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OPA380 with photodiode operation

Vijaya Kumar C
Prodigy 60 points

Other Parts Discussed in Thread: OPA380

Hello friends,

   I have connected the circuit as per Figure 6 - (a) page:12, in OPA380 datasheet(http://www.ti.com/lit/ds/symlink/opa380.pdf). It is working well and good. But i have reversed the photodiode (kept in Forward bias) with 5V at the cathode of photodiode. Now the photodiode is giving positive pulses, but opa380 output is not inverted.

  If negative pulses are given to inverting pin positive pulses of amplified signals are obtained. Similarly if i give the positive pulses i can get the negative pulses of amplified signals.

  Is there any circuit still need to modify. or this will not work?

Please reply me

  • 0
    Prodigy 100 points

    Typically photodiodes are used with either 0 voltage bias or reverse bias so that with no light hitting them, 0 current, flows. Reverse biasing reduces the photodiode capacitance and can speed up the rise and fall time if that is important.

    For transimpedance amps, Vout = I*Zf. With the circuit as shown in a), when light is converted to electron hole pairs, current flows from the cathode (held at positive V bias)  to the anode at ground. This current is supplied by the feedback network (Zf) from the output of the amplifier. Ideally, no current flows in or out of the very high impedance inputs such that a light pulse will cause a positive output response.

    Using the diode in forward bias mode to get negative going pulses on the output of the amplifier is problematic. Like shown on page 8, you can ground the Vbias pin so the there is 0 volt bias and then reverse the photodiode so that when light hits it, a current will flow toward the - input. Since it cant go into this high impedance point, it goes though the feedback to the output which goes lower to sink this current.

    You can use this amp  with +/- 2.5V supplies and in the above case the output would be 0V for no light and at some negative V=I*Zf when light is present.

  • 0 in reply to taylor Smith1
    Prodigy 60 points

    thankyou for your reply taylor,

         As you said this opa380 will accept positive pulses and gives negative pulses?

         But it is not happening!!! is it will work reply me. And one more is that i am using single source +3.3v only.

         And one more i tried is connect in non-inverting mode. But as per pageno:1, internal block diagram of OPA380 is not a single op-amp, it has one more op-amp. So Noninverting mode of operation is not possible.