EECS 105 Spring 2005 Lecture 29 R T Howe Lecture 29 Last time Transistor action large signal operation Today Ebers Moll model Small signal model of the npn bipolar transistor Dept of EECS University of California Berkeley EECS 105 Spring 2005 Lecture 29 R T Howe Transconductance cont Forward active large signal current iC I S e vBE Vth 1 vCE V A Differentiating and evaluating at Q VBE VCE Dept of EECS University of California Berkeley EECS 105 Spring 2005 Lecture 29 R T Howe Comparison with MOSFET gm Bipolar transistor MOSFET Typical bias point drain coll current 100 A Select W L 8 1 nCox 100 A V2 Dept of EECS University of California Berkeley EECS 105 Spring 2005 Lecture 29 R T Howe What about the Base Current Unlike MOSFET there is a DC current into the base terminal of a bipolar transistor I B I C F I S F eVBE Vth 1 VCE Vth To find the change in base current due to change in base emitter voltage iB v BE Dept of EECS Q iB iC iC v Q BE Q University of California Berkeley EECS 105 Spring 2005 Lecture 29 R T Howe Small Signal Current Gain o Dept of EECS University of California Berkeley EECS 105 Spring 2005 Lecture 29 R T Howe Input Resistance r r 1 iB v BE Q In practice the DC current gain F and the small signal current gain o are both highly variable 25 Typical bias point DC collector current 100 A Dept of EECS University of California Berkeley EECS 105 Spring 2005 Lecture 29 R T Howe Output Resistance ro Why does current increase slightly with increasing vCE Model math is a mess so introduce the Early voltage iC I S e vBE Vth 1 vCE V A Dept of EECS University of California Berkeley EECS 105 Spring 2005 Lecture 29 R T Howe Graphical Interpretation of ro Typical value Dept of EECS University of California Berkeley EECS 105 Spring 2005 Lecture 29 R T Howe BJT Small Signal Model Dept of EECS University of California Berkeley EECS 105 Spring 2005 Lecture 29 R T Howe BJT Capacitances Base charging capacitance Cb due to minority carrier charge storage mostly electrons in the base Cb g m F Base emitter depletion capacitance CjE 1 4 CjEo Total B E capacitance C CjE Cb Dept of EECS University of California Berkeley EECS 105 Spring 2005 Lecture 29 R T Howe Complete Small Signal Model Dept of EECS University of California Berkeley EECS 105 Spring 2005 Lecture 29 R T Howe IBM SiGe Heterojunction BJT Dept of EECS University of California Berkeley EECS 105 Spring 2005 Lecture 29 R T Howe SiGe BJT CMOS vs RF CMOS From IBM and Cadence collaborate to accelerate siliconaccurate design of advanced RF integrated circuits IBM Microelectronics Division March 11 2005 Dept of EECS University of California Berkeley EECS 105 Spring 2005 Lecture 29 R T Howe fT vs IC From J S Dunn et al IBM J Res Dev 47 101 138 2003 Dept of EECS University of California Berkeley
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