TransImpedance Amplifier circuit

Discussion in 'General Electronics Chat' started by saint_jay77, Sep 16, 2007.

Oct 19, 2006
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Does anyone know of any good OPAMP that could be interfaced with optical detectors. I need a good temperature stable chip first stage amplification chip.
I chose OPA380AIDG4 from TI; Unfortunately i have a negative going pulse that is required from the output but this chip can only be powered from 0-5V so thaqt means i always have to be positive. Please let me know how i can bias this chip so i can get negative pulses.
Does anyone know of any other chips???
Thanks..

2. SgtWookie Expert

Jul 17, 2007
22,183
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So, the output from the op amp is basically just going to be ON or OFF - as in a pulse?

If all you need to do is to get a pulse, you could put a capacitor in series with the output, as the cap will block DC, but pass the effects of AC, or changes in voltage levels.

On the far side of the cap, you could use a couple of resistors to establish your baseline voltage level between the rails and ground - or just tie it to ground via a resistor, if that's all you need.

The output side of the cap would then just reflect the transitions; if the output of the op amp went from say, +0.5v to +4.5v, held there a while, then went back to +0.5v and you had the cap's output tied to ground via a 47k resistor, the output side of the cap would go from 0v to nearly 4v briefly, then back to 0v. When the opamp output dropped back to 0.5v, the output side of the cap would drop from 0v to nearly -4v momentarily, then back up to 0v.

You would need to work out the values for the cap and resistor(s) using Ohm's Law, keeping in mind your expected pulsewidths and repetition times.

You also could connect what the IC expects to be Vcc to ground, and it's ground to -5v - it shouldn't know the difference; as everything's relative. Make sure your inputs and outputs don't exceed the specifications.

Oct 19, 2006
26
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Thanks SgtWookie!!! I will try that. I like the idea of connecting the VCC to gnd and -5 to GND. Is there any websites that you could forward me to that have this illustrated??
Attached is the original config of cct. The output swing is 0 to -6V or so. Now i replaced the op amp with opa380 which has a supply from 0 to 5V. I dont get any negative output swing.

Thanks..

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4. Distort10n Active Member

Dec 25, 2006
429
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OPA656 is a really good TIA. +/-5V supply.

5. SgtWookie Expert

Jul 17, 2007
22,183
1,728
Just to be specific, I meant for you to connect the op amp's Vcc pin to your signal ground, and the op amp's ground pin to the -5v.

However, now that I see the portion of the circuit you're working on - looks like you're also going to have to reference the ground side of the optocoupler to -5v, Vcc on R13 will have to be disconnected and re-connected to your signal ground, ground side of R14 to -5v, ground side of R6 to -5v, and the ground side of the variable resistor to the right of the relay will have to be reconnected as well - and I have no idea what might be on the other side of that resistor.

I took your PDF file and converted it to a .jpg image, and rotated it right-side up; attached below.

Not that I'm aware of. Keep in mind, everything is relative. Theoretically, you could have two potentials; 1,000,000 volts, and 1,000,001 volts. There's still a 1 volt potential between the two, and if you hook up a 1-ohm resistor between the two potentials, you'd get a 1 ampere current flowing between them. Now if you brought a 0 volt potential near them, you'd get quite an arc. But between the above two potentials, all you'd see is that one single volt.

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