Low noise amplifier.

[GK]

Are you going to try to run it on batteries?

Probably not as I can generally do without it. I did contemplate a small box with a small 12V gell cell and a well shielded and filtered SMPS to generate the +/-15V. At this power level the switching frequency could be very high to keep radiated noise out of the amplifiers pass band. But this doesn't currently fit into my priority list.

One thing to note is that the BF862 is now unfortunately obsolete and stocks from the major suppliers at least seem to have dried up. The are still a number of good candidates active though. My pick for a substitute would be 2SK3557. The gm appears to be a little bit less, so the overall noise will be a little higher, but the low frequency performance is much better with a 1/f corner of 100Hz or so. The BF862's from NXP typically cornered in the 1-2kHz range.   

For the potential experimenter I designed the amplifier to be flexible in the total drain current setting. I'm running my BF862s conservatively at 5mA each, but the design can easily dissipate three times this with smaller value resistors used in the current source. The heat is dissipated over a generously large area by many devices.
 

[T3sl4co1l]

There's a new JFET from On Semi that's apparently slightly better than BF862.  I forget the number offhand, but give it a search.

Tim

[CopperCone]

Having to position it in themiddle of the lab sounds annoying. Have you considered using thicker steel or iron for the enclosure? Or adding a seperate mu metal shield? I talked to someone about the whole hydrogen tempering of mu metal and he said its nice but not necessary. Might help

What gauge did you use btw? I am interested.

[CopperCone]

Also i think with a switching supply you can still get modulation of the switching frequency showing up in your pass band. And you have a large area so you might get modulated rfi rectification causing problems with a switcher.

You probobly want to power your lab power supply with a low capacitance isolation trasformer, verify home grounding and use shielded cables, including the cable between the lna and psu and psu and isolation transformer

[Cerebus]

Having to position it in themiddle of the lab sounds annoying. Have you considered using thicker steel or iron for the enclosure? Or adding a seperate mu metal shield? I talked to someone about the whole hydrogen tempering of mu metal and he said its nice but not necessary. Might help

What gauge did you use btw? I am interested.

Skin depth at 50Hz is about 1 cm, that's going to be one heavy enclosure...

[T3sl4co1l]

Having to position it in themiddle of the lab sounds annoying. Have you considered using thicker steel or iron for the enclosure? Or adding a seperate mu metal shield? I talked to someone about the whole hydrogen tempering of mu metal and he said its nice but not necessary. Might help

What gauge did you use btw? I am interested.

Skin depth at 50Hz is about 1 cm, that's going to be one heavy enclosure...

What is?  Mild steel is around 1mm.  Did you leave off permeability?

Induction in cables is a far more likely culprit; at these levels, even electrostatic coupling into poorly screened cables is significant.  (Braid + foil shielded cable is best.  Unfortunately, no common coax is really suited to deal with magnetic induction below 10s of kHz, where your best option is avoidance.)

Tim

[GK]

I considered making a double screened enclosure (a steel box within a steel box), but at the end of the day overly heroic efforts to screen the amplifier itself are pointless as you still have to deal with the interconnecting cable and the DUT itself. The amplifier and the DUT just have to be positioned away from sources of mains hum.

The sheet steel is 0.75mm.

[BrianHG]

What can I use such an insanely low noise amplifier for?

[GK]

Measuring stuffs.

[TiN]

are pointless as you still have to deal with the interconnecting cable and the DUT itself.

Typically for very low noise requirements the DUT in own shielded can will be placed inside the larger shielded box, together with the LNA.
In serious facilities this extends to making shielded rooms with metal walls, essentially putting test equipment, operator and related stuff into same shielded Faraday cage. 

[GK]

I finally have, at least, an initial webpage uploaded for this project: 

http://www.glensstuff.com/ulnma/ulnma.htm

I won't get around to finishing the page off with a brief write up and performance measurements this weekend, but for now the PCB Gerber files, schematic and PCB top overlay diagrams are all there.

[eu7bz]

Hello!
What is the current consumption of +15 volts and -15 volts?
Thank!

[Mrt12]

Hi Guys,
can someone explain a bit how this amplifier circuit with those paralleled JFETs work?
I would like to make a very low noise amplifier as well, for phase noise measurements.
As some of you probably know, Wenzel has published a low noise amplifier as well, exactly for this purpose (see: http://www.techlib.com/files/lowamp.pdf) and the circuit shown here in the initial post seems to be somewhat similar.
But I don't understand how it works at the moment, any hints?

