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TL074: Drop in output voltage when connecting multiple photodiodes

Part Number: TL074
Other Parts Discussed in Thread: TINA-TI

Hi I'm new in this forum

I'm not very good at english, I've searched about transimpedance amplifiers to read a quad photodiode, I'm using the basic circuit only using a feedback resistor to amplify the current to voltage with photoconductive mode connecting the cathode to ground. When I connect only one photodiode the output is near to the desired voltage value, in this case -3.3 V or 670 uA x 4.7k ohms, but when i connect one more photodiode the output drops to -2.8 V, connecting the fourth photodiode the output voltage drops to -2.2 V. What could be the reason why the voltage drops when connecting more photodiodes?

Thanks. 

  • Hi Jesus,

    can you show us a schematic? And please tell us which photodiode you use.

    Kai

  • Hello Jesus,

    Thanks for your question and welcome to the forum!

    Do not worry about your English.  We answer questions from all over the world and your English is perfectly fine.  If you'd like, we can discuss in Spanish instead.

    As Kai has suggested, would you mind sharing a schematic with us?  It would help to make sure that we are all on the same page.

    In the meantime, I suggest you take a look at the following resources that we have on this type of circuit.  They will guide you with some design practices.  In particular, I recommend you consider adding a feedback capacitor to help avoid instability.

    Cookbook Circuit

    Design Example with OPA128

    Reference Design

    We have more resources on this topic available online, if you search.  However, I do not want to overwhelm you with documentation.

    I'm looking forward to your response.

    Regards,

    Daniel

  • Hi thanks for the reply

    My schematic is the next one

     

    The quad photodiode is the next one (I don't see a thread in the FAQ about how to attach text to a link :(  )

    https://www.mouser.mx/ProductDetail/First-Sensor/QP58-6-TO5?qs=sGAEpiMZZMsw61Ac3T98P5llBm2JXOZj

    The current trough the photodiode in my application it's about 700 uA.

    I searched a lot about transimpedance amplifiers, I've seen circuits referring to noise & bandwith but I chose the most basic circuit to do the tests, I use the photoconductive mode because my apllication is related to the sun instead of lasers, I want preccision.

    In spanish

    En resumen ese es el esquemático con el que he hecho las pruebas, la corriente que obtengo es 700 uA por lo que decidí poner una resistencia de 4.7k para obtener -3.3V de voltaje de salida pero el voltaje se baja al conectar más fotodiodos.

  • Hi Jesus,

    the TL074 is no output rail-to-rail OPAmp. I saturates at about -3.5V output voltage:

    jesus_tl074.TSC

    Do you know TINA-TI? It's a free simulator and can be downloaded here:

    Kai

  • Hi Kai

    I had not heard before about that rail-to-rail topic, I already read about that topic and looking at the graph maximum peak output ooltage vs supply voltage in the datasheet I see what you mention but still I cannot find the relationship of that with the problem I am having. Is the voltage loss at the output increasing when connecting more amplifiers?. The fact that it can have a maximum output of -3.5V for my application does not worry me if I have that maximum voltage in the 4 outputs but I do worry about having a maximum output around -2.5V which is what happens when connecting the 4 photodides.

  • Hi Jesus,

    increase the supply voltage to +/-15V for a test and use this modified circuit:

    jesus_tl074_1.TSC

    The phase lead capacitance C2 is important to restore the phase margin eroded by the photo diode capacitance. Without C2 the circuit will be unstable:

    And don't omit R2. This 100R resistor isolates the output from capacitve loads.

    Kai

  • Hello Jesus,

    There is a lot to explain here. Let's start with your base issue and then move to Kai's suggestions.

    First, what kind of current output from the diodes are you expecting when you connect more than one?  Are you expecting the currents to add such that connecting two diodes will give you about 1.34mA of current?  If that is the case, then you'll run into an output voltage limitation.  The supplies of the amplifier are +/-5V.  If we then take the datasheet limitation of the output voltage (1.5V from the rail), then we can only output up to +/-3.5V.  If we have two diodes connected with 1.34mA of current running, then we'd expect to see (1.34m * 4.7k) 6.3V at the output.  That's beyond our output's capabilities.

    So you see, this is why Kai has suggested increasing the supply voltages to +/-15V for the test simulations.  Doing this shows how the output can be corrected.

    The ability of the TLV074 to reach its output all the way to the rail is limited.  Below is the datasheet specification for a variant of the TL074.  The limitation is there under typical conditions with a light load (10k or greater).  Things can degrade when the loading gets worse.  For this reason, it may be a good idea to consider a different device depending on your final design.

    Kai has also made the excellent suggestions to add components C2 and R2.  These are for stabilizing your circuit.  Without some form of stabilization, these circuits will often have undesirable oscillation and gain peaking.  Selecting values for these components is not difficult and can be done in simulation.  You can also leave these spots unpopulated and try different component values on your prototypes board.  This is a good way to tune the value.  Again, this is a good idea to consider and we have resources available on this topic that can help.

    Please let me know if you have any further questions.  Déjame saber si hay algo que le sigue causando confusión.

    Daniel

  • Hi Daniel

    I think there was a misunderstanding with my circuit and it was my fault for not explaining it well so I will try to explain it again and talk more about the main objective of the project.
    I want to know the location of the sun, the method is used in lasers, it is very easy to find information about this, use a photodiodes quadrant to know the location of a laser, the concept is something like like this image.


    The full schematic is the next one.


    I thought that this had been understood from the beginning but I did not explain well.

    I'm using a quad op amp because I want to read each photodiode individually, when I say to connect more than one photodiode it's not to the same input rather it's to connect a photodiode to another op amp. When I only have one photodiode connected the output is around 3.3V, but when I connect another photodiode to another op amp the voltage drops, when the 4 photodiodes are connected to their respective op amp, the current generated by the sun in the photodidodes it's in the range of 600uA-700uA.

    Si hay más dudas mejor contestaré en español que para el inglés no soy bueno.

  • Hi Jesus,

    maybe the intensity of direct sunlight is too high for your photodiodes? Keep in mind that the four photodiodes might share a common doped crystal region on die at the cathode side. This could cause unwanted feedback effects either by voltage drops or even by reducing the depletion depth of individual pn-junctions. This could change the sensitivity. 

    Put sun classes in front of the photodiodes and check again. But first modify your circuit by putting a 100pF cap in parallel to each potentiometer (from the -input of OPAmp to the output of OPAmp).

    I would take four BPW34 for your project. They are much cheaper and can also easily be arranged in a square. And they can handle direct sunlight (at least in Germany :-).

    Kai

  • Hi Jesus,

    Yes, this clarifies things now.  If you measure the current from each portion of the photodiode and note that it drops when each amplifier is connected, then I would assume the issue is probably with the diode as Kai has pointed out.  I can't imagine a property of the op amp that might cause this to occur.

    I've attached TINA-TI simulation files for the schematic with Kai's suggested stabilizing capacitor.  You can use them to run some sims and verify things if you'd like.  There should not be any stability issues.

    You may also try a different photodiode and see if the problem persists.

    Let us know if you need further help.

    Regards,

    Danieljesus_tl074_adjusted.TSCjesus_tl074_adjusted_stability.TSC

  • Thanks for your help.
    Since the problem is most likely due to photodiodes rather than op amp, I see no reason to continue with this question.

  • Jesus,

    I hope you are able to resolve your issue by pursuing the photodiode path.

    Please, feel free to reach out to us again in the future if you need help.

    Regards,

    Daniel