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Capacitor-feedback for low noise

J

John Woodgate

I read in sci.electronics.design that Winfield Hill
John Woodgate wrote...

Sheesh, John, I found the website and I must be 5x further
away from it than you!
Well, maybe I made a mistake in the URL, or something. I've lost the URL
now.
 
J

Jeroen Belleman

John said:
I read in sci.electronics.design that Jeroen Belleman



I couldn't even find the web site!

It's http://www.vacuumschmelze.de , but that site is a dog. I can't
find the datasheet, even though I know it's in there somewhere.
I've got a paper copy, fortunately.

Jeroen Belleman
 
W

Winfield Hill

Jeroen Belleman wrote...
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.

Well, could you spare me a dozen?
 
J

John Woodgate

I read in sci.electronics.design that Jeroen Belleman
It's http://www.vacuumschmelze.de , but that site is a dog. I can't
find the datasheet, even though I know it's in there somewhere.
I've got a paper copy, fortunately.
Thanks. I may have typed 'vacumm'. Or 'vaccum' (a neutered cow!)
 
W

Winfield Hill

Jeroen Belleman wrote...
Sure, sure. I took the hint.

Great, I'll trade you some small high-permeability
unobtainium cores you might find useful someday.
 
J

John Woodgate

I read in sci.electronics.design that Winfield Hill
Jeroen Belleman wrote...

Great, I'll trade you some small high-permeability
unobtainium cores you might find useful someday.
You might mention that the mu-r of unobtanium is negative, so coils
wound on cores made of it make excellent capacitors, values from 1 kF to
1 GF.
 
Z

Zigoteau

Hi, Jeroen,


I've looked at your webpage and at the paper whose reference you gave -
they're both for voltage amplifiers.
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?

I would appreciate your comments on the idea that the best
configuration for a transformer-feedback transimpedance amplifier using
low-noise op amps would have two stages, the first being a current
amplifier with the transformer primary connected between the output and
inverting input.

Cheers,

Zigoteau.
 
W

Winfield Hill

Zigoteau wrote...
I've looked at your webpage and at the paper whose reference you
gave - they're both for voltage amplifiers.

Yes, but you want a voltage-amplifier input stage, see below.
I would appreciate your comments on the idea that the best
configuration for a transformer-feedback transimpedance amplifier
using low-noise op amps would have two stages, the first being a
current amplifier with the transformer primary connected between
the output and inverting input.

OK, let's speak generally. Transimpedance amplifiers are made with
high-gain high-impedance opamps, etc., inside them. If you can find
an ideal JFET opamp to make your transimpedance amplifier, fine. As
I've discussed many times here on s.e.d., there's a straightforward
way to analyze the issues involved in selecting parts. Sometimes
you're forced to use a kind of "composite opamp," which has several
stages in series. The first stage is optimized for low input noise,
high input impedance, and provides some voltage gain (note, it's a
voltage amplifier), the later stages provide wideband voltage gain.

For the input stage, you're trading off the primary factors of the
JFET's gate-leakage current, voltage noise and capacitance, as you
select the JFET. Sometimes you're pushed toward an input stage
with less voltage noise than is available in commercial JFET opamps
(low voltage noise and low capacitance are two very critical aspects
for current amplifiers at higher frequencies, where they combine to
create a current noise, i_n = 2pi f e_n C).

Now let's speak specifically. Jeroen Belleman has an input stage
using one of the low-voltage-noise JFETs I was talking about. The
issue we'll address here is voltage noise. When you add a voltage
feedback path to the source of the JFET, the lower divider resistor
contributes Johnson noise. Keeping the contribution much lower than
Jeroen's JFETs would involved resistor values under about 20 ohms.
So if you wanted a gain of 10, your feedback resistor would be 180
ohms, an awkwardly-low value, requiring high current at high output
levels. Jeroen elegantly solved this vexing problem with a feedback
transformer, http://jeroen.home.cern.ch/jeroen/tfpu/LNA.shtml

When using such JFETs I have solved this issue with a higher gain
of 100, which allows using reasonable-value feedback resistors,
like 20-ohms and 2k. It also allows DC gain. But my application
did not require the high 75MHz bandwidth that Jeroen's did. If he
had used a gain of 100, he'd have suffered a reduced bandwidth.

Note, you could create an input stage without any internal voltage
feedback to the JFET, getting as much gain as it gives, and thereby
avoid a feedback-resistor Johnson-noise problem. But unpredictable
gain might create other problems elsewhere in a closed-loop design.
 
K

Ken Smith

Winfield Hill said:
Now let's speak specifically. Jeroen Belleman has an input stage
using one of the low-voltage-noise JFETs I was talking about. The
issue we'll address here is voltage noise. When you add a voltage
feedback path to the source of the JFET, the lower divider resistor
contributes Johnson noise. Keeping the contribution much lower than
Jeroen's JFETs would involved resistor values under about 20 ohms.
So if you wanted a gain of 10, your feedback resistor would be 180
ohms, an awkwardly-low value, requiring high current at high output
levels.

