Photodiode case

(e.g., compared with an OPA637 opamp).

I've never thought to compare you directly with the components you mention, but what the hell, you're right about being a lifesaver ;-)

-- Joe

Thanks, dad.

T'is exactly why I am holding back. I don't trust the managers there much anymore.

--
Regards, Joerg

http://www.analogconsultants.com/

No, of course not. It was just to say that I do use expensive parts. But only when absolutely necessary. The highest power I have ever generated at higher frequencies was 1.5kW (but that was continuous duty). Pulse generation was at higher powers but still not 2MW. There, we often use large arrays of cheap transistors. Not so much for cost reasons but because high-powered boutique parts have too often shown a habit of becoming unobtanium.

--
Regards, Joerg

http://www.analogconsultants.com/

The scope is 70's era (but I've got a 7000 series myself so no complaints). No beard.

Newer scope, beard is back. But recently you said the beard is gone. Now I am confused.

--
Regards, Joerg

http://www.analogconsultants.com/

Ok, shortening my hair was already done by nature, it's mostly not there anymore :-)

--
Regards, Joerg

http://www.analogconsultants.com/

Win,

The classical bootstrapped shield approach is very expensive in SNR unless you're careful (which I assume you are). How do you avoid a big noise peak due to concealing the RC rolloff?

Cheers,

Phil Hobbs

My long beard is gone. But I have adopted the Bruce Willis / Hollywood look, where 2 to 3 days is optimum. Actually, for me, this translates into fluctuating between 0 and 15 days. :-)

As long as SWMBO is agreeing with that ;-)

--
Regards, Joerg

http://www.analogconsultants.com/

Actually, the bootstrap is more quiet. The circuit is going to suffer from the opamp's en-Cin noise without a bootstrap, but with it, provided the bootstrap follower is quiet, the opamp's voltage noise is impressed equally on both sides of the diode capacitance and therefore it doesn't generate noise current in the capacitance (which would be seen as noise in the signal). In a sample case the opamp's e_n is 4.5nV, which is about 5x higher than the boostrap, so the amplifier enjoys a 5x improvement. As a bonus, the apparent bandwidth of the main opamp is greatly increased, or the detector's apparent capacitance is decreased, however you prefer to look at it, allowing dramatically more circuit bandwidth than would otherwise be possible with the opamp.

The bootstrap bandwidth greatly exceeds the transimpedance bandwidth, so there is no HF rolloff there. As far as the ac coupling is concerned (mine is 200Hz), the PD's light- current signal has to go through the feedback resistor/ capacitor, and hence get measured, no matter what the current path might be on the other side of the photodiode.

BTW, I prefer the bootstrap approach to your common-base transistor approach because it works well over a wider current range, all the way to zero DC photodiode current, where zero can be many many orders of magnitude below Imax.

[snip]

Bwahahahahaha! And I thought you had no sense of humor.

We BOTH sure look like Bruce Willis don't we ?:-)

(I use the shortest setting on my beard clippers... probably about

1/8"... I'd do the same to the hair on my head but, as Joerg notes, SWMBO objects to such a severe treatment :-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave

Win,

I know about bootstrapping the PD...I've used that approach often, and combined it with the common base idea as well. It's bootstrapping the cable shield I meant. The original 'ghost shield' approach from WWII sonar looks like magic but costs beaucoup SNR. The SNR of a bootstrapped RC front end is the same as the SNR of the same amp connected as a follower on the same RC rolloff--only the transfer function changes.

Wide range photometers have different problems than the laser measurements I'm typically interested in, so naturally different tradeoffs are reasonable. In general I'm worried about getting to within 1 dB of the shot noise in some relatively narrow range of photocurrents, maybe a 10:1 range at most. That's not too much of a restriction, though, since to get to 1 dB above shot noise, you have to drop at least 200 mV across Rf, independent of circuit topology. Even with asymmetric supplies or a big DC offset, there's at most a 20 dB (optical) range where a given linear op amp circuit can be quiet enough for my purposes. (Range switching doesn't count.)

