Phototransistor working principle applications

Phototransistor working principle applications

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Phototransistor Photoresistor
ECE 456 - Mechatronics Photodiode vs Phototransistor Photodiode Ideal for very fast signals but gives a much lower output signal Phototransistor Provided amplified signal but with relatively slow conversion T02 Sensors 59 © Bharat Balagopal, North Carolina State University, All Rights Reserved
ECE 456 - Mechatronics Mechanism of Photodiode Mechanism A PN-junction diode that consumes light energy to produce electric current Works in reverse bias mode of PN junction diode T02 Sensors 61 © Bharat Balagopal, North Carolina State University, All Rights Reserved V-I Characteristics of Diode pd I Op amp - + - f R E + - + - out f pd V R I = - ×
ECE 456 - Mechatronics Relation between current I and voltage V of photodiode V-I Characteristics equation for photodiode T02 Sensors 62 © Bharat Balagopal, North Carolina State University, All Rights Reserved [ ] 9 : 5 10 , ; 0.026, ; 3, ; log( 1) 1.3 , ; 0.1 , ; : 0, 500 . d t F t F S F F thermal voltage emissi where IS I A saturation current V V N V I I V V is the forward voltage I A is the forward current As on coefficient sume I A μ - = = ´ = = » + = = Î 1 t V NV I IS e æ ö = × - ç ÷ ç ÷ è ø
ECE 456 - Mechatronics Photodiode characteristics – Linear approximation Results T02 Sensors 63 © Bharat Balagopal, North Carolina State University, All Rights Reserved T02_VI_curve_photodiode.m Comments: 1. Once the current of photodiode reaches a certain amount, for example 100 μ A, we can assume the voltage drop on photodiode remains the same, or changes very little (less than 0.2V). 2. We can assume the linearity between incident light and current through photodiode when the terminal voltage of photodiode is constant or varies in small range.
ECE 456 - Mechatronics Example – Linear approximation Based on the V-I Characteristics equation and the related parameters given in previous slides, derive the equation for V as a function of I and other given parameters. Plot the I-V characteristics with current ranging from 0 to 600 μA. Suppose we are going to use the following piecewise linear function to represent the I-V characteristic of photodiode. Show the non-linearity error based on the given information. T02 Sensors 64 © Bharat Balagopal, North Carolina State University, All Rights Reserved 1 t V NV I IS e æ ö = × - ç ÷ ç ÷ è ø 3 0.7802, 1 0.2 10 0.78, 1 600 I V I I - = ì = í ´ × + < £ î
ECE 456 - Mechatronics Example – Solution Equation for I-V characteristics I-V characteristics of photodiode (actual curve and linear curve) Non-linearity error T02 Sensors 65 © Bharat Balagopal, North Carolina State University, All Rights Reserved [ ] ln 1 , 0, 600 . t I V NV with I IS æ ö = × + Î ç ÷ è ø 3 0.7802, 1 _ 0.2 10 0.78, 1 600 _ I V linear I I error V V linear - = ì = í ´ × + < £ î = - T02_VI_curve_photodiode.m