Subject: Re: Photodiodes, phototransistors and leds...
From: "wilf_nv" <wrigter@...>
Date: Mon, 03 Oct 2005 21:06:26 -0000
To: beam@yahoogroups.com
Hi Robert,
You have the device sensitivity ranking right for "photocurrent" but
when it comes to "photvoltaic" performance, there are subtle details
that can affect things.
We can define photocurrent as a resistive current which is
proportional to the light level and voltage across the device. This
is the current that flows when a reverse voltage is applied across a
photodiode,LED and solar cell. The photocurrent of a transistor is
due to the forward voltage applied across the collector and emitter
but internally there is actually a reverse voltage across the photo
sensitive collector base junction. The photocurrent from that
junction injected into the base is amplified by the current gain of
the transistor to increase sensitivity.
The LED has the lowest photocurrent sensitivity, generating no more
than a few uA. When two LEDs are connected in series, both reverse
biassed, across a power supply, they act like a voltage divider. The
output of this LED photobridge must be connected to a very high
resistance such as a CMOS input, to avoid output loading. Similar to
a photo transistor, the photocurrent of a LED can be amplified with a
transistor (2N3904/06) to conduct up to 1mA in bright light.
The exposing a semiconductor junction causes a photovoltaic effect
whereby photons push (tunnel) electrons across the potential barrier
of the junction. This generates a photovoltaic current which is
proportional to the light level. The maximum current is developed
when connected to a short circuit. The photovoltaic effect generates
a forward voltage which is limited (clamped) by the intrinsic voltage
of the junction and by the voltage drop caused by output current.
Maximum output voltage is developed with no load attached.
Silicon photodiodes generate 0.5V-0.6V in bright light. PDs can be
connected in series to increase output voltage and connected in
parallel to increase the output current.
LEDs are made from a semiconductor material that has a higher
intrinsic barrier voltage than Silicon and can generate up to 2V open
circuit. When a LED is connected as a photovoltaic generator it can
directly turn on the gate of a low threshold power mosfet. When
connected to base of a bipolar transistor, the photo current it
generates at 0.6V will be amplified by 100 in the emitter to
collector current.
As D PD and LEDs can be connected back to back (e,g, common cathodes)
between two common emitter transistor bases to add or subtract output
current and voltage. This is just like connecting two small batteries
in series with the positive terminals commoned. The voltage
difference across the negative terminals will be zero. But with LEDs
the voltage and current are proportional to the light level and the
output across the series back to back pair will vary between +2V and -
2V. When this pair is connected between the two transistor bases,this
is enough to turn one transistor on and the other off. The (-) and
(+) inputs of an opamp can be similarly driven by LEDs or PDs.
For example, two 2N3904 transistors with common emitters and 1M
collector resistors can be differentially controlled by two LEDs in
series across the bases. A reverse Si diode must be added across each
base emitter to provide a return path for the "negative terminal" of
the series LED pair.
Well I hope this ramble sheds some light on the subject but it's all
the time I have.
wilf.
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