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Super simple amp for an Infrared Detector

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resurgence2012

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Hi

I have been working on a project to demonstrate a measurement of the speed of light using infrared LED and detector.

I have used an avalanche transistor to produce short(is) (400 ns) pulses and at the moment have a circuit using a high speed IR detector diode and amp AD4001.

The AD4001 is giving me a maximum of about 200 mv when the emitter and detector are very close. In order to achieve a satisfactory result I need the emitter and detector at least 30 meters apart. At this distance it is well nigh impossible to get more than about 1 or 2 mv on the scope. So I want to use a really simple single (preferably) transistor to amplify the signal. Any suggestions please about the type of amp to use? I am considering the simplest of all 2n222 transistor in a common emitter mode, but not sure how I should bias it. I am a beginner (relatively), so please accept my apologies for asking such a basic question. Cheers
 

KlausST

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Hi,

What is your current schematic?

Klaus
 

resurgence2012

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Hi Klaus
Thanks for your reply. Here is the circuit that I tried using. (I am hoping the image has uploaded OK)

I also tried just connecting 18 volts in series with 10K ohm resistor and the IR detector and connecting the scope across the 10K resistor. I found that I could get a pretty good trace with that. I say pretty good, but the rise time was about 200 NS and the fall time much longer. However, the voltage output was high and this meant that over long distances of about 30 metres I could still get about 10 millivolts on the scope.

Using the transimpedencd amp though the pulse width is much better, but at the cost of a greatly reduce output. Ideally I would like to find a way of sticking just a single transistor at the output of the OP amp to boost the voltage before picking it up with the scope. Cheers

- - - Updated - - -

I did not see the image attached, so this is a second try

- - - Updated - - -

Hi,

What is your current schematic?

Klaus
Transimpedence Amp.JPG

Here is my third attempt at attaching the image.
 

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KlausST

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Hi,

Could you increase the value of rp? 50 ohms is a big liw.

And i'd try transimpedance? What circuit dud you test.

Klaus
 

SunnySkyguy

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Your conceptual design is flawed due to path loss.

If you use conducted pulses with no loss, you can see the prop delay at 2/3 speed of light due to dielectric constant if the substrate. Thus can either be a reflected pulse as in TDR cable fault testers or transmission delay in a long 50 ohm terminated cable viewing both input and output edges, so pulse width is irrelevant.

- - - Updated - - -

i.e, unless you use a laser with very fast rise times, you cant receive more than a few meters loss at these rise times and GBW result.

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To measure prop delay of speed of light the rise time ought to be less than the prop delay otherwise precision threshold time interval counters are needed.

I might suggest this LED for best emitter gain.
http://www.vishay.com/docs/83160/vsly5850.pdf

Or this http://www.vishay.com/docs/81002/cqy37n.pdf whichever matches the detector nm range

Then match your OTA gain and bandwidth to meet your path loss calculations for the required rise time.
 

uday mehta

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Hi

I have been working on a project to demonstrate a measurement of the speed of light using infrared LED and detector.

I have used an avalanche transistor to produce short(is) (400 ns) pulses and at the moment have a circuit using a high speed IR detector diode and amp AD4001.

The AD4001 is giving me a maximum of about 200 mv when the emitter and detector are very close. In order to achieve a satisfactory result I need the emitter and detector at least 30 meters apart. At this distance it is well nigh impossible to get more than about 1 or 2 mv on the scope. So I want to use a really simple single (preferably) transistor to amplify the signal. Any suggestions please about the type of amp to use? I am considering the simplest of all 2n222 transistor in a common emitter mode, but not sure how I should bias it. I am a beginner (relatively), so please accept my apologies for asking such a basic question. Cheers
use op-amp inverting amplifier in cascade(2 time). and add a buffer between them.
 

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