[Gerhard_dk4xp]

1. All FETs get the same input signal.
2. All FETs generate noise that has nothing in common to the noise of their neighbours.
    I.E. These noise sources are not correlated.
3. If you sum up the outputs of all FETs, the amplified input signal adds linearly.
4. A noise peak from one FET may happen when there is a noise valley from a
    different FET, so the noise does not add linearly but less. Partly, it cancels.
5. After some math ado: the noise adds up with the square root of the N FETs.
6. So, from 4 FETs in parallel you get half the noise voltage, relative to the output signal.

Some of the early publications on this are the PMI / AD MAT-02, MAT-03, MAT-04 data sheets
and the data sheet of the LT1028.

I also got into that via Wenzel's design. The pic is V2, OMG, there is a date code .
V1 was still more Wenzel-ish, but I wanted MORE!
The board has the ring mixer in the top left corner, but could be used as a general
purpose preamp. Everything was switched cold with relays: gain, input, frequency corners...

For phase noise measurements, there is no point to try to improve on Wenzel.
There comes enough noise from the ohmic resistance in the diode ring. An AD797 is enough.

[Gerhard_dk4xp]

I have buit in the mean time a lot of these massively paralleled JFET amplifiers.
Virtually all of them that I have checked shared one misfeature: Somewhere,
they show negative input impedance. If you don't have the right cable /
inductive source at the input, that may have no consequences.
But the vector network analyzer on the table is cruel and shows the shortcomings.
Bootstrapping seems to help to some extend, and also making the feedback loop
ultra fast.

You may think that the sources are near ground potential. After all, there is only
a 1 Ohm or so resistor from sources to ground. But that's not true. The feedback
makes the sources follow the gate AC.

In the view of the transistor, the drain is low impedance (looks into a cascode emitter
or mild load resistance) while the source follows the gate closely. Look, Ma, I'm a
follower!  And it oscillates like followers like to do.

Note that you cannot see that in the bode plot of the feedback loop. It is only the
FET that oscillates. The loop only creates the preconditions. And if it is slow,
the source voltage looks capactively loaded.

The usual, and the only easy way to remove the negative input impedance is a
gate stopper resistor. But you definitely do not want that in a low noise amplifier.

First pic is one of my massively par amplifiers. The added BUF634 is meant
to minimize the feedback delay, but it is still much worse than a VCVS in LTspice. 
The amplifier uses the new FETs from ON semi, individually cascoded with a
large pinch off JFET.

I'd say the new ON semi FETs are on par with the BF862, not better. Maybe a little
bit lower 1/f corner, but no reason to stop using BF862 as long as you have them.

The other pic uses 2 Interfet IF3602 FET pairs. The TO-5 coolers are just there
to provide thermal mass. 30 Hz 1/f, 300 pV/rt Hz per FET. Available from Mouser
for €55 each or so. The data sheet values seem quite optimistic; I need a lot more
current to get the 300 pV per transistor. The 4 of them could be measured at
180 pV/rt Hz. The FZT851 is the cascode; it is bootstrapped from the source.

While the normal feedback removes the gate-source capacitance, the bootstrapped
cascode also removes the gate-drain capacitance. We talk of several hundred pF
for each of these huge FETs. Minimizing capacitance has a very good influence on stability.

Everything is nice & well behaved until 1 MHz, where it all collapses. When I
damp everything down to, say, 250 KHz, it is probably unconditionally stable.
But I insist in 1 MHz bandwidth. 

I have removed the bias loop on this one. The input capacitor in these amplifiers
must be huge to effectively short the noise of the bias resistor throught the DUT.
The DUT must be low-resistance, or such a low noise amplifier would be futile
from start. The bias loop must be much slower than the input RC or it would interact.

A TL431 minus a diode drop was about right for temperature compensation.
The blue 10-turn-pot sets the OP. In the upper right of the board is a window
comparator that checks the operating point. When we are far off, as when we
just connected the preamp to a large DC voltage, the bias resistor is reduced
via a MAX393 analog switch so that we don't have to wait forever.

[Mrt12]

1. All FETs get the same input signal.
2. All FETs generate noise that has nothing in common to the noise of their neighbours.
    I.E. These noise sources are not correlated.
3. If you sum up the outputs of all FETs, the amplified input signal adds linearly.
4. A noise peak from one FET may happen when there is a noise valley from a
    different FET, so the noise does not add linearly but less. Partly, it cancels.
5. After some math ado: the noise adds up with the square root of the N FETs.
6. So, from 4 FETs in parallel you get half the noise voltage, relative to the output signal.

Some of the early publications on this are the PMI / AD MAT-02, MAT-03, MAT-04 data sheets
and the data sheet of the LT1028.

I also got into that via Wenzel's design. The pic is V2, OMG, there is a date code .
V1 was still more Wenzel-ish, but I wanted MORE!
The board has the ring mixer in the top left corner, but could be used as a general
purpose preamp. Everything was switched cold with relays: gain, input, frequency corners...

