Photodiode case

Oh, I'd use the BF862 where cheaper stuff won't cut it. If it weren't single-sourced. That has just bitten me and my clients too many times.

The laser diodes on one of my recent projects were in that ballpark as well. Ok, maybe half but we needed a few dozen. But they are driven by a three-cent transistor, works nicely 8-D

The free pic posting sites are great... supload, imageshack, many others. No registration required.

John

By 'embedded amp' I mean I am running the current mode amp as the output stage of the LMH6624. The OPA694 is set up in noninverting mode, and has a gain of about 2 and takes the load off the output of the LMH6624. The OPA694 is inserted between the output of the LMH6624 and the feedback loop for the transimpedance amp.

I'm looking for an input-referred noise in the 0.2pA/RtHz range (or better) at the photodiode with a gain of about 1.7e6 from the diode to my ADC. I have an optical signal that varies from 5nW to 1.3uW

I'd like to see that, please let us know if/when you get the schematic on the FTP site. What is the input-referred noise?

And thanks for all the help, I have a lot to think about!

So in essence the LMH6624 plus OPA694 would be the TIA? May I ask why you needed that buffer amp? If it's for amplitude reasons where the LMH can't swing it fast enough maybe you could run the TIA at a lower gain (sans OPA) and then follow that with a separate amplifier that isn't part of the loop. That avoids potential instabilities.

Ok, but you don't have to provide all that gain in one single stage. That makes the BW requirement tough to fulfill.

Or just use a.b.s.e. :-)

Ok, some ISPs' news servers don't serve up binaries but in this day and age there ought to be some web site that allows such access. While showing some ads ... why is it that the ads I see are mostly from some Italian dating service? It's all firewalled here, I am not Italian, my IP address should indicate I don't understand that language well, and I am married.

It was essentially to extend the bandwidth of the circuit.

Yep. But for some reason that escapes me right now, the analysis pointed towards putting as much gain in the first stage as possible to reduce the overall circuit noise. I do have a voltage gain stage after this, and a differential driver at the output to drive my ADC transformer.

It wouldn't help much with the BW except when you must drive huge output swings where the LMH runs out of slew rate.

That doesn't really work that way. The higher the gain, the smaller the overall bandwidth will be. For example, during my last high-speed PD design I kept things at 2.5mV/uA, IOW the amp ran at roughly 20dB. That's really all you need to overcome the noise, there isn't much SNR to be harvested with higher gains. I'd even have gone lower but this amp wouldn't have been stable then.

Yes. DC to establish the PD's bias and provide a return path for the low-frequency current. The ac signal path provides the bootstrap (which is only important at high frequencies where the opamp has lost tight control of the summing junction, and where Cdiode matters) and the return path for the high-frequency signal current.

How does that work for pdf files?

you may want to consider differential signal path to subtract thi s common mode noise, pick-up, ad lave only nice clean signal. That and manging the BW of the amplifier chain as well!

best Luck

Marc Popek

I think supload and imageshack will host pdf's or zips. Supload claims unlimited file size, Imageshack is 1.25 M for unregistered users... something like that. I don't know the mechanics of downloading... try it!

John

Can't you yank out the "under construction" sign and add a link to this site?

Or to this one?

Hey, you look a whole lot younger than on the AoE dust cover. How'd you do that? Can't just be the beard.

Photoshop? ;-)

Cheers! Rich

Thanks Win, then, the AC coupling is just for the bootstapped detector bias voltage. The "video" path is good to DC.

My detector amplifiers are not bootstrapped.

The last design has a diff pair of JFETS cascoded with a couple of BJTs. An OpAmp completes the forward path.

Thanks , Jure Z.

OK, a hint: Shaving off a beard and shortening one's hair indeed makes one look much younger.

The picture below was taken by somebody and given to me, after they edited it to haze another person in the picture. Anyway, there aren't many pictures I can use when one is needed, so I grabbed it.

That's an old photo, a newer one appears in the Rowland Institute Junior Fellow program brochure.

I wonder when damagers at NXP will EOL the BF862 ?

Winfield wrote: ...

November 06 on Electronica 2006, people at TI were quite optimistic to be back on par with the BB parts in spring 2007. And they apologized there too...