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OPTICAL SENSOR
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Project Objective• Minimum objective: Use the optical sensor to determine
the pulse rate in a finger.
http://www.homecaremag.com/sites/default/files/u4/Finger%20Pulse%20Oximetry%20Illustration.jpg
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Reflective Optical Sensor
Contains: • an infrared light-emitting
diode (LED)• a bipolar junction transistor
(BJT) that is missing the base connection (a phototransistor)
http://www.vishay.com/docs/83752/tcrt1000.pdf
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Visible Electromagnetic Spectrum
http://scienceblogs.com/startswithabang/files/2011/09/Visible-spectrum.jpeg
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Infrared Electromagnetic Spectrum
http://www1.infraredtraining.com/uploadedImages/InfraredTrainingcom/About/What_is_IR/EM_spectrum.jpg
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Absorption Spectrum of Silicon
• This is a plot of how well silicon (Si) absorbs light at different wavelengths.• It begins to absorb light at
~ 1.1 mm.• It strongly absorbs light
through the rest of the IR region into the visible spectrum and into the ultraviolet (UV) region.
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Light and Electron-Hole Pairs
• An electron and a hole are the two particles that move in a semiconductor. • Energy can be released
when an electron and hole destroy each other (recombination).• Light Emitting Diode
• Energy can be transformed into an electron and a hole (generation)• Phototransistorhttp://www.iue.tuwien.ac.at/phd/entner/img158.png
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Optical filter• There is an optical filter integrated into the optical sensor
package to prevent light at wavelengths other than the ones emitted by the LED from reaching the silicon transistor.
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Biasing an LED
• You should look at your notes from Microelectronic Systems as well as your notes from Fundamentals of Analog Circuits to determine how to limit the current and voltage applied to the IR LED.
http://www.vishay.com/docs/83752/tcrt1000.pdf
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BIPOLAR JUNCTION TRANSISTORS
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Three Terminal Device
• Terminals• Emitter
• The dominant carriers are emitted from the region (equivalent to the Source in a MOSFET)
• Base • These now minority carriers travel through the base region
• Some recombine in the base, forcing a base current to flow
• Collector• The remaining carriers from the emitter are collected from this
region (equivalent to the Drain)
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Types of BJTs• n-p-n
• Emitter is n+ type• Electrons flow from the emitter towards the collector
• Base is p type• Some of the electrons from the emitter recombine with the holes in the
base• Collector is n- type
• p-n-p• Emitter is p+ type
• Holes flow from the emitter towards the collector• Base is n type
• Some of the holes from the emitter recombine with the electrons in the base
• Collector is p- type
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Cross Section of npn Transistor
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Cross-Section of pnp BJT
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Circuit Symbols and Current Conventions
npn pnp
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CBE III
The one equation that will always be used with BJTs
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Circuit Configurations
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I-V Characteristic: npn Transistor
Measured in a Common Emitter ConfigurationModified from https://awrcorp.com/download/faq/english/examples/images%5Cbjt_amp_oppnt_bjt_iv_curves_graph.gif
IC = b IB when VCE > VCEsat
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Nonideal I-V Characteristic
ICEO – leakage current between the collector and emitter when IB = 0, usually equal to the reverse saturation of the base-collection diode
Effects from a change in the effective distance between emitter and collectorVA – Early Voltageb is not a constantBVCEO – breakdown voltage of the transistor
Modified from: http://cnx.org/content/m29636/latest/
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Current-Voltage Characteristics of a Common-Base Circuit
In Forward Active Region: IC = aF IE, where aF < 1
Modified from Microelectronic Circuit Analysis and Design by D. Neamen
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Simplified I-V Characteristics
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Modes of Operation• Forward-Active
• B-E junction is forward biased• B-C junction is reverse biased
• Saturation• B-E and B-C junctions are forward biased
• Cut-Off• B-E and B-C junctions are reverse biased
• Inverse-Active (or Reverse-Active)• B-E junction is reverse biased• B-C junction is forward biased
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npn BJT in Forward-Active
BE junction is forward biasedBC junction is reverse biased
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Currents and Carriers in npn BJT
iEn = iE – iEp
iCn = iC – iCp where iCp ~ Is of the base-collector junction
iEn > iCn because some electrons recombine with holes in the base
iB replenishes the holes in the base
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Current Relationships in Forward Active Region
F
FF
EFC
BFE
BFC
BCE
a
a
iai
ii
ii
iii
1
)1(
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DC Equivalent Circuit for npn in forward active
npn pnp
1nkT
qV
SE
BE
eII
1nkT
qV
SE
EB
eII
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Simplified DC Equivalent Circuit
npn pnp
VBE = 0.7V VEB = 0.7V
VCE > 50mV VEC > 50mV
IB > 0mA IB > 0mA
IC = bF IB AND IE = (bF +1) IB
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IC ~ ISC IC < bF IB
VBE = 0.75V VCE = 50mV VEB = 0.75V VEC = 50mV
Saturation
npn pnp
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Cut-Off
IC = IB = IE = 0
VBE < 0.6V VEB < 0.6V
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Phototransistor Characteristics
http://www.vishay.com/docs/83752/tcrt1000.pdf
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To detect and count the pulses
• Saturation• The transistor turns on
when light is reflected out of a finger back into the sensor.• The collector current is
limited only by the external resistors in the circuit when the base current is created by the
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To measure the amplitude and shape of the pulse
• Forward Active• The transistor turns on
when light is reflected out of a finger back into the sensor.• The collector current is a
function of the base current, which is determined by the amount of light that is reflected onto the sensor.
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Electronic Design Project• Design a circuit using the TCRT Optical Sensor:
• To bias the LED so that it emits light.• To bias the phototransistor in forward active where the maximum
light from reflected from a finger places the phototransistor close to or in the saturation region.