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Lecture 9_ Feb 7 2011

# Lecture 9_ Feb 7 2011 - Chapter 5 Bipolar Junction...

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Chapter 5 Bipolar Junction Transistors Lecture 9 Cover Sections: 5.11, 5.12

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Biasing for BJT Goal of biasing is to establish known Q-point which in turn establishes initial operating region of the transistor. For a BJT, the Q-point is represented by ( I C , V CE ) for an npn transistor or ( I C , V EC ) for a pnp transistor. The Q-point controls values of diffusion capacitance, transconductance, input and output resistances. In general, during circuit analysis, we use simplified mathematical relationships derived for a specified operation region, and the Early voltage is assumed to be infinite. Two practical biasing circuits used for a BJT are: Four-Resistor Bias network Two-Resistor Bias network Chap 5.11
Four-Resistor Bias Network for BJT V EQ = V CC R 1 R 1 + R 2 R EQ = R 1 R 2 = R 1 R 2 R 1 + R 2 V EQ = R EQ I B + V BE + R E I E 4 = 12,000 I B + 0.7 + 16,000( β F + 1) I B I B = V EQ - V BE R EQ + ( β F + 1) R E = 4 V- 0.7 V 1.23 × 10 6 = 2.68 μ A I C = β F I B = 201 μ A I E = ( β F + 1) I B = 204 μ A V CE = V CC - R C I C - R E I E V CE = V CC - R C + R F α F I C = 4.32 V F. A. region correct - Q-point is (201 μ A, 4.32 V) Chap 5.11 β F = 75

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Lecture 9_ Feb 7 2011 - Chapter 5 Bipolar Junction...

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