My favourite way to get better SNR is coherent detection, which also compresses the dynamic range by half (in dB). With coherent detection, you can pick the local oscillator beam power to be anything convenient, and still get *1 photon RMS* sensitivity on the signal beam. If I'm really stuck in the nanoamps, I'd rather use an avalanche photodiode and save my SNR. APDs have really improved in the last several years, so the noise penalty is much less than it once was.

Some of the reasons I emphasize the common-base approach in my writing are:

1. It's simpler than a bootstrap--one BJT, or one BJT and two resistors for the fancy model, and simpler is prettier. I share Joerg's aesthetics there.

1. Its characteristics are an excellent match for laser noise cancellers (q.v.), which use BJT diff pairs attached to photodiodes.

2. It's much easier to analyze, and thus it occasionally sneaks a bit of device physics into the heads of people who only know SPICE and a few textbook circuits.

It never ceases to amaze me how many EEs and applied physicists chicken out of analyzing the noise of a simple BJT circuit. Even new grads do this--people who were doing partial differential equations for class a year or two ago. The physics is dead simple, and it's about 5 lines of algebra, but the great majority of people I talk to simply _will_not_ do it. This is true even though BJTs follow their simple noise models essentially perfectly, and even when their jobs depend on the results. Pathetic.

I've had a few, fewer than 10 out of dozens and dozens I've talked to, who have gone through the math for a common-base stage. Good luck getting them to analyze a bootstrap.

Cheers,

Phil Hobbs

Wow, I had you both pegged as more like Willie Nelson. ;)

Cheers,

Phil Hobbs

Dang! A year ago a client could really have used one of those 10. Anyhow, if you happen to know a good analog guy who is part of that group of 10 and willing to work in Kahleefohniah (northern) let me know.

--
Regards, Joerg

http://www.analogconsultants.com/

Nah, then they'd both be driving a big truck with mud flaps and them big errialls ;-)

Or at least a Harley like them's guys out here who performed with him at times:

--
Regards, Joerg

http://www.analogconsultants.com/

Scary, very scary. I've ridden on a motorcycle once in my life--it was a Yamaha RD400 (two-stroke) and I was on the back. Problem was, the other guy was about half my size, and the bike went where *I* leaned, not where he did. We never even got out of the parking lot.

On the other hand, on bicycles I've nearly gotten myself killed more times than I can count. The closest one was when I was 18, riding down from Zermatt (Switzerland) on a Vienna-to-Paris tour. I was whipping along the edge of a two-lane road that snaked down the side of a mountain, having a great time. Then I hit an off-camber, decreasing-radius bend and started to slide into the oncoming traffic. There was a truck coming, followed by a line of cars, and about 4 feet from him to the guardrail next to the umpty-hundred-foot drop on the outside of the road. I had to cross in front of him and hit that little space--fortunately for me there was no gravel on the shoulder just there. Needless to say I went a bit slower for the rest of the day. Sometimes boring is beautiful.

Cheers,

Phil "made it to 48 so far" Hobbs

If the voltage noise of the bootstrap follower amplifier is the same as the voltage noise of the transimpedance opamp, there's not only no overall noise benefit, there's a small penalty. Yes. Although in many cases I imagine there can be a substantial practical bandwidth improvement without requiring an otherwise highly-complex circuit. Perhaps that was their appeal in those old tube days.

In the happy case of the bf862, which is easy to use in a high-performance follower bootstrap, its noise is much less than available JFET opamps, and it equals or approaches a BJT's performance without the dynamic-range limitations.

Right, fine, but for us a 10:1 or even 100:1 limit would still be a special case. For the common case of single-shot non-repetitive events, even though the weak signals may not be above their shot noise, it's still necessary to measure them and to know where zero is, as you agree, right?

I'm not into ratty-looking beards.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave

I DO own a pick-em-up truck (though it's a Frontier ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave