03-05-2012, 03:01 PM
Design Fundamentals for Phototransistor Circuits
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The common-emitter amplifier circuit (Fig. 1) generates an
output which transitions from a high state to a low state
when light in the near-infrared range is detected by the
phototransistor. The wavelength range for light in the nearinfrared
region is about 700 nanometers (nm) to 1100 nm.
The output is created by connecting a resistor between the
voltage supply and the collector pin of the component. The
output voltage is read at the terminal of the collector. It is
called an amplifier circuit because the current generated in
the component when light is detected is very small. However,
the component has an internal amplifier (in this case a
phototransistor) which magnifies this current to useful levels.
Figure 1. Common-Emitter Amplifier
The common-collector amplifier (Fig. 2) generates an output
which transitions from a low state to a high state when IR
light is detected by the phototransistor. The output is created
by connecting a resistor between the emitter pin of the component
and ground. The output is read at the emitter
terminal.
In both circuits the phototransistor can be used in two
modes, an active mode and a switch mode. Operating in the
active mode means that the phototransistor generates a
response proportional to the light received by the component
up to a certain light level. When the amount of light surpasses
that level, the phototransistor becomes saturated and
the output will not increase even as the light level increases.
This mode is useful in applications where it is desired to
detect two levels of inputs for comparison. Operating in the
switch mode means that the phototransistor will either be
”off” (cut-off) or ”on” (saturated) in response to the light.
This mode is useful when a digital output is required for
object detection or encoder sensing.