>-----Original Message-----
>From: Russell McMahon [mailto:apptech@PARADISE.NET.NZ]
>Sent: 31 March 2004 06:47
>To: PICLIST@MITVMA.MIT.EDU
>Subject: Re: [OT:] Why ate Ifrared LED receivers shown back to front?
>
>
>> 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.

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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