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From: Phil Hobbs
Subject: Re: the strange phototransistors circuit
Date: Thu, 17 Oct 2002 09:49:15 -0400
Organization: IBM T. J. Watson Research Center
NNTP-Posting-Date: 17 Oct 2002 13:49:18 GMT
X-Mailer: Mozilla 4.61 [en] (OS/2; U)
> | |------ SIGNAL IN
> | /
> LED | NPN
> | \>
> | |
> | />
> LED | NPN
> | \
> | |
> --- ------- SIGNAL OUT
> look at the circuit above. There two phototransistor in series. And
> the emitters are connected together. The signal goes from one
> phototransistor's collector to the other phototransistor's collector.
> It seems the circuit won't work,but it works well in certain
> Why does the circuit use two phototransistors in series instead of
> just one?
> How can the circuit work?
This is a cute method for allowing a phototransistor to be used with
AC. A more common method would be to connect a bridge rectifier's AC
terminals to the signal in/out lines, and put a single phototransistor
across the DC terminals, but that method has a volt or so higher drop
than this one.
Transistors actually work in both directions--you can exchange base and
collector and they'll still work at some level. Such inverted
transistors are occasionally useful, because the Vce_sat is very low (as
low as 5-10 mV). The reason all switching circuits aren't built this
way is that the beta of an inverted transistor is very low (like 5), and
the breakdown voltage is only a few volts.
Phototransistors are biased by the photogenerated carriers, without
needing an external base connection, so they'll work in inverted mode.
What you've got here is one inverted and one normally biased
transistor--the normally biased one does the dc blocking in the
OFF-state, and the inverted one basically just keeps out of the way.
When the applied voltage is positive, the top transistor is normally
biased, and when the applied voltage is negative, it's the bottom
The leading disadvantage I can think of is that the inverted transistor
will probably pass a lot less current than the normally-biased one for
the same light level (due to the low beta of the inverted connection),
so that if the load resistance is too low, it may drop enough voltage
that it gets damaged.
IBM T. J. Watson Research Center
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