But what happens if you don't? It's a big R in series? Is that right? 'scribble' Let's say 10 nA of current (100mV @10M ohm) so it's V_thermal/I = 40 M ohm... I guess I could live with some offset at the highest gain, most of the time there's uA's of current. Why not a bootstrap first? well second, after the FB capacitance... maybe I need to boot strap that first!
Yes. The input impedance Re at the emitter of the cascode is 25/Ie, where Ie is transistor current in mA. So at 1 uA, the impedance is
25K. What will happen is that diode conduction and maybe leakage current will pull the emitter positive in the dark.
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So when the light hits, the emitter current is low, the Cpd*Re time constant is long, and the current builds up slowly as the emitter current creeps up. Adding Rb pulls the emitter down and keeps Re low.
You can add Rx, or something downstream of the TIA amp, to null out the current in Rb.
Here's an over-the-top version:
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Booting is great for slowish (MHz range) high capacitance photodiodes. It's usually not a good idea for fast stuff.
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John Larkin Highland Technology, Inc
lunatic fringe electronics
In some alternate universe where 40 ns is blazingly fast. ;) UV ones are faster than that, and if you really bomb a high-current IR one, you can get the recombination to enhance enough to get them down to 100-ns territory.
LEDs are slow compared with even cleaved-cavity lasers, which will do 200 ps pretty easily. VCSELs are much faster than that.
The biased side of the PD needs an RF-compliant small ceramic cap to ground. 0.1uF in 0603 package or similar, very short path to the ground plane. Short meaning a via right next to its ground pad. The RF impedance of an electrolytic cap is like roulette.
The onset can be quite fast, just not the trailing edge:
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When I pulsed LEDs I never tried it with the really old ones I still have from my teenage days (when I still had hair on top). Maybe I should. Those need at least 10mA to be reasonably visible during the day.
Phil is the expert on that stuff. He might know tricks to milk more BW at such high current/voltage ratios.
IMHO you should not force all that gain into one stage. It may be better to reduce to 1-5k in the TIA and then follow with a regular amplifier stage to do the rest.
This is the best we've done with a butterfly-packaged fiber-coupled laser:
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To get that sort of pulse, you have to tease the pulse width and amplitude to get a sort of optical avalanche effect. Lots of lasers have so much inductance that they can't go fast. Or they were intended for telecom, and the impedances are wrong for pulsing.
Here's the driver, which has a built-on one-shot to make the pulse width:
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John Larkin Highland Technology, Inc
lunatic fringe electronics
I guess I'm talking/thinking about adding some driven shield plane around the inverting input, try to block some of the ~1.5pf of FB capacitance. 1MHz is fine, I could use a dual opamp.
Hmm, I can try adding a ceramic to the bias supply. There's no ground plane in this circuit. Well the whole thing lives in a grounded aluminum box, ~1"x1"x2" maybe a bit bigger. Inside, capacitance is not your friend at 1 meg ohm. (I've been measuring this outside of the Al box, I wonder if it will change by much inside?)
Yikes! Then it's almost a wonder that a 200MHZ amp runs in there. Although a 40MHz scope won't necessarily show you if it does tarantella dances.
The RF behavior can't be ascertained well from outside. Or even inside, without a ground plane. Trying nanosecond stuff that way doesn't make much sense, I'd rebuild that with a ground plane. For prototyping a piece of copperclad will do.
Depends on the desired dynamic range, and use a low noise amp in the first stage. Most of all, build it with proper RF guidelines in mind and that's where this project could use some upgrading. Otherwise anyone with a GSM phone walking around could disturb the signal especially if the amp has BJT structures in its input area.
James, yeah the opamp TIA drives the coax too. It was oscillating happily when I had only 330 ohms of TIA R. :^) But it seems OK when terminated. So as you say, the ringing is due somewhat to the cable capacitance.. I guess, I'm not really sure how to think of 7' of coax.. transmission line or capacitance?
Ground planes and such. Joerg I'm not sure, what I'd like is a big ass photodiode at 1 M ohm gain and 1 MHz BW. (typically ~1 uA of current.)
1 MHz is sorta border line RF, and with 1 Meg ohm a pF of capacitance can ruin your whole day. (I've got hacked pcb's where I learned this... ground plane sanded off around PD and TIA... ugly.)
I bought a couple of THS4361's so maybe I'll build up a pcb w/o the switch and see what that look like.
Oh the numbers I post upstream are not right. I'm a little bothered by the high gain end, cause I think I was getting more (HF) gain there with a slower opamp, so I need to redo this more carefully.
Build on solid ground plane with good RF bypassing.
Use Manhattan for non-critical components.
Air wire the sensitive node.
Do you really need to drive the coax directly? Can you add a series resistor to reduce the loading? Any ground bounce from poor supply bypassing can give feedback to the inputs.
The datasheet Application Information section has a lot of useful information on feedback resistor selection, layout, bypassing, alternate configurations, etc.
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