1. Optical Sensor

2. Project Objective
Minimum objective: Use the optical sensor to determine the pulse rate in a finger.

3. Reflective Optical Sensor
Contains:
an infrared light-emitting diode (LED)
a bipolar junction transistor (BJT) that is missing the base connection (a phototransistor)

4. Visible Electromagnetic Spectrum

5. Infrared Electromagnetic Spectrum

6. 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.

7. 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)
Phototransistor

8. 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.

9. 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.

10. Bipolar Junction Transistors

11. Three Terminal Device Terminals Emitter Base Collector
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)

12. Types of BJTs n-p-n p-n-p Emitter is n+ type Base is p 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

13. Cross Section of npn Transistor

14. Cross-Section of pnp BJT

15. Circuit Symbols and Current Conventions
npn
pnp

16. The one equation that will always be used with BJTs

17. Circuit Configurations

18. I-V Characteristic: npn Transistor
IC = b IB when VCE > VCEsat
Measured in a Common Emitter Configuration
Modified from

19. 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 collector
VA – Early Voltage
b is not a constant
BVCEO – breakdown voltage of the transistor
Modified from:

20. Current-Voltage Characteristics of a Common-Base Circuit
For an n-p-n BJT, a particular value of IE is set, VBC is swept by changing V+ and V-, and IC is measured.
In Forward Active Region: IC = aF IE, where aF < 1
Modified from Microelectronic Circuit Analysis and Design by D. Neamen

21. Simplified I-V Characteristics

22. Modes of Operation Forward-Active Saturation Cut-Off
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

23. npn BJT in Forward-Active
BE junction is forward biased
BC junction is reverse biased

24. 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

25. Current Relationships in Forward Active Region

26. DC Equivalent Circuit for npn in forward active
pnp

27. Simplified DC Equivalent Circuit
IC = bF IB AND IE = (bF +1) IB
npn
pnp
VBE = 0.7V VEB = 0.7V
VCE > 50mV VEC > 50mV
IB > 0mA IB > 0mA

28. Saturation IC ~ ISC IC < bF IB pnp npn
VBE = 0.75V VCE = 50mV VEB = 0.75V VEC = 50mV
pnp
npn

29. Cut-Off
IC = IB = IE = 0
VBE < 0.6V VEB < 0.6V

30. Phototransistor Characteristics

31. 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

32. 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.

33. 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.