For phase noise measurements, there is no point to try to improve on Wenzel.
There comes enough noise from the ohmic resistance in the diode ring. An AD797 is enough.

Hi Gerhard,
I already know you are one of the low noise experts here, I have already read your posts ;-)
Thanks for the hints with the data sheets. I will check these out.

Maybe my question was not clear: for me it is obvious, that the noise of the transistors is uncorrelated and thus, when paralleling them, it is somewhat averaged out because it adds geometrically. This point was clear to me. But it is still not clear how the circuit consisting of the JFET and OpAmp works. How is the gain adjusted, etc. pp. I do want to make my own design, and not just copy the Wenzel circuit without completely understanding what it does ;-)

[TERRA Operative]

Apologies for the thread bump, but I am interested in attempting to build this amp, except for the lack of availability on the BF862 JFET's.
Can anyone point me in the direction of a suitable replacement?

[TERRA Operative]

To answer my own question, I'm going to try 2SK3557 in place of the BF862 parts.
I have made my own layout from the schematics with almost all SMD parts, sized to fit in a few common enclosures I can get here in Japan. Picture of what I have so far attached below.


I have some more questions though if anyone can answer.

What do the trimpots RV1 and RV2 do? It looks like RV2 is to balance the balanced output, but I'm not sure about RV1, I just want to make sure that I twiddle them properly when I power it up.

Also, the capacitors on the input, output and power jacks, I assume they are probably around 0.1uF, except the disk ceramics that look to be 22nF, with the ground of the input jack connected directly to the chassis?

[Kleinstein]

In the last schematics from this post RV1 sets the exact gain. They are both not really critical.

2SK3557 looks like a reasonable replacement for the BF862. It is higher input capacitance and a lower 1/f noise cross over. So not as good for the higher frequencies but likely better for low frequencies.
The capacitor values are given in the plan. The capacitor in series with the input should be more like 1 µF (it sets the low frequency limit) and should be a low loss type, so likely PP film  type.

The circuit needs reasonable well matched FETs to really used all fets equally.

This type of FET amplifier may need some extra capacitance or RC series connection at the input to ensure stability under all circumstances / source impedance. Something on the order of 50 pF + 100 ohms in series. A simulation could give a better guess. 

[TERRA Operative]

Thanks for the info!

For the capacitors, I don't mean the input 1uF cap, but the capacitors soldered point to point style directly on the input, output and power jacks.
I assume they are not critical in any way, so I assume 0.1uF would work ok?

[Kleinstein]

The capacitors directly at the power jacks should be not critical and 100 nF sound like reasonable.

For the caps at the output - these must be much smaller if a fast response is needed. So more like 100 pF, maybe 1 nF.
One could also consider to wind the input wires through a ferrite ring as a common mode filter.
I don't see a cap at the input - here one may have some 10-100pF that could also help against possible negative input impedance.

[TERRA Operative]

Great, thanks.

One more question about a detail that I noticed in the schematic.
If you look where I highlighted in the attached image below, it seems one diode in package D3 is shorted ground to ground. Does this look correct? It looks like it will affect the signal on one half of the waveform compared to the other?

I also attached a 3D render of my version, sized to fit a Takachi brand enclosure, almost all SMD with no wiring or flyleads required. I think it should work ok. 

[awallin]

If you look where I highlighted in the attached image below, it seems one diode in package D3 is shorted ground to ground. Does this look correct? It looks like it will affect the signal on one half of the waveform compared to the other?

looks like a diode clamp that clips the input voltage to a maximum of either +2 or -2 times a diode forward drop.

do you (or someone else?) have kicad sources for this somewhere? looks like a fun project
previously I made an op-amp version with ADA4898 roughly based on the 'lono' (http://www.hoffmann-hochfrequenz.de/downloads/lono.pdf) ...

[TERRA Operative]

Yeah, a voltage clamp is what I thought, but what's up with that ground between one side of the diode arrays won't that make the clamped voltages lopsided so-to-speak?


I have the diptrace layout I drew up that I can supply once I have it tweaked nice, and here's the webpage of the original design: http://www.glensstuff.com/ulnma/ulnma.htm

[GK]

The clamp isn't symmetrical about ground because the JFET gate potential sits at a few hundred mV negative and needs headroom for the possibility of lower transconductance JFETs or running a lower Id.
The diode of that pair with both A and K tied to ground simply isn't used.

When you first apply power don't be alarmed that the output is railed out and that the lower two LEDs aren't initially glowing. That's normal because the JFETs are saturated and grounding the emitters of the cascodes until the servo loop has stabilized the quiescent DC operating point. It takes 30 or 40 or 60 seconds or so. Sorry I don't have a BOM. Don't go changing any resistive dividers or time constants anywhere.