Long-tailed JFET pair question

Hello,

I'm designing a low-noise, low-frequency current-to-voltage converter (TIA). Since the really nice JFET-input, low-noise OpAmps such as the OPA627 are scarce and expensive I decided to build the input stage with a discrete dual JFET followed with a bipolar OpAmp. The idea is to keep the gain of the input stage just high enough to "hide" the voltage noise of the OpAmp (if the JFET has, say, 4nV/rtHz and the OpAmp has 16, a gain of 5 should be enough).

Out of curisoity I opened up a commercially available TIA to see how they did it and if I could learn from their design. I found a dual JFET running with Ids=3.5mA and Ugs=-1.7 V. It's probably something like a 2N5912 (it's in a 7-leg TO78 can, part # scraped off). The odd thing is: the drain resistors are only 220 ohms off a 9V rail. The parts (at least the ones I can find) that fit this Vgs/Ids combo have a few mS transconductance which would get the input stage's gain barely over unity. Parts with significantly higher gm run at tens of mA in this gate voltage range.

What I don't understand: If unity gain is enough, why not just use the JFETs as source followers?

Disclaimer: I'm not ripping off anybody's product (and besides a JFET diff amp ain't rocket science). It's just instructive to look at other people's ideas, but in this case I can't make much sense of it.

robert

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Robert Latest
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Robert Latest a écrit :

No, you can't have unity gain. Noise is summing quadratically, so your opamp input refered noise has to be, as you first expected, the input stage gain has to be at most 1/3 of the diff pair noise, so that the opamp accounts for at most 5% of total noise voltage density.

Now, in its flat region a JFET will give you a 2/(3*gm) noise resistance which is sqrt(8kT/3 gm) noise.

To achieve 4nV/rtHz you'll need 2.8nV/rtHz JFETs which translates to

470R noise resistance and at least 1.4mA/V transconductance.

Now that you have a JFET input stage, you can use a bipolar opamp which are easier to get in the relatively low noise range and 4nV/rtHz is easy. For ex. the OPA227/228 is a good and cheap one.

But at 2mA/V you'll still at least 1.5K/2K drain resistors for the JFETs to noise to be dominant.

For the TIA you opened, 220R resistors means:

- either that the noise performances aren't optimal

- either that the JFETs have higher gm, which isn't optimal either.

--
Thanks,
Fred.
Reply to
Fred Bartoli

Look at the AD8628. Not exactly cheap but low noise.

In case this is for a photodiode: FETs are often only used to decouple the high photodiode capacitance from the TIA that follows at the drain node of the FET. Buys bandwidth.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

It's probably just an impedance converter, running into a much quieter bipolar op amp--easier to stabilize and just as good. Using a diff pair instead of two source followers would improve the negative supply rejection a bit, I expect, as well as saving one resistor. With BJTs it would improve the Johnson noise as well, since the small-signal emitter resistance has a noise temperature of T_ambient_/2, but I really don't know about JFETs.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs
[snip]

Phil, Would you be Philip C. D. Hobbs?

Stheno ?:-)

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Yeah, but it's all my mum's fault.

Cheers,

Phil(ip C. D.) Hobbs

Reply to
Phil Hobbs

Hello Phil,

Tell your mom she did good :-)

Just reading your "Photodiode Front Ends, The Real Story" because I have to build one. This one has to go up to 100MHz or so. Thinking about doing it all discrete again but other newsgroupers here have suggested modern opamps like the OPA847. The noise specs look ok. Would you trust these new amps?

The photodiode will be AC coupled to the TIA, the DC is handled elsewhere. Since the PD might be switched to another type some day I'd cascode it even if going with an amp like the OPA847. That way the feedback stability doesn't go to pots. My main concern is the usual, long term availability of the chips.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

Son-of-a-gun! I've just been reading all your papers ;-)

(Working with the other Phil ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Same here. So you are really doing HW design now? The stuff where you hold a hot piece of iron to some stuff that comes off a spool and melts?

