Photodiode case

Oh no, will it come to this? Beer that isn't so bitter it's undrinkable? ;-)

Tim

-- Deep Fryer: A very philosophical monk. Website @

Great news. People put too much hops into beer as it is.

John

I didn't think the US had any legally qualified beer, by german standards anyway

Martin

Try Marmite

Martin

Sheesh, we seem to be on the opposite side of several issues today. In fact, bootstrapping, properly done, which isn't very hard, works marvelously well. Keep in mind some PDs have VERY high capacitance, even when back biased to the max. (For example, some of those used for UV detection: try 400pF at 10V bias.) At high freqs one is bitterly fighting i = w-en-Cin noise, and there's a limit to how far down one can push en with available IC opamps. But one can easily make a 0.8nV 80MHz-bandwidth guard / follower (e.g., with a bf862 JFET input stage), dramatically lowering Cin in the noise formula above. I can be a real lifesaver, giving a serious 3x to 5x performance improvement (e.g., compared with an OPA637 opamp). I'd say it's well worth the trouble.

Ahum!

We even have a local Altbier-Brewery and AFAIK they go by the Reinheitsgebot.

Their is hope yet!

Colbert 2008..... the least dishonest politico in the US

Martin

^^^^^

Aha!

Agree, for hi-C PDs. But the OP said his is only 2.5pF. That should be good even for a Gb/sec FO link.

Getting 80 MHz bw from a 2.5 pf diode certainly doesn't need bootstrapping, and bootstrapping is asking for stability problems. I doubt that a laser ranging application is going to be dominated by amplifier noise anyhow.

None of the really fast o/e converters, in the GHz or 10's of GHz range, are bootstrapped. Bootstrapping has its place, but probably not here.

John

I'm planning to experiment with a PD amp using OPA637B configured with the BF862 bootstrapping as shown in Linear's app note "Low Noise Amplifi ers for Small and Large Area Photodiodes Design Note 399".

I bought some of the last remaining OPA637BPs on the planet from Newark a few months ago. There's still a small handful of the 'A' grade out there.

What the heck is going on with TI's Burr Brown parts manufacturing?

Good day!

Joerg,

Its a GPD GAP300 variant. I am using an LMH6624 as the TIA with an embedded gain stage (OPA694) and the front end cascode and bootstrap (emitter-follower) are BFG25A/ X. The TIA stage has a gain of about 35k, and I use a capacitive Tee with a tuning cap to stabilize the TIA.

It seems 'OK' for a first cut. I have been playing around with the capacitances at the TIA trying to reduce the noise gain. I could use a

That's a bit confusing, could you post a schematic? What do you mean by embedded? The LMH6624 loses steam quickly with slight capacitive loading (page 7 in the data sheet).

That's a bit puzzling since the LMH6624 is a pretty good performer here.

My circuit is a bit more complex, to insure low-Z HF bootstrap drive and provide DC PD biasing, while maintaining the 0.8nV bf862 voltage-noise capability.

If you ask TI, they'll give you a straight answer: They badly miscalculated and didn't replace Burr-Brown's automated test sets far enough in advance to keep up with the fact the old ones were failing and could no longer be fixed. The new ones are in place, but the programming is going slowly. They're bringing back the products several at a time and they hope to have everything back by next spring. Oh, they do apologize.

Can you post a schematic?

Meantime, us guys have moved on ....

It's got 16 parts - too many for an ASCII schematic. I'll see about putting it up on our public ftp site. BTW, the parts are all inexpensive. :-)

Basically it's the bf862 with a ferrite in the gate, a source resistor to Vee for running at 5mA, a drain resistor plus bypass cap to gnd to reduce the drain voltage to under 8V (see datasheet), a coupling cap to an emitter-follower stage, which is biased at say -10V, depending on the PD, an LC filter to quiet the bias-voltage source, a pos/neg bias-voltage header, a low-noise EF BJT, with resistor to run at 4mA, etc., with back-back collector pmbfJ309 JFETs to limit the current in the event of a PD-sensor short. Finally there's another ferrite at the output to damp parasitic RF oscillations (small resistors, commonly used for this purpose, aren't allowed because of their Johnson noise). The bootstrap follower has an 80MHz bandwidth, probably limited by the ferrites, and is capable of driving the guard shield in a triple-coax cable, etc. Armed with a pencil, one should be able to draw the schematic.

It's got 16 parts - too many for an ASCII schematic. I'll see about putting it up on our public ftp site. BTW, the parts are all inexpensive. :-)

I have found this circuit to be quite stable and robust when used in various ways (five different ckts so far).

Basically it's the bf862 with a ferrite in the gate, a source resistor to Vee for running at 5mA, a drain resistor plus bypass cap to gnd to reduce the drain voltage to under 8V (see datasheet), a coupling cap to an emitter-follower stage, which is biased at say -10V, depending on the PD, an LC filter to quiet the bias-voltage source, a pos/neg bias-voltage header, a low-noise EF BJT, with resistor to run at 4mA, etc., plus back-back collector pmbfJ309 JFETs to limit the current in the event of a PD-sensor short. Finally there's another ferrite at the output to damp parasitic RF oscillations (small resistors, commonly used for this purpose, aren't allowed because of their Johnson noise). The bootstrap follower has an 80MHz bandwidth, probably limited by the ferrites, and is capable of driving the guard shield in a triple-coax cable, etc. Armed with a pencil, one should be able to draw the schematic.

Interestingly enough, the electronics course where I both had the AofE as a text, and finally "got it", to the extent that I have ever gotten it (the more I learn, the more I know how vast an area my ignorance covers), was the one in Physics that was aimed at getting students to get their experiments to work. The EE courses all had sketchy books self-published by the department, and were entirely barf-back based (memorize this equation for a circuit that looks like this - and don't ask why). AofE led to the same equations in a manner that made sense, and you could figure out again without having to barf anything back. Mind you, this was in the era when there was no second edition yet...

Ok, I think I grok it now. Pencil broke off a few times ...

I love inexpensive stuff although the BF862 doesn't quite fall into that category (yet).

Yep, I think we've got you're number, 1 to 5 cents is OK, but 18 cents is not. Seems a bit overly contrived.

One of my recent projects, a 2MW sine-wave generator, used $850 transistors (thankfully I got used ones on eBay for a fraction of that). Sometimes one has to bite the bullet and leave all the cheap stuff behind.