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OPA380 based transimpedence amplifiers

Karan Mehta
Prodigy 10 points
Other Parts Discussed in Thread: OPA380

Hello all,

I want to make my first transimpedence amplifier using opa380, the circuit as shown if the figure no. 1 of datasheet with BPW34 photodiode from vishay. But the problem is that I get no variation in output if I illuminate photodiode. The output remains constant at 5V only. The photodiode is working as i tested it using it in series with a resistor and measuring voltage drops.  I want arround 3 MHz bandwidth. But currently I am testing the output by just turning on and off the light on photodiode. Is it ok to do so ? Will I get change in output ?

What can be possible reasons for this ? 

I am using 1M feedback resistance.

  • 0
    TI__Genius 12631 points

    Karan,

       What are the power supplies you are using? When the PD is unilluminated what is the value at the inverting pin and the output of the amplifier? How much current are you expecting from the PD when it is illuminated? Please drop the feedback resistance to about 10K and retest the circuit.

    Samir

  • 0 in reply to Samir Cherian

    Hey Samir!

    Did you notice any difficulties in the production of the OPA380? I was using one for for a regular transimpedance circuit for frequencies from 1MHz to 10MHz. In the beginnig it was working quite well and gave quite a nice gain to my photocurrent(10pF capacitance in diode) . While experimnting I was crashing my OPA due to too much voltage. After reconstructing the basic circuit I tried several new OPA380 but none would work good in high frequencies. While it was working great at 100kHz there was more or less no Signal beginning by 1MHz. I tried to be careful for ESD, so I reckon it's not about that. Did you experience similar problems?

    Regards and thanks for the help in advance,

    Hannes

  • 0 in reply to Hannes Emmerich
    TI__Genius 12631 points
    Hannes,
    There should be no issues with the OPA380...I cannot foresee a situation wherein the bandwidth of the amplifier reduces and everything else works normally. Do you think something is up with the photodiode and its capacitance increased? An increased capacitance will reduce the bandwidth of the system.

    Another possibility is that you added a feedback capacitance, which also ends up reducing the bandwidth of the TIA.
    -Samir
  • 0 in reply to Samir Cherian

    Hey Samir,

    I checked the photodiode again and the capacitance is still the same (10p). The Feedback-Capacitance I didn't even use since it would be again worse of a a Signal. Another thing that might be intersting to note, is that this one now just gives a Little Signal with pretty high resistor. Everything less than 1MOhm Shows even less. I don't think though, that there is problem with the photocurrent, since I changed also the diodes...but if you didn't experience a behaviour like this, might be just some weird electromagnetic field around or sth. else.

    Thanks for your time though.

  • 0 in reply to Hannes Emmerich
    Prodigy 230 points
    Haans,
    I made a simulation (with micro-cap) of the circuit of fig 1.
    1. With 1M ohm resistor you will get no more then ~1.5Mhz BW (no cap).
    In the real world you will have ~ 0.05pf of stray cap so your BW will be less.
    2. When trying to simulate with the 0.5V biasing it does not work. I change the biasing to 2.5V and now it is working.