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

Sure.

Naaaah! They have a technician (Bob Toler) from West Virginia to do that ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

That'll open a whole new world. Now you can use inductors larger than a few picohenries :-)

Great. Working on two client projects right now. One client has a nice large group of technicians, the other has none. Either I'll have to solder the whole prototype enchilada myself or hire a temp. But how do you find a good temp?

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

A temp agency might be one choice.

Best regards, Spehro Pefhany

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"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Reply to
Spehro Pefhany

I have used them in the past. Was ok for not too difficult jobs but when in need for someone with skills to solder 0402 parts and hotrod RF transistors the results were mixed. Hiring engineers that way did not work at all.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

Re-reading, you mean. ;)

Wow, they must have got some more VC money if they can afford you. I hope he still owns some stock by the time they're over the hump. Interesting gizmo, that. (For everyone else: Stheno Corp. is a startup that has an interesting technology for measuring chirality in biological molecules, by way of very small amounts of optical activity. It ought to separate the NIH from a small boatload of money if they can get it into production.)

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Joerg,

That sounds pretty sensible, as long as you don't let the 'elsewhere' dominate the noise--which can easily happen, e.g. with simulated inductors or current mirrors. (I'm perfectly confident that you wouldn't make that mistake, but I've seen others do it pretty frequently.)

The cascode trick decouples the TIA performance pretty well from the fine details of the amplifier, as you say, so if the chip goes away you ought to be able to find another one without undue pain. Maybe qualify a couple or three?

I've been doing device work for the last 5 years or so, so my stock of parts is getting embarrassingly antique--I still have some AD639 trig function chips that I bought in 1988, as well as some Raytheon 8-bit flash converters.

The other approach is to pick a low-noise 50-ohm amplifier and connect the PD right to the input--I've done that very successfully with 60-cent Mini Circuits amps. Since you're AC-coupling anyway, a transformer is another approach.

How big a photocurrent are you expecting?

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

One of the toppers I saw lately was a BIG inductor with way too much inductance. Actually more of a choke. It happened to resonate smack dab where it hurt the most.

I'll have a nice small UHF capable inductor first and then a lil' larger one towards the DC circuitry.

I have read not so good things about the AD8099 in that it was hard or next to impossible to get it stable. That leaves the OPA847 or maybe one from the THS series. But they are all different, not drop-in replacements, picky about the supply voltage and when someone dares to place more than a few volts across the inputs, bzzzzt-poof. That's why I was thinking discretes. There are nice cheap GHz transistors that boast noise figures of around 1dB. Doesn't have to be a real rocket like the BFP640. A few BFR505 would do fine.

Looks very enticing, have to check their noise figures. I have never used one because they are single-sourced but Mini-Circuits is quite a reliable company.

So far they told me 1mA max. It'll always be above 300uA or so. The challenge is to detect very minute changes in that, hence the low noise requirement.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

I like their abbreviation for Magneto Optical Enantiomeric Detection. They call it MOPED :-)

Have to confess that I don't even know what enantiomeric is. So are we now in a race about who can create the lowest noise source/receiver?

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

If you have 300 uA to work with, life is easy. A 200-ohm load will get you to the shot noise limit--so something like a Mini Circuits T4-1 transformer running into a garden variety 50-ohm amplifier (with no load resistor) will be the bee's knees from a SNR point of view. Gain stability will probably be the limitation then.

A cascode stage running into the summing junction of a current-feedback amp would be another approach. That AD847 doesn't look as though it has the GBW for a 100-MHz application.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

Hello Phil,

[ ... ]

Maybe I am too obsessed with noise here then. Gain stability won't be such a problem because it's only changes we have to detect. The absolute current value doesn't have to be logged since that is handled in the DC section.

It's the OPA847. That thing can do a GHz. Basically the new ones are all too fast for this application. But that could be tuned down via Cf.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

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