• Resolved



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. 



  • In reply to Mehdi Noormohammadi:

    Yes, we mentioned both of those points a few posts back:

    Many of the same comments apply although the bias current of the OPA111 is only 2pA which would function in this circuit. However the OPA111 requires a minimum supply voltage of +/-5V and does not feature a rail-to-rail input stage. 

    The +/-11V input common-mode range is only valid for a +/-15V supply voltage.  The minimum supply voltage for the OPA111 is +/-5V so with a 0-5V supply your system is trying to operate the OPA111 with less than 1/2 the required supply voltage as Paul pointed out.

    Collin Wells
    General Purpose Amplifier Applications