Your circuit is probably way better than is justified by the photodiode. The response of a pd varies wildly with wavelength, temperature, angle of illumination, and light distribution over the surface. Big ones can be leaky too.
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John Larkin Highland Technology, Inc
lunatic fringe electronics
We try to drive the SJ conductance to infinity. One can imagine a scheme where an extra factor is picked up, but it's hard to imagine getting 10^8 or 10^10 operating range, you pick a number.
This fellow's creation can do that. OK, time for another hint, if it isn't obvious by now, there's one input amp, and one feedback resistor chain.
The entire circuit has output ranges, yes, but multiple ranges are simultaneously active. You can have as many ranges, with whatever scaling factors, 10, 30, 100, 300, etc., as you like. We picked 300 for star-to-sunlight, save parts.
Well the perhaps silly idea is to have to current go through R1 and R2. Say R1 = 10 k and R2 = 1 Meg, and there is 1 uA of current, then ignoring offset voltages the output of the second opamp should be 1.010 V, and the first 10 mV... I guess I'll have to spice to check it.
I seem to recall some chained TIA circuits, but I couldn't find them online, and forgot what they looked like.
When the current is high enough in R2 to rail the 2nd opamp, how can the first one go to its proper value? But, yes, the current has to go through both R1 and R2, at least for low currents (oops, another hint).
Yes, that would give you dynamic range, just like range-switching the feedbackresistor. But the cool thing about this design is, there's no range switching. Multipule ranges are active at the same time, so there's never a momentary loss of data. BTW, it also works going from positive to negative and back.
On a sunny day (Wed, 26 Jun 2019 04:48:16 -0400) it happened "Tom Del Rosso" wrote in :
It would oscillate, but you could only use the value that makes sense, start: nothing railed: use the higest gain if 1 railed, use the next one up else if 2 railed activate JFET1, test again if still 2 railed also activate JFET2 and test again. if still 2 railed sound the super-nova alarm else use the one that is not railed .. .math.. output go to start
The first stage will have to lowest gain.. Still one could probably make it rail with a laser. I want to see what the various step responses look like.
Win, If we all promise to buy the X-chapters, will you post 'scope shots? (or do I have to build it myself. :^)
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