W09 314 HW08 all - EECS 314 Winter 2009 Homework set 8 EECS...

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EECS 314 Winter 2009 Homework set 8
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EECS 314 Winter 2009 Homework set 8
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EECS 314 Winter 2009 Homework set 8
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EECS 314 Winter 2009 Homework set 8 Student’s name ___________________________ Discussion section # _______ (Last, First, write legibly, use ink) (use ink) Instructor is not responsible for grading and entering scores for HW papers lacking clear information in the required fields above © 2009 Alexander Ganago Page 1 of 4 Problem 2 Photodiode circuits as light sensors The Big Picture Here we introduce the photodiode – a non-ohmic semiconductor device used as a light sensor (detector of light). The circuit in this problem takes advantage of the leakage current I PD through a reverse-biased photodiode: I PD increases when the ambient light gets brighter. Note that I PD is much, much smaller than the current through a forward-biased diode. Since photodiode is a non-ohmic element, the voltage across it cannot be found from Ohm’s law; in the circuit shown here it can be calculated from KVL around the loop that includes the source, the resistor and the photodiode. Recall that the current through the Gate terminal of the MOSFET transistor can be neglected. The current through the Source and Drain terminals can be large; the same current flows through the lamp. If the voltage V GS is below a certain threshold V GS < V T the MOSFET does not conduct, or acts as an open switch thus the lamp does not glow. If the voltage V GS exceeds the threshold V GS > V T the conducts, its resistance R DS becomes very small thus current flows through the MOSFET as a closed switch, and the lamp glows. Suppose that the circuit is in the dark so that the leakage current I PD through the photodiode is practically zero. Then we neglect the current through the resistor, and the entire source voltage V S is applied across the diode. Due to the connection, the same voltage V S is applied as V GS , which makes the MOSFET conduct and the lamp glow. On the other hand, if the circuit is illuminated so that I PD > 0, there develops a voltage drop across the resistor, and the voltage V GS is decreased. If V GS drops below the Part 1 2 3 Total
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EECS 314 Winter 2009 Homework set 8 Student’s name ___________________________ Discussion section # _______ (Last, First, write legibly, use ink) (use ink) Instructor is not responsible for grading and entering scores for HW papers lacking clear information in the required fields above © 2009 Alexander Ganago Page 2 of 4 threshold, the lamp is switched OFF. Thus MOSFET is used as the switch for the lamp in this circuit. For comparison, consider the circuit with two voltage dividers (so called, Wheatstone bridge) and a comparator, familiar from previous HW and shown here for your convenience. Note that the photodiode plays the
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This note was uploaded on 10/28/2009 for the course EECS 314 taught by Professor Ganago during the Winter '07 term at University of Michigan.

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W09 314 HW08 all - EECS 314 Winter 2009 Homework set 8 EECS...

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