[SOLVED] LED/PHOTODIODE waveform

[kdg007]

I gave square across LED but the the photodiode diode detects sawtooth waveform.. which is clearly not a square wave,,why is that >??

 

[FvM]

Most likely time constants in your circuit are causing the waveform distortion. Without referring to circuit details, the question can't be answered.

 

[kdg007]

the waveform across the photodiode looks like a sawtooth...but how come when it is connected to op-amp the wave changes to square ?
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i simply modulated an LED(connected to frequency generator 10khz)no matter what waveform i give,the shape across the photodiode is the same...


for example..for 100hz,the waveform across photodiode is square but when i increase the frequency,the waveform gradually changed to the sawtooth....

 

[FvM]

That sounds plausible. Generally the photo diode capacitance forms a low-pass with load the load resistor. With direct oscilloscope connection, the load resistance is 1 or 10 MOhm, with transimpedance amplifier it's Rf/OP-Gain, a value in a Ohm to kOhm range.

 

[kdg007]

hmm...maybe i should start from the beginning...how do i modulate a single LED properly...at what points i have to keep the connection of frequency generator >?might need a capacitor too

 

[FvM]

A standard pulse generator with 50 ohm input impedance is perfectly suited to drive a LED, if you observe maximum forward current and reverse voltage.

 

[kdg007]

i am newbie..can you explain me what u mean by observing forward current... so,if i keep a voltage supply to an LED,i cant keep a frequency generator ?
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can you provide me a proper circuit to test an LED ?@20mA current
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so that i can modulate LED and can see the output from LED in a oscilloscope..

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http://www.edn-europe.com/anledsintrinsiccapacitanceworksina650mvlrccircuit+article+5154+Europe.html

i found this article...very interesting

 

[FvM]

I mean you need to calculate the required generator voltage for 20 mA and set it accordingly.

 

[kdg007]

i gave 5v supply.. for 85ohm ,the LED is getting 20mA.. i want the function generator to drive LED directly..but is it possible to feed the function generator in the above circuit in the picture ??

 

[ZekeR]

As FvM said, the reason the voltage across the photodiode appears somewhat triangular is because of the slow time constants involved. When receiving light, a photodiode behaves essentially as a weak current source, which slowly charges a capacitance (in your case, the photodiode's junction capacitance in parallel with your oscilloscope probe's capacitance). When you connect the op-amp to it (in a transimpedance amplifier configuration), the op-amp uses negative feedback to prevent the voltage across the photodiode from changing; thus, the photodiode's weak current is no longer forced to charge or discharge a capacitance. Instead, its weak current is conveyed directly through a resistor (Rf), and the resulting voltage can be measured using an oscilloscope. This action can be relatively fast, since the only node that is required to change voltage (the output) is being driven by strong currents.

This principle —*of using negative feedback to increase response speed by preventing slow things from occurring — is common: op-amps, hot wire anemometers, cascode amplifiers, and current-feedback amplifiers all operate on the same idea.

My recommendation is to continue using the op-amp circuit. If you need to operate the circuit even faster, you can use a combination of reverse-biased photodiode, PIN photodiode, and faster op-amp.

 

[kdg007]

can you give me some material related to this so that i can study ?thank you