Capacitor-feedback for low noise

[Joerg]

Hello Zigoteau,

John and Joerg, no, I don't think I can use a JFET to discharge,
because I will then be stuck with the reverse leakage current of the
gate-channel junction, which is typically a few nA. ...

I thought about MOSFETs, not JFETs. You can get nice arrays such as the
SD5400. That allows some neat tricks to compensate for charge injection.
This array is going up in price and often not in stock so I have the
uneasy feeling it may be headed for lala-land. Sigh... But there are
other arrays from Japanese mfgs.

Regards, Joerg

 

[Joerg]

Hello Zigoteau,

Capacitors have no noise because there is no dissipation. Jeroen is
right that inductors also have no noise.

Be careful. Depending on what type of core they have that might not
always be the case.

Regards, Joerg

 

[John Woodgate]

I read in sci.electronics.design that Joerg

But there are other arrays from Japanese mfgs.

That should allay your anxiety.(;-)

 

[Joerg]

Hello John,

There's also the old trick of collapsing the opamp power supplies
briefly, letting the esd diodes discharge the cap.

That reminds me of a strip chart recorder from the 80's. It's reset
button literally shorted +5V (!) to GND. Bzzzt.... worked every single
time until the switch contacts gave out.

Regards, Joerg

 

[Joerg]

Hello John,

That should allay your anxiety.(;-)

It does but it's tough to get small qties for a prototype build. Same
for Euro parts, just got a mighty black eye there. Took me hours on the
phone to get the parts.

Regards, Joerg

 

[Ken Smith]

Capacitors have no noise because there is no dissipation. Jeroen is
right that inductors also have no noise.

Insert the word "ideal" in front of both of those. Many capacitors and
inductors make nice little microphones.

All the high K ceramic capacitors have this problem. Stick with NPO/COG
in low noise circuits, or don't try to go surface mount.

Inductors make noise due to stress on the cores and also pick up external
fields.

The resistance of an inductor is noisy like all resistances.

 

[Ken Smith]

No timeouts. All we need now is a very low-noise switchable current
source!

Someone, I think it was IBM, made a device some years back that could gate
through a single electron at a time. You need one of them and a very
jitter free clock.

 

[John Larkin]

Someone, I think it was IBM, made a device some years back that could gate
through a single electron at a time. You need one of them and a very
jitter free clock.

That would be nice.

Or you could charge pump: zot in one small unit of charge just after
you digitize each sample, gated by the software when it's in the mood.
It's then pretty simple to compute the amount of charge very precisely
(just average a lot of shots) and the charge injection will be very
low-noise (if you do it right.) One of those old Jim Williams/Bob
Pease v/f converter tricks should work.

Hell, that might even be practical.

John

 

[Jeroen Belleman]

[Transformer feedback] sounds very interesting. Do you have a reference?

Sure, here is one:

J. Lepaisant, M. Lam Chok Sing, D. Bloyet, "Low-noise preamplifier
with input and feedback transformers for low source resistance sensors"
Rev. Sci Instrum. 63(3), March 1992, p2089

This is basically a differential amplifier with series transformer
feedback in the input. Its bandwidth is 5Hz-100kHz. It has 65pV/rtHz
and 1.5pA/rtHz noise, so you'll want to change the transformers to
trade current noise against voltage noise.

In a project of mine, I used a different scheme, a folded cascode
with transformer feedback to the sources of the input FETs. Here, the
bandwidth is 10kHz-75MHz and the noise voltage is 650pV/rtHz. I did not
measure the noise current, but it should be in the 30fA/rtHz ballpark.
You can see short description at:
http://jeroen.home.cern.ch/jeroen/tfpu/LNA.shtml

There is quite a bit of flexibility trading off voltage against current
noise, and there's the bandwidth to play with too.

Jeroen Belleman

 

[Zigoteau]

Hi Jeroen,

Sure, here is one:

J. Lepaisant, M. Lam Chok Sing, D. Bloyet, "Low-noise preamplifier
with input and feedback transformers for low source resistance sensors"
Rev. Sci Instrum. 63(3), March 1992, p2089

This is basically a differential amplifier with series transformer
feedback in the input. Its bandwidth is 5Hz-100kHz. It has 65pV/rtHz
and 1.5pA/rtHz noise, so you'll want to change the transformers to
trade current noise against voltage noise.

