Prelim 2 spring 08 - April 8, 2008 ECE 315 Prelim # 2...

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April 8, 2008 ECE 315 Prelim # 2 Solution Spring 2008 This exam is closed book but with 1 page (8.5 by 11 inches) full of notes of your choosing. Use calculator to get numerical answers. Show all work! Take 90 minutes. 1. Consider a power n channel MOSFET whose output characteristic in saturation or active mode is given by: I D = k P 2 ( V GS - V T ) 2 (1 + λV DS ) . Now consider that we want to include an internal series resistance in the drain part of the transistor in the model as shown below in Figure 1. Figure 1. The nmos device with R ser added to the model. a) (10 pts) Modify the above equation to include the effects of the internal series resistance R ser . In other words find an equation for I R D (the drain current with the series resistance included) which depends on V GS , V DS and R ser . First we need to express V DS as a function of I D so that ohm’s law can be used to add to the series resistance drop to the drain source voltage. Re arranging the equation above, we can write: V DS = 1 λ ± 2 I D k P ( V GS - V T ) 2 - 1 ² . Now we add the ohmic drop on R ser and replace I D with I R D : V DS = 1 λ ± 2 I R D k P ( V GS - V T ) 2 - 1 ² + I R D R ser . Solving this equation for I R D yields the answer as: I R D = 1 + λV DS 2 k P ( V GS - V T ) 2 + λR ser . The output characteristics for the model excluding the internal series resistance is plotted below in Figure 2 for a value of V GS = 2 volts.
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ECE315 April 8, 2008 Page 2 Figure 2. Power nmos output curves with V GS = 2 volts. b) (10 pts)
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This note was uploaded on 05/27/2008 for the course ECE 3150 taught by Professor Spencer during the Spring '07 term at Cornell University (Engineering School).

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Prelim 2 spring 08 - April 8, 2008 ECE 315 Prelim # 2...

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