you minimize Ic, beta may drop steeply
> depending on the transistor. Then you may be worse off, over all.

I'd neglected beta's dependence on collector current; thanks for the
reminder.

As for the base shot noise, Motchenbacher includes this in the complete
noise formulation and then discards it for his simplified voltage noise
expression, leaving just collector shot noise and base thermal noise. He
claims these are the two quantities limiting noise performance at
midband frequencies.

After running some numbers I'm inclined to agree. The base thermal noise
is proportional to the root of the base resistance whereas base shot
noise is proportional to the first power of base resistance. For T=300,
ic=10mA, beta=100, the base shot noise doesn't catch up to base thermal
noise until rbb hits about 500ohms. That should be well out of low-noise
BJT territory. Even with lower beta the equalizing rbb is still pretty
high.

> I go to measurement with the intended circuit. There are other
> constraints
> too, like supply voltage, required gain...
> Spice models are seldom good wrt noise. Nobody seems to care.
> A lot of these have historically been made with Orcad parts.exe.
> They all seem to have the same RBB, be it 2N3055 or BC109.

Ah, yeah I was worried about something like that. The fact that
datasheets barely provide any information about noise performance didn't
bode well. Are people not designing low-noise first stages with
discrete transistors?

> Since you mentioned AOE: their ribbon microphone preamp meets its
> 70 pV/rt Hz goal as promised. Of course, there is a lot of noise
> current.
> OTOH, 70pV/rtHz makes only sense with a low impedance DUT.
> It is a nice tool in front of the FFT analyzer.
> I've built it in a single-ended version. That reduces the # of
> transistors from 64 to 16. That costs a lot of input coupling capacitors
> which is ugly, but then, most interesting signals ride on a DC
> and a dc coupled differential input is no help then.
> Vcc must be _really_ clean.

I'd come across that but not given it a detailed look yet. I'll look
closer. I think that's confined to audio frequencies, though, whereas
the prompt for this question was looking at Bruce Griffith's
common-emitter Norton buffer amplifier for distributing a clean 10 MHz
source.

I've been investigating alternative transistors to posssibly lower the
noise further. I also wanted to add a second stage to increase the
reverse isolation. That raised the possibility of running the first
stage at lower collector current.

I've been going around in circles a bit trying to use datasheet and
spice values in calculations. It sounds like I need to just build a few
variations and test them at this point. The only problem is that I'm not
sure I have a test setup sensitive enough to measure the difference...

Matt