In a project of mine, I used a different scheme, a folded cascode
with transformer feedback to the sources of the input FETs. Here, the
bandwidth is 10kHz-75MHz and the noise voltage is 650pV/rtHz. I did not
measure the noise current, but it should be in the 30fA/rtHz ballpark.
You can see short description at:
http://jeroen.home.cern.ch/jeroen/tfpu/LNA.shtml

There is quite a bit of flexibility trading off voltage against current

Many thanks for the references.

I've been doing a bit of thinking - quite painful when it comes to
noise analysis of multinode circuits. While your examples are
invaluable, I will have to understand the theory to optimize the design
for my particular application. Am I on the right track?

My requirement is for a transimpedance amplifier, whereas your example
has a high-impedance input. I think that my first stage must be
configured as a current amplifier, with conversion to voltage occuring
first in the second transimpedance stage. Compared to your circuit, my
bandwidth requirement is much more modest, and I plan to use
low-current-noise op amps for both stages.

Using op amps, which are inherently voltage in-voltage out, rather than
discrete components, I have to deviate significantly from your
configuration, but I think I have come up with a viable one, where the
transformer primary is between the input and output of the stage 1 op
amp, and its secondary, with much fewer turns, feeds directly the input
of the stage 2 op amp with feedback resistor Rf2. Seen from the stage 1
op amp, the feedback circuit is essentially the magnetizing inductance
M of the transformer primary, in parallel with Rf2*N/A2, where A2 is
the DC voltage gain A2 of the second op amp and N is the turns ratio of
the transformer.

The low-frequency roll-off happens at the break frequency of
Rf2*N/A2/2/pi/M, and there are no stability problems at that frequency.
The stability problems occur near the frequency 1/2/pi/sqrt(M*Ci1),
where Ci1 is the input capacitance of op amp 1. This gives a resonance
in the feedback fraction, which must be damped by a resistance
somewhere. I think the only way you can provide this resistance without
contributing to the input node noise is via the apparent resistance
Rf2*N/A2.

I think that the noise generated by this apparent resistance Rf2*N/A2
is much less than the Johnson noise from a real resistor of this value.
It will essentially be the total noise current at the input-node of op
amp 2, divided by N. Do you agree?

The resonant frequency 1/2/pi/sqrt(M*Ci1) provides a strict upper limit
to the bandwidth of the overall amplifier, and it might be hard to get
within an order of magnitude of it. This puts an upper limit on the
magnetizing inductance M of the transformer.

Thanks again for your excellent suggestion.

Best regards,

Ziggy.

 

[Winfield Hill]

Jeroen Belleman wrote...

[Transformer feedback] sounds very interesting. Do you have a reference?

Sure, here is one:

J. Lepaisant, M. Lam Chok Sing, D. Bloyet, "Low-noise preamplifier
with input and feedback transformers for low source resistance sensors"
Rev. Sci Instrum. 63(3), March 1992, p2089

This is basically a differential amplifier with series transformer
feedback in the input. Its bandwidth is 5Hz-100kHz. It has 65pV/rtHz
and 1.5pA/rtHz noise, so you'll want to change the transformers to
trade current noise against voltage noise.

In a project of mine, I used a different scheme, a folded cascode
with transformer feedback to the sources of the input FETs. Here, the
bandwidth is 10kHz-75MHz and the noise voltage is 650pV/rtHz. I did not
measure the noise current, but it should be in the 30fA/rtHz ballpark.
You can see short description at:
http://jeroen.home.cern.ch/jeroen/tfpu/LNA.shtml

There is quite a bit of flexibility trading off voltage against
current noise, and there's the bandwidth to play with too.

0.65nV of voltage noise corresponds to a noise resistance of 26 ohms.
So a x100 JFET amplifier with a 10-ohm lower feedback resistor would
retain low noise operation. However, for a gain of only 10x, like
your preamp, the required 90-ohm upper feedback resistor would be
awkwardly small, and would limit the maximum output swing.

