# Photodiode circuit Op Amp and amplification failure

I am trying to build a photodetector circuit to detect and amplify the output signal of a fiber laser. I started with testing my fiber pigtailed photodiode on a breadboard as shown.

simulate this circuit – Schematic created using CircuitLab

It worked as I expected. Voltage increased as the light output increased etc. My first question is,

When I increased the light output, the voltage value increased with a nice sensitivity but as the output increased voltage value started to change only with broad output intervals and then at one point, increasing light output did not change anything.Here are my measurements:

Up to 2.3 mW light output power voltage quickly reaches 9.4 V then until 14 mW power it only rises to 9.6V and it stays around 9.62 V for further increments.

Am I missing something very fundamental here? I was expecting the voltage value to keep rising as I increased the light.

Next I wanted to build an amplifier circuit to amplify the signal. The circuit I tried is the same as the one in the figure. I didn't use the optional resistor. Specs of the circuit are:

• Rf= 10k to 1M Ohms

• Cf= 1pF
• Photodiode Bias: -9V
• OpAmp +-Vss: +-5V

simulate this circuit

However when I connected everything, power supply showed that -Vss was using more than 7 Volts and OpAmp started to get very (smoking)hot. And before it got hot I didn't get any useful value from the output. It showed something around -5 volts which was surprising because the signal must have been inverted and therefore it had to be positive. Nothing changed as I changed the light output.

I tried the same circuit by using 7V batteries as Vss sources with a new OpAmp. Again I did not get anyting. The voltage at the output was lower than the value with only 10k test circuit and as I increased the Pf up to 1M, voltage dropped. This is again very interesting to me because I know that Rf determines the gain and as it increases output signal should incrase. One other thing is that the output voltage is always around the Vss. Could this be a clue for something?

What is wrong with my circuit? What causes these weird results? Is there something wrong with my OpAmp? This seemed something quite easy to build but this is my second wasted week on this. Please explain things as clear as possible. I feel like a caveman in a spaceship. I can't proceed further. Please help this poor man.

Edit: To be more clear, all the voltage values which describes -Vss is negative. I did not neither gave nor read any positive voltage value concerning -Vss of OpAmp.

• Where should that extra voltage come from? You have a 9V supply plus a tiny bit of photovoltaic... Mar 20, 2018 at 12:23
• Start with "the op amp got smoking hot"! and work out from there. Why do you have +5V on -Vss? As an aside, your circuit is set up to provide a negative voltage output with increasing light. Mar 20, 2018 at 12:48
• The photodiode will produce current that goes toward the negative input terminal, which is pinned at 0V. This is forced through the feedback resistor, producing a negative output voltage. Mar 20, 2018 at 13:36
• No. You built a transimpedance amplifier -- which is the appropriate amplifier for your diode. If anything, you might consider turning the diode around and biasing with +9V. If you don't need speed, in fact, you could just hook the anode to the negative input, and the cathode to the grounded positive input. Leave the feedback resistor in place. Mar 20, 2018 at 14:26
• Sorry-Can't help. There's nothing wrong, per we, as lkkng as the wild can take that reverse bias, and the circuit is wired as you drew it here. You're in "mistake" territory, I think. Start again with fresh parts, unless you can convince yourself the PD is still working. Certainly a fresh op amp is in order. Mar 21, 2018 at 0:20

I think I can help you out with the first part of your question, if you see the I-V characteristics of the photodiode, you will find that the current increases linearly for some time, then it becomes non-linear and saturates as you approach the bias voltage, and as you are getting the voltage across the 10k resistor,the maximum voltage that you can obtain is I(Sat)*10k.

As for the Op-amp circuit, your photodiode will be acting like a current source,so I think you should try it by using a resistor in series with the photodiode, so that you can give an equivalent input to the opamp.(Not sure about this part)

• Thank you! Finally some light appeared at the end of this very dark and long tunnel. Now I know why I get that weird result during the test part. Photodiode gets saturated. Mar 20, 2018 at 13:37
• Saturation might not be the reason for your weird results coming from your op amp.I m not really sure about what might the reason maybe, hopefully, someone from the community will be able to answer that Mar 20, 2018 at 13:40
• Oh I was talking about the first part at which the voltage value on 10k increased and then almost stopped increasing with increasing light output. I checked a few documents and the situation fits perfectly. Thank you. Mar 20, 2018 at 14:09

I didn't use the optional resistor.

Problem #1: If you actually open-circuited the feedback loop, you caused IN- to go to -9V, which may have destroyed the opamp (since -9V is outside the range +/-5V of your supplies, it could cause a large current to flow through the input protection diodes).

Problem #2: You probably shouldn't be using the opamp ADA4897-1. This is a high frequency opamp with 200MHz bandwidth, meaning that without proper physical circuit layout it will go into oscillation. This would cause it to heat up and possibly destroy it. As a beginner I would strongly advise you to use something more forgiving like TL082 (which also comes in DIP package).

It is also worth noting that RF oscillation is a probable cause for the strange step-like changes in DC bias you saw.

Problem #3: You need to put a resistor in series with the output of this circuit. Coaxial cables will present a capacitive load to the opamp and could cause it to oscillate. A standard 50 ohm resistor should suffice.

Comment: I am highly skeptical of this photodiode saturation idea, as I have never seen this occur in practice and I have done a lot of optical setups.