# LM358 Transimpedance Amplifier

Discussion in 'The Projects Forum' started by hsherwood68, Nov 23, 2013.

1. ### hsherwood68 Thread Starter New Member

Oct 29, 2013
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0
Hi!

I've been trying for a while to construct a circuit to convert and amplify the current generated by a photodiode with the output as a voltage which ranges from 0-3.3V.

My original mistake was in terms of op-amp selection, as I had picked a model whose supply voltage was too high.

Having now got hold of a low-voltage op-amp, I realise that the configuration of my circuit is also a problem. I cant seem to find any consistent/doable schematics for a Transimpedance Amplifier online, and I was wondering whether any of you could offer any expertise to improve my circuit (which I realize is simplistic/wrong, better to add things than take away)

A summary of the circuit's intended operation:

The tiny current generated by the photodiode (http://docs-europe.electrocomponents.com/webdocs/08b2/0900766b808b25c6.pdf) is converted to a voltage by an LM358 where the "dark voltage" is around 0V and the "light voltage" is around 3.3V. The supply voltage can be 5V or 3.3V.

Thanks!

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2. ### #12 Expert

Nov 30, 2010
18,076
9,676
The concept seems right. The devil must be in the details. Quantify. Label the drawing.

There is probably a better chip to amplify this because the input bias current of the LM358 chip can be as high as 5000 times the dark current and the light current is only 40 times the inherent bias current. Still it's doable with those margins.

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3. ### hsherwood68 Thread Starter New Member

Oct 29, 2013
4
0
Thanks! So what would you recommend for values of R1 and R2? I could have it completely wrong.

Thanks

4. ### #12 Expert

Nov 30, 2010
18,076
9,676
And here I thought you would give me a clue as to what doesn't work and what went wrong.

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5. ### John P AAC Fanatic!

Oct 14, 2008
1,727
258
Your circuit needs a little more consideration.

The photodiode pulls the negative input of the op amp high. The positive pin is at ground, so the output of the op amp will go negative to pull the negative pin down to ground also--but the low-side power supply for the amplifier is ground. It can't go negative. Try looking up "Single supply transimpedance amplifier" for more ideas.

All right, I'll do it. This circuit is similar to yours; can you tell what it has that yours doesn't?
http://www.maximintegrated.com/images/appnotes/5129/5129Fig02.gif

Anyway, the LM358 is a horrible component for anything involving sensitive specifications or high speed. Use it at your peril.

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6. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
510
With a tiny current, you could not have picked a worse op amp: the LM358 is a bipolar op amp. Use a FET amp like LF356 so it won't load the input source.

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7. ### #12 Expert

Nov 30, 2010
18,076
9,676
Face palm. It's a negative amplifier without a negative supply voltage. Completely missed that.