I am using an OPA2111KP as a transimpedance amplifier (TIA) with a very large resistor (1 giga-ohm) in feedback for fluorescence signal detection applications. I have attached the TIA schematic's picture. I am using a phototransistor as a detector, thus its voltage need to be set to 0.8V as shown in the schematic. I set the terminal three to 0.8V. Furthermore, I can not use double supply voltage (-/+V), only the positive supply voltage is allowed in the system.
The circuit does not work, its output remains at 0V because feedback does not operate to regulate the terminal input voltage, and the terminal voltages (terminal 2 and 3) are in different voltage.
I am looking for a small footprint opamp to work with a single supply voltage in a TIA configuration, with a large resistor (about 1 giga-ohm) at a bandwidth of 5 KHz or more, at a mid-supply range output common mode voltage, input common mode voltage at around 0.8V, and low-noise high-precision. I would apprecaite it if you guide me how I can select such an opamp for the special application.
A 1GOHM feedback resistor is an extremely large value to use in practical op amp circuits. The input bias current through the feedback resistor will create a large DC offset that will saturate the output stage. In the case of the OPA211 the 60nA input bias current would create a +/-60V signal through a 1GOhm feedback resistor.
I suggest you limit the TIA gain to less than ~10MOhms and then use a post amplification stage to increase the signal amplitude further.
Regards,Collin WellsPrecision Linear Applications
In reply to Collin Wells:
It was very helpful and I think I did not carefully make an attention into the device. However, if you look at its datasheet, there is a schematic which shows a 1Gohm resistor utility.
My input signal is a very weak (less than one pA) and need a very low input noise input TIA. To the best of my knowledge, the input referred current noise is inversely proportional to the feedback resistor value. Thus, I chose the large value resistor to achieve the target.
Could you please introduce me an opamp to work with input common mode of 0.8V or less, the large resistor with a bandwidth of about 2KHz or more, and only need a single supply voltage (+V)?
In reply to Mehdi Noormohammadi:
I scanned the OPA211 datasheet but can't find the schematic showing the 1GOhm resistor. Could you point it out to us? Also, I think you're a little backwards as the input referred current noise is directly proportional to the feedback resistance (V = I*R). These subjects are covered in detail in the TI Precision Labs videos on op amp noise.
Consider using the LMP7721. It's one of the lowest input bias current amplifiers available and my work in your application.
You can find in page 10 of the datasheet in the following link:
Meanwhile, its schematic is attached.
Low Power Amplifiers and Comparators (LPAC) Applications
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