AN-3005 Datasheet, PDF (1/2 Pages) Fairchild Semiconductor – Design Fundamentals for Phototransistor Circuits

www.fairchildsemi.com
Application Note AN-3005
Design Fundamentals for Phototransistor Circuits
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 near-
infrared 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. How-
ever, the component has an internal amplifier (in this case a
phototransistor) which magnifies this current to useful levels.
V_CC
R_C
V_OUT
GND
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 com-
ponent and ground. The output is read at the emitter
terminal.
V_CC
V_OUT
R_E
GND
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 sur-
passes 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.
By adjusting the load resistor in the amplifier circuit one can
set the mode of operation. The correct value for the resistor
can be determined by the following equations:
Active Mode: V_CC > R_L x I_CC
Switch Mode: V_CC < R_L x I_CC
Typically a resistor value of 5kΩ or higher is adequate to
operate the phototransistor in the switch mode. The high
level output voltage in the switching mode should equal the
supply voltage. The low level output voltage in the switch-
ing mode should be less than 0.8 Volts.
The circuits just described can be applied to all two pin IR
phototransistor components that Fairchild Semiconductor
offers. They can also be applied to three pin phototransistor
components that have a base lead.
A third phototransistor circuit (Fig. 3) involves only the three
leaded components that have a base connection. Access to
the base allows a base-emitter resistor to be connected. A
high RBE value will prevent low levels of light from trigger-
ing the phototransistor and help provide a more digital out-
put. The collector and emitter terminals can be connected in
the same way as described above. Fairchild Semiconductor
offers the three leaded component in a hermetic (metal can)
package only.
Figure 2. Common-Collector Amplifier
REV. 4.00 4/30/02