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| Photodiodes saturating in ambient light |
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| tggzzz:
--- Quote from: aussie_laser_dude on May 14, 2020, 11:46:42 am ---OMG... Witchcraft! I had no idea op amps could amplify a current directly. I'll need to study transimpedance amplifiers in detail. I would have thought the current in the secondary coil is being forced through a very high resistance load, but apparently it just goes around the op amp. --- End quote --- The way to think about voltage opamps[1] (in the normal operating regime) is that the output will perform gymnastics to keep the two inputs at the same voltage, and that no current goes into the opamp. So if you have an inverting amp, all the current through the input resistor goes through the feedback resistor. If there is no input resistor, any current (e.g. from a photodiode) still goes through the feedback resistor. The gain is then measured in units of Vout/Iin. I would still consider reverse biassing your photodiode, to reduce capacitance and increase linearity (if that matters). If you can choose amplifier gain and power supply so that you get adequate signal output without the ambient light saturating the amplifier, I'd consider omitting the transformer and AC coupling the amplifier's output to the next stage. [1]Current feedback opamps are a different kettle of fish, with differing advantages and disadvantages. They are not relevant to your problem. |
| NiHaoMike:
--- Quote from: twospoons on May 13, 2020, 11:23:00 pm ---Also you can do this with just one opamp and one resistor. And this will get you your best bandwidth. - neg input is set up as a virtual earth, so any current in the secondary of the transformer will be forced (by the opamp) to flow in R1. So VOUT = I x R1 : thats your transimpedance gain equation. - The tfr secondary sees a very low impedance, which is what you want for best bandwidth. You will note this is the same as your circuit topology, but with R2 set to zero. R1 becomes unnecessary, as it is 'shorted' by the virtual earth. Remember this is a current signal. You dont actually have to use a transformer - an inductor will do too. --- End quote --- That configuration will magnify the offset voltage of the opamp. Easiest fix for that is to connect the photodiode directly. Then add an integrator and another resistor to cancel out DC. |
| David Hess:
--- Quote from: tggzzz on May 14, 2020, 12:28:07 pm ---[1]Current feedback opamps are a different kettle of fish, with differing advantages and disadvantages. They are not relevant to your problem. --- End quote --- They might be relevant. Current feedback operational amplifiers are used for higher performance transimpedance amplifiers in photodiode applications. |
| tggzzz:
--- Quote from: David Hess on May 14, 2020, 03:01:58 pm --- --- Quote from: tggzzz on May 14, 2020, 12:28:07 pm ---[1]Current feedback opamps are a different kettle of fish, with differing advantages and disadvantages. They are not relevant to your problem. --- End quote --- They might be relevant. Current feedback operational amplifiers are used for higher performance transimpedance amplifiers in photodiode applications. --- End quote --- I haven't seen that done, and Phil Hobb's book doesn't mention it. But that's a weak argument :) I think that book will be sufficient for the OP, and give them more than enough to think about. |
| StillTrying:
--- Quote from: aussie_laser_dude on May 14, 2020, 11:46:42 am ---Even with a $100 optical bandpass filter sunlight is still ~3x higher than the laser, this is better than the 50x ratio without the filter. --- End quote --- I thought you had only about 5mV of signal on top of 1.5V of ambient, but your signal to noise/ambient is 100 times better than that. :) 50uA of photodiode current signal is quite a lot, even if the ambient is still higher. Do some simulation. :scared: The Green plot is simulated photodiode current, 50uA 5us wide pulses on top of 0 to 400uA of ambient changing at 1kHz. The Yellow plot is attempting to separate the pulses from the changing ambient by the AC coupled method. The Red plot is using an inductor method. The last three are close ups of the first three. I don't know how to do simulated transformer versions. |
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