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S06hw4s - EE3220 Homework#4 Solutions March 3 2006 8.55...

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EE3220 Homework #4 Solutions March 3, 2006 8.55 Consider the BJT differential amplifier below. When a common-mode signal is applied to v 1 and v 2 simultaneously, the voltage of node E will rise and fall with a value equal to the applied signal minus a single voltage drop V f contributed by the parallel base-emitter junctions of Q 1 and Q 2 . How negative can v icm become before Q 3 ceases to operate as a dc current source? Use r 0 = 50 kΩ, V f = 0 . 6 V and β 0 = 100. V EE Q 2 Q 1 V CC v OUT1 v OUT2 Q 3 R A R B R E R C 3.9 k 9.1 k 470 5 k 15 V -15 V R C 3.9 k Find the emitter voltage of Q 3 V B 3 = V EE - V EE R B R A + R B = - 1 . 3 V V E 3 = V B 3 - V f = - 1 . 9 V The amplifier will stop working when the collector voltage reaches V C 3 = V E 3 + V sat = - 1 . 9 V + 0 . 2 V = - 1 . 7 V 1
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This voltage will be produced by the common mode input and base-emitter junction volt- ages of Q 1 and Q 2 V B - V f = V C 3 V B = - 1 . 1 V Thus, if the signal reaches -1.1 V, Q 3 will stop operating as a current source. 8.57 For the current-source bias circuit below: R E R C Q 1 Q 2 v IN R A D 1 D 2 v OUT V CC V EE 10 V -10 V 50 k 1 k 180 a) For R A = 50 kΩ and R E = 180 Ω, determine the value of the bias current I C 2 . Using the constant-current approximation V B = V EE + 2 V f = - 8 . 6 V V RE = V B - V f - V EE = 0 . 7 V I C 2 I E 2 = V RE R E = 3 . 9 mA b) What are the resulting values of I C 1 , V CE 1 , and V OUT ? I C 1 I C 2 = 3 . 9 mA V C 1 = V CC - I C 1 R C = 6 . 1 V V CE 1 = V C 1 + V f = 6 . 8 V V OUT = - V f = - 0 . 7 V c) Find the gain v out /v in .
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