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bjtbias

# bjtbias - c Copyright 2008 W Marshall Leach Jr Professor...

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c ° Copyright 2008. W. Marshall Leach, Jr., Professor, Georgia Institute of Technology, School of Electrical and Computer Engineering. The BJT Bias Equation Basic Bias Equation (a) Look out of the 3 terminals of the BJT and make Thévenin equivalent circuits as shown in Fig. 1. Figure 1: Basic bias circuit. (b) Write a loop equation for the base-emitter loop. V BB V EE = I B R BB + V BE + I E R EE (c) Use the relation I C = βI B = αI E to express I B and I E as functions of the current desired. Let I C be the current. V BB V EE = I C β R BB + V BE + I C α R EE (d) Solve for I C . I C = V BB V EE V BE R BB β + R EE α An “educated guess” of the value of V BE must be made to evaluate this. Typical values are 0 . 7 V for IC transistors, 0 . 6 0 . 65 V for low-power discrete transistors, and 0 . 5 0 . 6 V for higher-power discrete transistors. (e) Check for the active mode. For the active mode, V CB > 0 . V CB = V C V B = ( V CC I C R CC ) ( V BB I B R BB ) or = ( V CC I C R CC ) ( V EE + I E R EE + V BE ) Example 1 V BB = V + R 2 + V R 1 R

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