Lecture 5 ANNOUNCEMENTS HW1 will be considered as extra credit HW3 will be posted tonight due Tuesday 9 18 Monday 3PM 247 Cory discussion section has room OUTLINE BJT cont d Transconductance Small signal model The Early effect BJT operation in saturation mode Reading Chapter 4 4 3 4 5 EE105 Fall 2007 Lecture 5 Slide 1 Prof Liu UC Berkeley Notes on PN Junctions Typically pn junctions in IC devices are formed by counter doping The equations provided in class and in the textbook can be readily applied to such diodes if NA net acceptor doping on p side NA ND p side ND net donor doping on n side ND NA n side ID A I D I S e qVD kT 1 Dn Dp I S Aqni L N L N n A p D 2 VD V EE105 Fall 2007 Lecture 5 Slide 2 Prof Liu UC Berkeley Transconductance gm The transconductance gm of a transistor is a measure of how well it converts a voltage signal into a current signal It will be shown later that gm is one of the most important parameters in integrated circuit design dI C d VBE I S exp gm dVBE dVBE VT 1 VBE g m I S exp VT VT IC gm VT EE105 Fall 2007 Lecture 5 Slide 3 Prof Liu UC Berkeley Visualization of Transconductance gm can be visualized as the slope of the IC vs VBE curve The slope hence gm increases with IC EE105 Fall 2007 Lecture 5 Slide 4 Prof Liu UC Berkeley Transconductance and IC For a given VBE swing V the resulting current swing about IC2 is larger than it is about IC1 This is because gm is larger when VBE VB2 EE105 Fall 2007 Lecture 5 Slide 5 Prof Liu UC Berkeley Transconductance and Emitter Area When the BJT emitter area is increased by a factor n IS increases by the factor n For a fixed value of VBE IC and hence gm increase by a factor of n EE105 Fall 2007 Lecture 5 Slide 6 Prof Liu UC Berkeley Derivation of Small Signal Model The BJT small signal model is derived by perturbing the voltage difference between two terminals while fixing the voltage on the third terminal and analyzing the resultant changes in terminal currents This is done for each of the three terminals as the one with fixed voltage We model the current change by a controlled source or resistor EE105 Fall 2007 Lecture 5 Slide 7 Prof Liu UC Berkeley Small Signal Model VBE Change EE105 Fall 2007 Lecture 5 Slide 8 Prof Liu UC Berkeley Small Signal Model VCE Change Ideally VCE has no effect on the collector current Thus it will not contribute to the small signal model It can be shown that VCB ideally has no effect on the small signal model either EE105 Fall 2007 Lecture 5 Slide 9 Prof Liu UC Berkeley Small Signal Model Example 1 The small signal model parameters are calculated for the DC operating point and are used to determine the change in IC due to a change in VBE IC 1 gm VT 3 75 r 375 gm EE105 Fall 2007 Lecture 5 Slide 10 Prof Liu UC Berkeley Small Signal Model Example 2 In this example a resistor is placed between the power supply and collector to obtain an output voltage signal Since the power supply voltage does not vary with time it is regarded as ground reference potential in smallsignal analysis EE105 Fall 2007 Lecture 5 Slide 11 Prof Liu UC Berkeley The Early Effect In reality the collector current depends on VCE For a fixed value of VBE as VCE increases the reverse bias on the collector base junction increases hence the width of the depletion region increases Therefore the quasineutral base width decreases so that collector current increases EE105 Fall 2007 Lecture 5 Slide 12 Prof Liu UC Berkeley Early Effect Impact on BJT I V Due to the Early effect collector current increases with increasing VCE for a fixed value of VBE EE105 Fall 2007 Lecture 5 Slide 13 Prof Liu UC Berkeley Early Effect Representation EE105 Fall 2007 Lecture 5 Slide 14 Prof Liu UC Berkeley Early Effect and Large Signal Model The Early effect can be accounted for by simply multiplying the collector current by a correction factor The base current does not change significantly EE105 Fall 2007 Lecture 5 Slide 15 Prof Liu UC Berkeley Early Effect and Small Signal Model VCE VA VA ro VBE I C I C I S exp VT EE105 Fall 2007 Lecture 5 Slide 16 Prof Liu UC Berkeley Summary of BJT Concepts EE105 Fall 2007 Lecture 5 Slide 17 Prof Liu UC Berkeley BJT in Saturation Mode When the collector voltage drops below the base voltage the collector base junction is forward biased Base current increases so that the current gain IC IB decreases EE105 Fall 2007 Lecture 5 Slide 18 Prof Liu UC Berkeley Large Signal Model for Saturation Mode EE105 Fall 2007 Lecture 5 Slide 19 Prof Liu UC Berkeley BJT Output Characteristics The operating speed of the BJT also drops in saturation EE105 Fall 2007 Lecture 5 Slide 20 Prof Liu UC Berkeley Example Acceptable VCC Range In order to prevent the BJT from entering very deeply into saturation the collector voltage must not fall below the base voltage by more than 400 mV VCC I C RC VBE 400mV EE105 Fall 2007 Lecture 5 Slide 21 Prof Liu UC Berkeley Deep Saturation In deep saturation the BJT does not behave as a voltage controlled current source VCE is relatively constant EE105 Fall 2007 Lecture 5 Slide 22 Prof Liu UC Berkeley
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