EE105 Fall 2006 Microelectronic Devices and Circuits Prof Jan M Rabaey jan eecs Lecture 24 Bipolar Junction Transistor Small Signal Model Chapter 7 5 The Early Effect IC Forwardactive VBE3 Saturation VBE2 VBE1 VA VCE Current in Forward Active a Function impacted by VCE Main reason Base width modulation Hence iC I S evBE Vth 1 vCE VA 2 1 Small Signal Models Analogy from MOSFET s s model iD f v GS v DS v BS iC f v BE v CE 3 Transconductance gm The transconductance is analogous to diode conductance 4 2 Transconductance cont Forward active large signal current iC I S evBE Vth 1 vCE VA Differentiating and evaluating at Q VBE VCE diC dvBE Q q I S eqVBE kT 1 VCE VA kT gm diC dvBE Q qI C kT 5 Comparison with MOSFET Typical bias point drain coll current 100 A Select W L 8 1 nCox 100 A V2 qI C I C BJT gm kT I 100 gm C 4mS Vth 25m MOSFET gm gm Vth 2I D VGS VT 2I D W 2 Cox I D 2 100 8 100 400 S VGS VT L 6 3 BJT Base Currents Unlike MOSFET there is a DC current into the base terminal of a bipolar transistor I B I C F I S F e qVBE kT 1 VCE VAth To find the change in base current due to change in base emitter voltage iB vBE Q iB iC iC Q vBE Q 1 gm 7 Small Signal Current Gain iC F iB 8 4 Input Resistance r r 1 iB vBE Q 1 iC vBE Q gm 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 9 Output Resistance ro Why does current increase slightly with increasing vCE Collector n WB Base p Emitter n Model introduce the Early voltage iC I S e vBE Vth 1 vCE V A 10 5 Graphical Interpretation of ro slope 1 ro slope 11 The Early Effect IC Forwardactive VBE3 Saturation VBE2 VBE1 VCE VA Current in Forward Active a Function impacted by VCE Main reason Base width modulation Hence iC I S evBE Vth 1 vCE VA ro 1 IC VA 12 6 BJT Small Signal Model 13 14 7 BJT Capacitances Base charging capacitance Cb due to minority carrier charge storage mostly electrons in the base C b g m F Base emitter depletion capacitance CjE 1 4 CjEo Total B E capacitance C CjE Cb 15 16 8 17 18 9 19 20 10 21 Complete Small Signal Model 22 11 IBM SiGe Heterojunction BJT 23 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 24 12 fT vs IC From J S Dunn et al IBM J Res Dev 47 101 138 2003 25 13
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