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Looking for a low noise amplifier (<1nV / rt. Hz)
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Gerhard_dk4xp:
That's because I underestimated the need for a large input capacitor.

A 100 or 200 uF foil capacitor would have been enough from a frequency
response point of view, but that resulted in a rise of noise at the bottom end
that is much stronger than 1/f.

Sub-nV/rtHz amplifiers make only sense when their input signal source is
smaller than, say, 60 Ohms. 60 Ohms delivers 1nV/rt Hz thermal noise at
room temperature and your complete system could never be better than that.

OTOH the input impedance of the preamp itself is high and determined
mostly by the 10K bias resistor. But these 10K also create noise like a 10K
resistor. That completely spoils the good noise behavior of the preamp itself.

Here the low signal source impedance can help. AC-wise, it is in parallel to
the bias resistor, but the coupling C is in series to it. That works nicely in the
middle of the passband of the amplifier.

But at the lower -3dB point, Xc is equal to Rbias, so there is not much shorting
left. From a noise viewpoint, Xc must be small against the signal source impedance
and not only small against Rbias.

That is very unpleasant. You no longer need 100 uF but 5000 uF.
That cannot be made with foil caps. I already had a dozen or two of WIMA
10u/50V in parallel. The largest foil caps I could get and already costly.

So i used the wet slug tantals. 5000u cost about €70 if you are very lucky.
Could be more than twice that. The cheapest I have found was AVX from Digikey.
Vishay was MUCH more than twice.

The wet tantalums still have a guaranteed ESR of 400 mOhm. Quite a lot, IMHO.
I choose the wet slug tantal after reading Jim Williams' article on measuring the ref noise
of the LT6655.

BUT, even in the 70 pV amplifier in the Art-of-Electronics-III-style i could not find
an obvious drawback using good quality aluminium electrolytics by Panasonic. YMMV.
That design was more a proof-of-concept for a chopper amplifier. At the 500 KHz
chopping frequency, the capacitors are harmless.

In FET amplifiers you may see gate resistors in the 100 Meg range and one is
tempted to see that as a drawback, noise voltage-wise. But that's not true.
The 100 Meg are easily ac-shorted.


Pic: chopper amp, might feature 100 pV rt Hz at << 100 mHz, but I cannot yet verify
that because of the 1/f noise of my 89411A FFT analyzer. The CPLD creates the
chop clock and the delayed unchop-clock after the 500 KHz filter.

Echo88:
Interesting details, thanks. Didnt know you design discrete chopper amps yourself. Did you consider using two LNAs and cross-correlation to further reduce your LNA-noise? Like the battery noise measurement at NIST for example.
I have a cookie-box with two 0.1-10Hz-LNAs and would like to employ the cross-correlation via Python, but lack the knowledge/documents to do so.
Gerhard_dk4xp:
I have played shortly with the cross correlation in my Agilent 89441A.
The effect was so strong that I had doubts about the results. That
machine has so many things to set up that you need a program
for it (or at least a checklist).

I have just married a LT2500-32 ADC to a Beaglebone Black; that
has probably much better 1/f behavior than the 89441A. I'm just
getting first time series; still a lot of software to do. Eventually I'll do
cross correlation also, maybe over xmas.

It seems that numpy can do cross correlation, but my programs
are in C.

cheers, Gerhard

David Hess:

--- Quote from: Marco on November 14, 2018, 02:37:32 am ---A discrete differential pair front end with say 9 BJTs on each leg?
--- End quote ---

It does not even require parallel devices to do this.  Large area audio driver transistors like the BC327/BC337 can operate below 1nV/SqrtHz assuming that the relatively high bias current is acceptable.  A precision amplifier is used in parallel to remove DC offset.  If low noise at low frequencies is necessary, then a chopper stabilized amplifier will be needed to remove 1/f noise.

The BC327/BC337 is not used for its power or current handling capability.  The large area is a way to lower the base spreading resistance which limits low noise operation.  Using lots of smaller transistors in parallel is more complicated because emitter ballasting cannot be used without raising the noise level.  This points to the problem JFETs have achieving low noise with their lower transconductance from their higher source resistance.

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