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Ch10Sec1 - THE FIBER FORUM Fiber Optic Communications...

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Dr. Joseph C. Palais 10.1 1 THE FIBER FORUM Fiber Optic Communications Dr. JOSEPH C. PALAIS PRESENTED BY
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Dr. Joseph C. Palais 10.1 2 Chapter 10 Modulation
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Dr. Joseph C. Palais 10.1 3 Section 10.1 LED Circuits
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Dr. Joseph C. Palais 10.1 4 10.1.1 Analog Modulation t P SP (dP/di) = a 1 P dc I dc I SP t i i s Power P
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Dr. Joseph C. Palais 10.1 5 i I dc i s i = I dc + i s i = I dc + I SP sin ϖ t ϖ = modulation frequency P = P dc + P SP sin ϖ t modulation factor: m’ = I SP / I dc Analog modulation circuit:
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Dr. Joseph C. Palais 10.1 6 For 100% modulation: I sp = I dc and thus m’ = 1 t I dc I sp m’ = 1 0 i In general 0 ' 1 m
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Dr. Joseph C. Palais 10.1 7 The optic modulation factor is m = P SP / P dc Thus P = P dc + mP dc sin ϖ t P = P dc ( 1 + m sin ϖ t ) We earlier noted the frequency response of an LED. It was: ( 29 ϖτ = + 1 SP SP 2 a I P 1
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Dr. Joseph C. Palais 10.1 8 τ = carrier lifetime (average recombination time) a 1 = slope of the power vs. current curve Thus m = P SP / P dc = m = ( 29 1 ϖτ + 1 SP 2 1 dc a I a I 1 ( 29 ϖτ + 2 m' 1 m / m’ ϖ 1
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Dr. Joseph C. Palais 10.1 9 Transistor amplifier LED driver: R a R in 50 R b 5 k R e 60 i b i c LED (LD) b c e 2k Signal v in i E V dc = 5 V C
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Dr. Joseph C. Palais 10.1 10 50 30 10 0 1250 750 250 1 2 3 4 v ce (volts) i c (mA) i b ( μ A) Load Line = Q point (operating point)
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Dr. Joseph C. Palais 10.1 11 Current amplification: β = ( i c / i b ) = [(30 - 10)mA / (750 - 250) μ A] = 40 When there is no signal (v in = 0), the transistor operates at the Q-point.
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