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Unformatted text preview: -1- Biasing the BJT for Discrete-Circuit Design (see also section 5.5, S&S 5th Ed.)There are many techniques in use, each with its own advantages & disadvantages (as elaborated in section 5.5 of the 5th edition of the text by Sedra & Smith); the following discussion focuses on the "classical" technique using 4 resistors and a single power supply, followed by an overview of some common variations on this approach. Objective: The bias current(s) should be relatively insensitive to variations in temperature and to the large variations in ¡commonly encountered in transistors of the same type. Consider the prototype circuit shown on the LHS below: RE+VCCRCR1R2IBIEIC=RE+VCCRCIBIEICVBBRBBThe Thévenin equivalent of the Base circuit will be used here to simplify analysis of the prototype (as shown on the RHS above) where VBB=R2R1+R2¡VCC, and RBB=R1R2R1+R2. Kirchoff's Voltage Law applied to the base circuit then yields VBB=IBRBB+VBE+IERE=IE(1+¡)RBB+VBE+IEREwhich in turn yields the following expression for IE: IE=VBB¡VBERE+RBB(1+¢)....
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This note was uploaded on 12/21/2010 for the course EECE EECE 254 taught by Professor Robinturner,purangabomasumi during the Spring '09 term at The University of British Columbia.
- Spring '09