photodetector amplifier - help needed

[harryd]

photodetector amplifier

i am working on a amplifier for a photo detector( a simple common collector photo transistor circuit )

the circuit of the photo detector is working perfectly bu the amplifier is not working and i am dying to know. could you help me plz?


the circuit is just 2 inverting amplifiers ( the first one gives a gain of -10 and the other -1) i am sure that everything is connected correctly but i dont know what if this circuit is suitable to work as an amplifier for a photodetector or there is something else i should consider?

 

[Bin_Wang]

Re: photodetector amplifier

Could you paste your circuit and simulation result ,so we can help you to analysis the problem .
BTW, What is your photodetector output signal ? Current or Voltage?
What is the relation between the Output and Input Photo Signal?

 

[harryd]

Re: photodetector amplifier

i am measuring voltage and the output is ( when the laserbeam is right infront of the base of the transistor) around 4.7 voltage.

the problem is i dont have pspice to smiulate it, i will try to download it. in the meanwhile, the cirucit consists of of an IC ( 4 op amps and i am only using 2 of them ) 2 inverting amplifiers in cascade to give a gain of +10

ic = lm 324
resistors used 1k and 10 k in the first amplifer and 2 x 1k in the second

please help i am should submit this asap and i dont know why it isnot working

 

[harryd]

Re: photodetector amplifier

btw what i am trying to do is measure the beam width of a guassian beam so i am trying to design a photodetector with an amplifier

 

[FvM]

Re: photodetector amplifier

Your intention is rather unclear. By placing a laser in front of a photo transistor, it will fully saturate (as your measurement
clearly shows), there's nothing to amplify in this case.

Without knowing the power supply, nothing can be said about failure of your amplifier. Also you should show the intended
photo detector connection. What makes you think to need a multi stage amplifier with your photo detector? What's the intended light
source and expectable intensity?

Photo transistors have data sheets, you should be able to calculate the photo current for a specific measurement setup and
determine, if an amplifier is needed, or if the intensity is possibly too high without any amplifier, as in your test.

 

[keith1200rs]

photodetector amplifier

You haven't shown the whole circuit - e.g. the photo-transistor connection and the power supply connections. Your circuit won't work unless you have a negative supply for the LM324 depending on the way you have connected the photo-transistor - the output of the first opamp will try to swing below ground.

Keith.

 

[harryd]

Re: photodetector amplifier

i will use an optical fiber between the laser and the detector in the experiment . I was just saying that the detector is working and it gives reading when i place the laser infront of the transistor.

ok i am gonna tell you everything and please try to help me as much as you can

i am doing an experiment on measuring the beam width of a guassian beam. currently i am working on the photodetector, which is a simple common collector. i chose the resistor in the circuit to satisfy the equation vcc=5v> icc=2m X RL=220ohm . The circuit is working and it reads voltage when i apply any source of light to it ( sun light, laser anything ).

Now i am required to make an amplifer to amplifiy the volatge coming out of the photodetector so i can read it thru an oscliscope or anything.
i have the datasheet but i dont know what i am looking for to calculate the photocurrent

please help

Added after 7 minutes:

ok the circuit is as follows:

the photodetector

 

[keith1200rs]

photodetector amplifier

In which case the problem is that the amplifier tries to amplify a signal relative to ground. So, if the input voltage is, say, 2V, the output wil try to go to -20V! It cannot go below ground.

In general, I would say it is better to use a photodiode rather than photo-transistor in your application. Even if you try to amplify your signal relative to +5V, you still have two problems - an unknown offset voltage and the LM324 won't amplify signals close to its positive supply.

Keith.

 

[harryd]

Re: photodetector amplifier

ok what other options do i have if i must work with a photo-transistor. do u suggest any other circuit design for the the amplifier

 

[keith1200rs]

photodetector amplifier

My guess is the photo-transistor emitter signal is normally close to ground (I don't think you mentioned what voltage range it works at) in which case a non-inverting amplifier configuration should work.

Keith.

 

[harryd]

Re: photodetector amplifier

i tried it with a non inverting amplifier but it didnt work. it always gave 0 reading and some people told me that it is unstable.

could you explain what is excatly the issue with 2 inverting amplifiers. i didnt get what u said earlier

 

[keith1200rs]

photodetector amplifier

It would be helpful if you let me know what the voltage from the photo-detector is. However, you have an inverting amplifier. The non-inverting input is connected to ground. So the output will be -Vin*10. So, if you put a positive input voltage into it, it will try to go negative. It cannot go outside the power supplies of the opamp. So, it cannot go below ground. So, the output of the first opamp will always be zero (ground). Is that what happens?

Keith.

 

[harryd]

photodetector amplifier

well the input from the photo-detector will varie because i am going to measure 10% - 90% of the beam( but most probably will range from 0.5 to 4 v)

if thats the case what is the point of the inverting amplifier if any voltage will come out of it will be -ve.

and another thing is 5vcc good for the ic ?

 

[keith1200rs]

photodetector amplifier

If you have a signal that is 0.5V to 4V why do you want to amplify it?!

The LM324 is not great at 5V as the input voltage range is only 0V to 3V with a 5V supply. So, you cannot even buffer the signal with it.

Keith.

 

[harryd]

photodetector amplifier

so u sure i can read it clearly on the oscliscope ?

 

[keith1200rs]

photodetector amplifier

Of course! Haven't you tried it?

Keith

 

[harryd]

Re: photodetector amplifier

but what about the optical fiber in between?? i am sure the power disspation will affect the reading

 

[karesz]

photodetector amplifier

Hi,
You have possibly no problem with power dissipation in the fiber_only with couplings efficiency!
I mean, if you put only a fiber into tha laser light; you must calculate (accept) with smaller signals as direct rayed detector!
How much smaller_good question.
Depends of your fiber data (MM/SM/aperture/ surface polishing quality & angle of fiber surface to Laser axe), wavelength...
I would assume a possible couplings efficiency of maybe some at 50%, but you have enough signal...
K.

 

[keith1200rs]

photodetector amplifier

Do some measurements and then you will know the size of your signal and whether or not you need gain.

Keith

 

[harryd]

photodetector amplifier

ok but i have a question.. how should i detect the beam coming out of the fiber?? i mean i can barley see the a red dot from the end of the fiber... is that normal ?