examples - b) Let us find first I EQ V CC – V BE = I EQ R...

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ELE311 EXAMPLES 1. For the circuit shown below, determine the differential and common mode input resistances calculate R id and R ic respectively. (2002 Midterm I) ANSWER: I 0 = I REF = (V CC – V BE – V EE ) / R 1 = (5-0.7+5)/18.6k = 0.5mA I E1 = I E2 = I 0 / 2 = 0.25mA h ie (h fe + 1) V T / I EQ = 101 x 26mV / 0.25mA = 10.8k Ω In the differential mode, i.e. v i2 = -v i1 . R id = 2h ie = 2 (10.4k) = 21.6k Ω In the differential mode, i.e. v i2 = v i1 . Let us define r o1 = 1/h oe1 , r o2 = 1/h oe2 , r o4 = 1/h oe4 R ic = (h fe +1)(r o4 || r o1 || r o2 ) = (100 + 1)(200k || 400k || 400k) = 10.1 M Ω . Q 1 Q 2 h fe = h FE =100 V BE = 0.7V 1/h oe1 =1/h oe2 = 400k Ω 1/h oe4 = 200k Ω
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2. For the circuit shown below, a) Identify the type of feedback b) Find input resistance R if c) Find the gain A f = v o / i s . (2002 Midterm I) ANSWER: a) Voltage-parallel feedback
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Unformatted text preview: b) Let us find first I EQ V CC – V BE = I EQ R E + I BQ R B I EQ = (5 – 0.7) / (500 + 100k/(50+1) ) = 1.75mA h ie ≅ (h fe +1)V T / I EQ = 51 x 26mV / 1.75mA = 758 Ω Let write down, the open-loop (i.e. without feedback) input resistance R i R i = R 1 || R 2 || h ie = 10k || 100k || 758 = 700 Ω , the open-loop gain A A = v o /i s (open-loop) = -(R C || R 2 ) h fe i b / [h ie i b / R i ] = -909 x 50 / (758 / 700) = -42 k Ω and the feedback gain β β = i f / v o = -1/R 2 = -100 μΩ-1 R if = R i / (1+ β A) = 700 / (1 + 100 μ x 42k ) = 700 / 5.2 = 135 Ω c) A f = A / (1+ β A) = -42k / (1 + 100 μ x 42k ) = 42k / 5.2 = -8.08k Ω h fe = h FE =100 V BE = 0.7V...
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examples - b) Let us find first I EQ V CC – V BE = I EQ R...

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