 

[Winfield Hill]

Jeroen Belleman wrote...

In a project of mine, I used a different scheme, a folded cascode
with transformer feedback to the sources of the input FETs. Here,
the bandwidth is 10kHz-75MHz and the noise voltage is 650pV/rtHz.
I did not measure the noise current, but it should be in the
30fA/rtHz ballpark. You can see short description at:
http://jeroen.home.cern.ch/jeroen/tfpu/LNA.shtml

BTW, which Vitrovac core did you use in that amplifier?

 

[Jeroen Belleman]

Jeroen Belleman wrote...



BTW, which Vitrovac core did you use in that amplifier?

I used the T60009-E4006-W650. That's a 6.5x3.5x2.1 mm (od x id x h)
tape-wound toroid core with Al=13uH/turn^2.

Jeroen Belleman

 

[Jeroen Belleman]

Hi Jeroen,
Many thanks for the references.

I've been doing a bit of thinking - [snipped line of thought]

Hola now! I have no time now to work out if your line of reasoning
is correct, alas. Can we pick this up again next Wednesday?

Cheers,
Jeroen Belleman

 

[Winfield Hill]

Jeroen Belleman wrote...

I used the T60009-E4006-W650. That's a 6.5x3.5x2.1 mm (od x id x h)
tape-wound toroid core with Al=13uH/turn^2.

Thanks, Jeroen.

What were your selection criteria, size and permeability?
Is that an obsolete part now? I didn't see any that small.

 

[Jeroen Belleman]

Jeroen Belleman wrote...



Thanks, Jeroen.

What were your selection criteria, size and permeability?
Is that an obsolete part now? I didn't see any that small.

Size and permeability, exactly. Despite the very high permeability,
you can make transformers that work up to a few hundred MHz with
these cores.

I don't think the cores are obsolete, but it's true I've been
unable to find them on their web site. I last ordered a thousand
in June, from Sekels GmbH, Dieselstrasse 6, 61239 Ober-Morlen,
Germany. They dropped on my desk just today. I paid 2.72 Euros/pc.
They used to be cheaper.

Regards,
Jeroen Belleman

 

[Winfield Hill]

Jeroen Belleman wrote...

Size and permeability, exactly. Despite the very high permeability,
you can make transformers that work up to a few hundred MHz with
these cores.

I don't think the cores are obsolete, but it's true I've been
unable to find them on their web site. I last ordered a thousand
in June, from Sekels GmbH, Dieselstrasse 6, 61239 Ober-Morlen,
Germany. They dropped on my desk just today. I paid 2.72 Euros/pc.
They used to be cheaper.

A thousand? Sheesh, what are you going to do with a thousand
cores? Give some away? :>)

 

[Jeroen Belleman]

Jeroen Belleman wrote...


A thousand? Sheesh, what are you going to do with a thousand
cores? Give some away? :>)

Well, they work so well I tend to end up using them in many
applications where I used ferrite before.

Perhaps you remember the thread some years ago, where I
reported on my attempts to build a four decade bandwidth hybrid
transformer? Using these cores, four decades was a snap. In fact,
I got 5kHz-500MHz (-3dB, 50 Ohm). I had to use two network
analysers to cover the bandwidth . Another neat device was
the 7 decade bandwidth 1:4 transformer.

I'm in the process of building 150 amplifiers that contain two of
these cores each. I also use them to make more hybrid transformers
and there's another 150 going in there. I could go on.

In fact, I believed that the thousand I bought in 2002 were a
lifetime supply, but we've been going through them faster than
I thought.

Regards,
Jeroen Belleman

 

[John Woodgate]

I read in sci.electronics.design that Jeroen Belleman

I don't think the cores are obsolete, but it's true I've been
unable to find them on their web site.

I couldn't even find the web site!

 

[Winfield Hill]

John Woodgate wrote...

Jeroen Belleman wrote ...


I couldn't even find the web site!

Sheesh, John, I found the website and I must be 5x further
away from it than you!