You can also take the feedback from the output of a second stage. An over
simple example:

+V
!
/ R3
\ +V
/ !
! !/c
+----!
! !\e
! !
! +------- to next stage
!-- /
-------->! \ R1
!-- /
! !
+------
!
/
\ R2
/
!
VX


Note +V must be very quiet.

In real life, the circuit ends up a lot more complicated than this but
this drawing shows the concept. The overall feedback of the
transimpedance amplifier trys to make the output of this section have zero
swing so the output swing of this section only really has the high
frequency components on it.

If your input is such that the circuit always stays in the linear mode,
the VX of the JFET stage can be servoed to bring the gate voltage to run
at exactly zero. This lets you get a DC accurate and low noise input at
the same time.


[....]
Note, you could create an input stage without any internal voltage
feedback to the JFET, getting as much gain as it gives, and thereby
avoid a feedback-resistor Johnson-noise problem. But unpredictable
gain might create other problems elsewhere in a closed-loop design.

Make that "might" into "is almost certain to at some time". The circuit
bandwidth ends up different from unit to unit and depends on temperature.
 
W

Winfield Hill

Ken Smith wrote...
Winfield Hill wrote:
[...]
Now let's speak specifically. Jeroen Belleman has an input stage
using one of the low-voltage-noise JFETs I was talking about. The
issue we'll address here is voltage noise. When you add a voltage
feedback path to the source of the JFET, the lower divider resistor
contributes Johnson noise. Keeping the contribution much lower than
Jeroen's JFETs would involved resistor values under about 20 ohms.
So if you wanted a gain of 10, your feedback resistor would be 180
ohms, an awkwardly-low value, requiring high current at high output
levels.

You can also take the feedback from the output of a second stage.
An over simple example:

+V
!
/ R3
\ +V
/ !
! !/e
+----! Q2
! !\c
! !
Q1 ! +------- to next stage
!-- /
-------->! \ R1
!-- /
! !
+------
!
/
\ R2
/
!
VX

Note +V must be very quiet.

I must not have been very clear, this is the JFET configuration I was
talking about, thanks for the drawing! Except of course, you meant Q2
to be a PNP, as I marked above. To see the problem with this circuit,
consider the value for R2, and the 0.65nV spot noise of Jeroen's JFETs.
R2's noise adds (sqrt sum of squares) to the JFET noise. If we choose
say 0.5nV as the noise budget for R2, to avoid too much noise increase,
then R2 has to be no more than 15 ohms! Do you see the problem now?
To get a wide 75MHz bandwidth Jeroen specified a gain of 10x, but this
would mean R1 would only be 135 ohms! You see the problem now, right?
Well, Jeroen's problem anyway; We don't need a 75MHz bandwidth and can
use a higher preamp gain. For 100x, R1 = 1.5k, which is fine.
In real life, the circuit ends up a lot more complicated than this
but this drawing shows the concept. The overall feedback of the
transimpedance amplifier trys to make the output of this section
have zero swing so the output swing of this section only really
has the high frequency components on it.

Not necessarily true at the highest frequencies, where the post-JFET
stage gain is low, yet full output may be desired. The gains can be
setup differently, with less JFET-stage gain and more gain in the
output stage at high frequencies, but then you end up with increased
high-frequency noise, e.g., as in the LT1028 opamp above 200kHz.
If your input is such that the circuit always stays in the linear
mode, the VX of the JFET stage can be servoed to bring the gate
voltage to run at exactly zero. This lets you get a DC accurate
and low noise input at the same time.

Yes, exactly, but Jeroen's ac-coupled transformer version can't.
[....]
Note, you could create an input stage without any internal voltage
feedback to the JFET, getting as much gain as it gives, and thereby
avoid a feedback-resistor Johnson-noise problem. But unpredictable
gain might create other problems elsewhere in a closed-loop design.

Make that "might" into "is almost certain to at some time". The circuit
bandwidth ends up different from unit to unit and depends on temperature.

Right, that's why I like to run the JFET preamp stage in its own
feedback mode, even though it's inside a larger feedback loop.
 
K

Ken Smith

Ken Smith wrote...

I must not have been very clear, this is the JFET configuration I was
talking about, thanks for the drawing! Except of course, you meant Q2
to be a PNP, as I marked above.

Yes the PNP is the correct transistor in this case. The circuit is also
an over simplified version of the final version.

To see the problem with this circuit,
consider the value for R2, and the 0.65nV spot noise of Jeroen's JFETs.
R2's noise adds (sqrt sum of squares) to the JFET noise. If we choose
say 0.5nV as the noise budget for R2, to avoid too much noise increase,
then R2 has to be no more than 15 ohms! Do you see the problem now?

No, not exactly. R2 does have to be 15 Ohms. That is correct. See
below:
To get a wide 75MHz bandwidth Jeroen specified a gain of 10x, but this
would mean R1 would only be 135 ohms! You see the problem now, right?


No, this where I don't see that there must be a problem.

First off, R1 = 135 Ohms does mean that Q2 is running at a large current
if we assume something like (oh, lets say) 13.5V across it, we will have
100mA flowing in Q2.

Yes that is a lot of power but that is why Gawd invented the heat sink. I
don't remember the power budget being limited. Was it? 1.35W is not that
hard to get rid of.

