Hi everyone. I have a photodiode, photoconductive mode, and I need to amplify the signal comming out of it. How is this done? Can I just use an op-amp? Thank you.

 

You can definitely use an opamp.

What exactly is your application?

hgmjr

 

Hello hgmjr. I am using pulsed LEDs into a photodiode as an input into a logic block. When hardwired, the pules are sent directly and the circuit operates as it should. I then added the LED and photodiode and the circuit does not work. This makes me think that the signal is not strong enough. I have not recorded the output voltages of the photodiode yet, I was just wondering if amplifying the output of a photodiode is common. I was thinking a non-inverting opamp would work? Thanks for your help!

 

Can you provide us with a schematic of your circuit so that we can try to help you diagnose your problem?

hgmjr

 

A photodiode has an extremely low output current. Of course an opamp is needed to amplify it to a useable level.
The photodiode can be used in two different modes:

 

Hi hgmjr, I don't currently have a schematic drawn up yet. It is a fairly large circuit so it would take much to do so. I will do so if the amplifier does not solve the problem.

Audioguru - this looks like what I need. Could you provide me with the link to the website you got this from? So a LM741 would work for A1 in the photoconductive mode? Also, why is the photodiode in this picture connected to -30V rather than ground? I am a newbie to photodiodes so I do not understand this. Thanks again for the help.

 

The photodiode is connected to -30V to reverse bias the pn-junction. Since the pn-junction is reversed biased (larger junction), then the capacitance decreases. This is called photoconductive mode. As a rule of thumb, the amplifier will remain stable over a larger bandwidth. Capacitance at the summing junction is bad.

I would not suggest the LM741, as I believe it has high bias currents and could swamp your photodiode signal. The OPA129 is what I would suggest.

 

robby991,

What supply voltage or voltages are you proposing to use in your design?

hgmjr

 

The pulses into my logic block are 1.4V (hardwired), which is what I need the output of the photodiode to be. The photodiode I am using is this:

http://www.osioptoelectronics.com/products/13-14_PhotoCondSer_OSIOpto.pdf

It is the PIN-10D.

To test it what I did was ground it by tying a wire around the base of it, and then connected a wire to the output pin in the back of it where i recorded the voltages. When doing this I got .3V output with just the light around me shining in, and .324 V when I shine the red LED I am going to use into it. Is this the correct way to use a photodiode? How do I supply 30V to it if there is no place to power it?

 

The opamp in the Photoconductive Mode for the photodiode has a positive and negative supply. Simply connect the anode of the photodiode to the negative supply instead of to -30V.

 

What power supply voltages do you have available to work with?

hgmjr

 

Hello. The supply voltages for the opamp are around +/-18V, the required input to my logic block has to be around 1.3V, and 5V powers the logic gates. I think the power supply I am using goes up to 30V. Is this what you are looking for hgmjr? I am going to build an amplifier tomorrow and let you know how it works. Thank you!

 

Hi everyone. I ran a preliminary test using an LM741 opamp and my photodiode (with BNC connector) and I would like to hear what people think about what is going wrong. My set up is the photovoltaic mode from page 3 of the following document.

www.aptechnologies.co.uk/PDF/PD Theory of Operation.pdf
LM741 data sheet: http://www.datasheetcatalog.com/datasheets_pdf/L/M/7/4/LM741.shtml

I am supplying +15 V into pin 7 and -15 into pin 4, as well as a 47k resistor between pin 2 and the output pin 6.

Even before I attach the photodiode to the opamp I get about .3V when I probe it with the multimeter in a well lit room.

Following the circuit diagram above, I attach the anode to pin 3 and the cathode to pin 2. My output voltage is -13.72, which is incorrect. When I reverse the anode and cathode I get 15V, the supply voltage. I am new to electronics so I feel like I am missing a crucial element in this set up. Anyone have any thoughts?!



p.s. I also set up the photoconductive mode circuit in the same document (using -15V as Vr) and I got the supply voltage, 15V, and with the anode and cathode reversed I got 10.6V

Thanks for your help!!!

 

The 0V of the power supply must connect to pin 3 of the opamp.
In the photo-voltaic mode, the photo-diode is a solar cell.

With the 47k resistor connected but no photo-diode then the output of the opamp should be 0V.
A 741 opamp is too old and has an input current much too high for this function. Its bandwidth is very narrow.

I think you got the anode and the cathode of the photo-diode mixed up. In the photo-conductive mode it is supposed to be reverse-biased then it leaks a small current when light shines on it. Yours must have been forward biased for it to conduct hard enough for the output to be +15V then it worked normally when you reversed it.