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OPA2381, is Vbias required?

Vitali Lach
Expert 1120 points
Other Parts Discussed in Thread: OPA2381, TINA-TI, OPA381

Hello,


I intend to use OPA2381 as a transimpedance amplifier for a photodiode (Everlight PD638B) in my project. The impendance of that photodiode is 333M (dark)....200K (1mW/cm²). In TINA-TI the amplifier doesn't work properly without bias voltage on the positive input, the output voltage is always about 17,7mV in this case.

If I apply a Vbias to positive input (0,5V) the amplifier seems to work (output 2.06V…4,59V). Could you please explain me the reason why Vbias is required? I attached a TINA-TI simulation file with the schematic.

Many thanks in advance.

Best Regards


VL

OPA2381.TSC
  • 0
    TI__Genius 12325 points

    Vitali,

    Photodiodes are more commonly modeled as current sources rather than variable resistances. I am curious why you chose to model your photodiode as a changing resistance?

    With the photodiode modeled in this fashion, the OPA381 is configured as a non-inverting amplifier with respect to the voltage at the non-inverting input. The input offset voltage of an op amp is modeled as a voltage source in series with the non-inverting input, and most importantly, can be positive or negative! Consider that if the OPA381 has an input offset voltage of -7uV, the output of the amplifier will attempt to produce negative voltages. Ideally it would want to produce -357uV when your photodiode is 200k ohms. This is not possible for it to do because you do not have a negative supply voltage. Therefore the op amp output saturates as close to its minus supply as it can get. 

    Adding a bias voltage to the non-inverting input biases the output away from the negative rail which avoids this problem.