If we assume the JFET is running at an ID of about 10mA it only implies a
HFE in Q2 of 10 or more. Q2 doesn't need to be a Darlington to do that.


[ .. low swing on the output ..]
Not necessarily true at the highest frequencies, where the post-JFET
stage gain is low, yet full output may be desired.

Does the OPs case require full swing at the high end? I don't remember.
The gains can be
setup differently, with less JFET-stage gain and more gain in the
output stage at high frequencies, but then you end up with increased
high-frequency noise, e.g., as in the LT1028 opamp above 200kHz.

Yes, a prpblem that bit e before LT added information about that to the
data sheet.
Yes, exactly, but Jeroen's ac-coupled transformer version can't.

It may be posible to arrange things so that there is a DC path and an AC
path provided the thing doesn't ever go non-linear. Harris used to make
some fast op-amps like this. They had two amplifiers inside. The specs
looked very nice but never mensioned the overload recovery which was
dreadful. We tried them in a charge sensitive amplifier the results were
not good :<


[.. JFETs make your circuit oscillate ...]
Right, that's why I like to run the JFET preamp stage in its own
feedback mode, even though it's inside a larger feedback loop.

I learned the hard way with a pair of 2SK170s leading into an op-amp.
 
J

Jeroen Belleman

Zigoteau said:
Hi, Jeroen,

[...]

I would appreciate your comments on the idea that the best
configuration for a transformer-feedback transimpedance amplifier using
low-noise op amps would have two stages, the first being a current
amplifier with the transformer primary connected between the output and
inverting input.

Sorry to have been a bit long responding. I can't be reading usenet
all the time...

So, if I correctly interpret your (more complete) description of
August 24, the circuit would look like this:

+--R-----+
| |
1 | |\ |
GND----UUUU--+--| > --+--- out
==== |/
+-UUUU---+ A2
| N |
| |
| |\ |
Iin -+---| > -+
|/
A1
(With A1 and A2 negative and largish).

So ignoring the bandwidth limitations due to the shunt inductance of
the transformer and its under-unity coupling factor, and also ignoring the
limitations of the amplifiers, the transconductance of the overall circuit
would be (N A2 R)/(1-A2). Its input impedance (R N^2)/((1-A1)(1-A2)), and
its input-referred noise current sqrt((4 k T)/(N^2 R)), ignoring the
contributions of the active circuitry.

So the transformer indeed confers a noise advantage: The transconductance
scales with N and the noise current with 1/N.

If this had been the classical single-stage transconductance amplifier, with
N * R as the feedback resistor to get the same transconductance as in the
above two-stage circuit, the noise current would have scaled with 1/sqrt(N).

So, yes, I think this shows promise. Now to turn this into a working circuit...

Best regards,
Jeroen Belleman
 
Z

Zigoteau

Hi, Jeroen,

Sorry to have been a bit long responding. I can't be reading usenet
all the time...


Thanks for getting back to me with your detailed comments. Worth
waiting for.

So, if I correctly interpret your (more complete) description of
August 24, the circuit would look like this:

+--R-----+
| |
1 | |\ |
GND----UUUU--+--| > --+--- out
==== |/
+-UUUU---+ A2
| N |
| |
| |\ |
Iin -+---| > -+
|/
A1
(With A1 and A2 negative and largish).


Yes. In fact my initial ideas were slightly confused, and I have
realized that this is not classical negative feedback. The more
conventional configuration would in fact be:

N
+--------UUUU---GND +-----R--+
| |\ ==== | |\ |
Iin -+--|-> --UUUU---------+--|-> --+--- out
|/ 1 |/
A1 A2


I have part-analyzed both circuits, but have not yet decided which is
better.


<snip>
So, yes, I think this shows promise. Now to turn this into a working circuit...

I liked the idea of your high-mu toroidal cores. My experience with
ferrite cores is that it is extremely difficult to get more than three
orders of magnitude bandwidth.

I have been in touch with Vacuumschmelze, who say that the
T60009-E4006-W650 is still on their books. The salesman apologized for
the search facility on their website, which does not yet find catalog
numbers. There is a minimum order, but they will send me a couple of
samples, one slightly bigger than the T60009-E4006-W650. By the way,
they say to get in touch and they will buy you a beer to thank you for
the recommendation.

I will have to think about trade-offs and such, but initial
calculations suggest that, as a result of the high impedance level, the
primary may have to consist of 10000 turns or more of very fine wire. I
do not fancy the job of winding by hand. To test the concept, I may
have to accept a lower 3-dB frequency somewhat higher than the final
application requires. The alternative, which I have not yet thought
through, is to lower the impedance level at the A2 summing junction, by
making up A2 from two op amps, playing tricks to achieve stability,
perhaps something like.


+--------R-----------+
| +-----Z2--+
| |\ | |\ |
Iin-+--|+\ +---|-\ |
| >-Z1-+ | > -+--out
GND--|-/ +-|+/
|/ | |/
A2 GND A3


^^^^^^^^^^^^^^^
transresistance, transresistance...


Errare humanum est, ignoscere divinum . . .

Cheers,

Zigoteau.
 
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