amping a photodiode help

[blackbunny]

I need to amp up an rgb photodiode (3 diodes for each channel), however the diodes are connected with common cathode. All the amp circuits that I have looked at connect cathode as a unique channel. Therefore I can’t extract each channel.

Can I please have some suggestion?

Vf = 0.5-1.3V
Vr = 10V

 

[Torben]

What exactly do you mean by "amp up"?


Torben

[Edit: Can you provide a datasheet or part number for the LED?]

 

[blackbunny]

just a transresistance amplifier, to convert tiny current into a reasonable voltage that i can do something with.

however the 3 diodes have a common cathode ffs (4 pins, 1 cathode, 3 anode). if they would have had common anode then i would have easily used a simple photovoltaic amplifier.

**broken link removed**

however i cant use that circuit as i would be connecting all cathodes. i want to somehow extract each channel.

 

[Roff]

The circuit will still work if you reverse the photodiode. Your output voltage will be negative instead of positive. If you need it to be positive, add an inverting op amp at the output.

 

[blackbunny]

The circuit will still work if you reverse the photodiode. Your output voltage will be negative instead of positive. If you need it to be positive, add an inverting op amp at the output.

i dont think current will flow if the diode is reverse biased :S. correct me if im wrong

 

[Sceadwian]

The diode won't be biased at all connected like that.

 

[Roff]

With the circuit you posted, the diode has no voltage across it, due to the virtual ground on the cathode. If you turn it around, it will still have zero volts across it. Light will cause current to flow, always from cathode to anode (conventional current). If you reverse bias it, photocurrent will flow, and in fact the response will be faster than in the zero bias mode, as in your circuit. The advantage of zero bias is that you will have zero dark current. With reverse bias, you will still get leakage current, even with no illumination.

 

[blackbunny]

With the circuit you posted, the diode has no voltage across it, due to the virtual ground on the cathode. If you turn it around, it will still have zero volts across it. Light will cause current to flow, always from cathode to anode (conventional current). If you reverse bias it, photocurrent will flow, and in fact the response will be faster than in the zero bias mode, as in your circuit. The advantage of zero bias is that you will have zero dark current. With reverse bias, you will still get leakage current, even with no illumination.

so what would be an appropriate amplifier circuit for this sensor?

datasheet

 

[audioguru]

Any low noise opamp will work with the sensor.

 

[Roff]

so what would be an appropriate amplifier circuit for this sensor?

datasheet

What are you actually trying to do, i.e., where do your RGB (light) signals come from, and what are you going to do with the outputs of the amplifiers?

 

[blackbunny]

What are you actually trying to do, i.e., where do your RGB (light) signals come from, and what are you going to do with the outputs of the amplifiers?

rgb lights come from sources such as an lcd screen for example.

ill hook up the rgb outputs to 3 adc pins good old atmega88.

im still not sure about the amplifier circuit for this thing. as it has common cathode. how can i extract each channel?

 

[mneary]

You just need to turn your amplifier up side down

Connect the cathodes and + inputs of the opamps to Vref. Light on the diodes will generate a voltage which subtracts from Vref, but that conversion is simple arithmetic.

You need to set Vref within the common mode range of the opamp. Use almost anything that includes the positive rail in its common mode range. A quick glance suggests to me that a TL084 would do with Vref=Vcc=+5V. LM324 might do if your Vcc is 5V or more and your Vref is less than 3V.

 

[blackbunny]

You just need to turn your amplifier up side down

Connect the cathodes and + inputs of the opamps to Vref. Light on the diodes will generate a voltage which subtracts from Vref, but that conversion is simple arithmetic.

You need to set Vref within the common mode range of the opamp. Use almost anything that includes the positive rail in its common mode range. A quick glance suggests to me that a TL084 would do with Vref=Vcc=+5V. LM324 might do if your Vcc is 5V or more and your Vref is less than 3V.

u mean something like this?

 

[Roff]

u mean something like this?

I thought you had a common cathode device.
How much speed do you need (risetime, bandwidth, or whatever)? Can you tolerate offset due to dark current?

 

[blackbunny]

I thought you had a common cathode device.
How much speed do you need (risetime, bandwidth, or whatever)? Can you tolerate offset due to dark current?

yeah i do have common cathode, so u mean just switch the diode around in that diagram?

dark current is insignificant, other things like risetime etc are also insignificant from my knowledge.

 

[Roff]

yeah i do have common cathode, so u mean just switch the diode around in that diagram?

dark current is insignificant, other things like risetime etc are also insignificant from my knowledge.

Bandwidth/risetime requirements depend on the application. You didn't actually tell us what you plan to do with this, so we can't determine bandwidth requirements. The choice of op amps, diode biasing, feedback resistance, etc., all depend on bandwidth requirements.

 

[blackbunny]

Bandwidth/risetime requirements depend on the application. You didn't actually tell us what you plan to do with this, so we can't determine bandwidth requirements. The choice of op amps, diode biasing, feedback resistance, etc., all depend on bandwidth requirements.

all i simply want is to have this diode amplified with separate channels, output some voltage that i can connect to the three adc pins on my avr chip to do something with it.

 

[rezer]

The 3 amplifier circuit does not look like it will work. All three diodes are in parallel. What effects one branch will effect all branches. If this is what you want, just use one op-amp. It will do the same thing.
Why not use 3 seperate photo diodes with seperate op-amps?

 

[Roff]

What power supply voltage(s) do you have available? Do you have a preferred op amp? It needs to have FET inputs.

 

[Roff]

so what would be an appropriate amplifier circuit for this sensor?

datasheet

The datasheet says it is common anode.