Transimpedance Op-amp, little gain for small signals, large gain for large signals

Thread Starter

AFool

Joined May 21, 2014
11
Decided to try a transimpedance then a non-inverting voltage amplifier stage. This appears to be giving me the correct gain, without the wild oscillation. I seriously under-stepped the feed-back resistor in the first stage to a mere 330kOhm. Still a bit of noise. It's not complete fuzz though and can likely be managed. I think I may leave most of the gain on the voltage amplifier stage rather then the transimpedance stage. As it is harder to stabilize the transimpedance circuit. It does appear that my voltage is 'off-set' by a set amount but this actually isn't a problematic thing for me. It's really all about coping with the noise and having the right gain.

I also do want to mention that I observed the effect of air transmission to the photodiode. What a crazy phenomena. I was testing with the volt meter(holding leads between fingers) and accidently touched one of the bolts(with another finger) which is at least a centimeter out of contact with the photodiode. Watched the signal spike to 1V from 0.090V! So fingers not air but still really interesting. I'd love to know the thermodynamic or I guess electronic reasoning behind why this is the case. Skin is normally a decent resistor.
 

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Thread Starter

AFool

Joined May 21, 2014
11
Wanted to update and say I am so close with the dual stage that it isn't even funny. Wouldn't of gotten this far without all the advice much appreciated. Just need to see where the tiny bit of noise is coming from before putting this thing onto some perf board. Dual stage was the way to go. The impedance load I was putting on the transimpedance stage must of been way way too high. I will post a final circuit diagram once this is completed.
 

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