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'[OT:] Why ate Ifrared LED receivers shown back to '
2004\03\31@002651 by Hopkins

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part 1 636 bytes content-type:text/plain; (decoded quoted-printable)

I have seen several infrared receiver circuits were the anode is connected to the negative rail and cathode connected towards the positive rail.

What am I missing here?

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Roy Hopkins   :-)

Tauranga
New Zealand
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part 2 15764 bytes content-type:application/octet-stream; (decode)

2004\03\31@014818 by Russell McMahon

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> I have seen several infrared receiver circuits were the anode is connected
to the negative rail and cathode connected towards the positive rail.

> What am I missing here?

The transistor polarity is normal.

The transistor functions as an emitter follower with a gain of (essentially)
1.
This acts as a buffer for the photodiode so it "sees" a high impedance load.
The output is lower impedance than the diode by itself (but not very low).

The 47uF and 10k act as a filter to remove power supply noise.



       RM

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2004\03\31@014819 by Brent Brown

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> I have seen several infrared receiver circuits were the anode is
> connected to the negative rail and cathode connected towards the
> positive rail.
>
> What am I missing here?

Because that's how photodiodes work. Reverse leakage current increases
with indcident light.

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2004\03\31@020516 by hael Rigby-Jones

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Right answer to the wrong post Russell? ;')

A photo-diode can be operated in one of two ways.  It can be used as a small
"solar cell", i.e. it will generate a small current when exposed to light.
If used in this manner, the device would be forwards biased.

The other property of photodiodes is that the "leakage" current when
reversed biased is proportional to the intensity of the light it is exposed
to.  This is how the device is used in the circuits you have seen.  This
method has the advantage of moving carriers from the depletion region out
very quickly, and gives the device a much faster response than using it
forwards biased.

Typicaly you can get PIN based photodiodes that can operate with a bandwidth
of 7GHz or even higher for high speed data over optical fibres.

Regards

Mike




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2004\03\31@140728 by Hopkins
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STOP THE PRESS - thanks guys did not know that - WOW have learnt something.
> A photo-diode can be operated in one of two ways.  It can be used as a
small
> "solar cell", i.e. it will generate a small current when exposed to light.
> If used in this manner, the device would be forwards biased.
>
> The other property of photodiodes is that the "leakage" current when
> reversed biased is proportional to the intensity of the light it is
exposed
> to.  This is how the device is used in the circuits you have seen.  This
> method has the advantage of moving carriers from the depletion region out
> very quickly, and gives the device a much faster response than using it
> forwards biased.
>
> Typicaly you can get PIN based photodiodes that can operate with a
bandwidth
> of 7GHz or even higher for high speed data over optical fibres.
>
> Regards
>
> Mike



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